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DEPT. OF HEALTH AND HUMAN SERVICES Date: 10/18/85 Number: 41 PUBLIC HEALTH SERVICE FOOD AND DRUG ADMINISTRATION Related Program Areas: Drugs *ORA/ORO/DEIO/IB*
ITG SUBJECT: EXPIRATION DATING AND STABILITY TESTING FOR HUMAN DRUG
PRODUCTS
BACKGROUND
Publishing of 21 CFR Part 211 - Current Good Manufacturing Practice
for
Finished Pharmaceuticals established requirements concerning the
expiration date on a drug product and stability testing to assure the
appropriateness of that date. Each drug product may be a unique article
because of, for instance, differences in (1) chemical and physical
properties of the active ingredients or the excipients, (2)
manufacturing procedures, (3) formulations, (4) containers and
closures, (5) proposed storage conditions, and (6) the stability of
the
article to maintain its quality or purity through the use of
antioxidants or preservatives. Because of the uniqueness of each drug
product, it is virtually impossible to provide one set of rules that
can apply to all situations. The CGMPs were purposely written broadly
to allow for such unique differences.
EXPIRATION DATING (21 CFR 211.137)
A. Absence of an Expiration Date
The absence of an expiration date on any drug product packaged
after September 29, 1979, except for those drugs specifically
exempt by 211.137 (e), (f), and (g), is cause to initiate
regulatory action against the product and/or the responsible firm.
B. Exemptions
OTC drug products meeting the exemption of 211.137 (g) may utilize
accelerated testing programs to support the requirement that they
are stable for at least three years. Information obtained from old
stock, not previously the subject of stability studies, may also be
utilized.
C. Products Intended for Reconstitution
Any drug product intended for reconstitution and not bearing an
expiration date for the unreconstituted product and another
expiration date for the product after reconstitution is considered
to be out of compliance with 211.137 (c). There must be separate
stability studies to support each expiration date.
STABILITY TESTING (21 CFR 211.166)
A. Written Stability Testing Program
The absence of a written protocol for stability testing is cause to
initiate regulatory action against the product and/or the
responsible firm.
B. Supportive Stability Data
1. Number and Size of Batches
Initial stability testing by accelerated testing may be
performed on a batch smaller than the normal production size as
long as the batch is produced by similar equipment as would be
used for regular production.
Generally, the placing of three initial batches into the long
term stability program is considered minimal to assure batch
uniformity for establishing an expiration date. Since a dosage
form is a complex unit and there are continued variables in the
production process, such as change in personnel, raw material
lots and suppliers, and equipment, it is imperative that
stability studies are not limited only to initial production
batches but a portion of annual production batches be the
subject of an ongoing stability program.
2. Accelerated Studies
When accelerated stability studies are performed, one batch may
be adequate in order to establish a tentative expiration date.
This is acceptable since it is not the purpose of an
accelerated test to determine batch uniformity but rather to
test for kinetic degradation.
The use of accelerated testing data to establish a tentative
expiration dating period of greater than three years is
discouraged when it is based solely on accelerated data.
Combining data compiled at room temperature and at accelerated
temperature is possible to justify an expiration dating period
of over two years. This can be done, as an example, by taking a
sample product that has been at room temperature for one year
and subjecting that sample to accelerated temperature
conditions. The expiration dating period used would then be the
sum of that justified individually at each storage condition.
We do not believe it is reasonable to perform accelerated
testing at very high temperatures for a very short time and
expect to extrapolate results to a very long expiration dating
period since the actual mechanism of degradation at high
temperature may be different than at room temperature.
3. Test Intervals
It is commonly recommended that stability testing be performed
initially, than every three months for the first year, then
every six months for the second year, and then annually
thereafter. However, more frequent testing near the end of the
anticipated expiration date is often likely to give better
information about the actual stability of the finished product.
Nonetheless, testing at least annually is considered minimal
for compliance with CGMPs. Some firms have chosen, for
economical purposes, random dates to test all stability
samples of a given product. As long as there is at least one
test performed annually, this approach can be quite
satisfactory.
4. Storage Conditions
If a product was stored under controlled conditions, those
actual conditions (temperature and humidity) should be
recorded. Merely stating that a product was stored at room
temperature is not sufficient for purposes of determining
stability. The USP defines controlled room temperature as
being between 15 and 30 C (59 and 86 F). A product stored
for stability at or near 15 C may have quite a different
quality profile at its expiration date than a product stored
at or near 30 C. Based on published information, it appears
that 24-25 C is a reasonable reference for thermal exposure
at room temperature.
Stability studies should be conducted on product stored under
normal storage conditions or, preferably, under exaggerated
conditions. Products liable to degradation by light or moisture
should be stored either in a lighted area or under conditions
of high humidity unless it can be demonstrated that the
packaging will prevent deterioration by that condition of
interest. For example, a product liable to degrade by light
need not be stored in a lit area if it is normally packaged
and stored for use in an opaque container.
5. Test Methods
While 211.166 (a) (3) merely requires that test methods be
reliable, meaningful, and specific, section 211.165 (e) gives
more guidance by stating that the accuracy, sensitivity,
specificity, and reproducibility of test methods employed by
the firm shall be established and documented. Section 211.194
(a) (2) further requires that all testing methods used shall
be verified under actual conditions of use. Testing procedures
must include a stability indicating test which will distinguish
the active ingredient from any degradation products and be
able to make a reliable estimate of the quantity of any
degradate. The stability indicating test does not have to be
the assay method used to determine product strength.
Manufacturers, who contract with analytical laboratories to
perform either end product testing or stability studies, or
who produce product under contract for other firms are
ultimately responsible for the quality of the product and must
have copies of all analytical procedures employed and the
appropriate documentation to assure their validity on file.
Likewise, repackers who rely on stability studies performed by
the manufacturer must have copies of all analytical data
necessary to support the expiration dating period.
Although specific methods are critical to determine product
stability, they do not have to employ any specific technique.
The use of quantitative analysis, where limits are known, such
as thin layer chromatography, may be satisfactory. While many
USP tests are specific for the drug or its degradates and may
be used for stability testing, some USP monographs do not
incorporate stability indicating tests. Additionally, it may
be unreasonable to expect a manufacturer to develop specific
methodology for each component of some multi-component drugs
containing ingredients of botanical origin such as benzoin,
Peruvian balsam or tolu balsam.
6. Container-Closure Systems
The requirement that stability testing be performed in the same
container-closure system as that in which the drug product is
marketed has been subject to interpretation. The courts ruled
in U.S. vs. Kaybel that when a "new drug" was repackaged,
the
repacker did not have to obtain pre-market approval of the
repackaged product or the firm's repacking procedures. However,
the repacker is subject to applicable current good
manufacturing practices.
Although stability studies were performed on the dosage unit
in the original manufacturer's container, the event of placing
the dosage unit into a different storage unit may and often
does affect the product's shelf life. It is the policy of the
Center for Drugs and Biologics to allow repacking into
container-closure systems that can be demonstrated to be at
least as protective or more protective than the original
system without performing new stability studies prior to
marketing.
Satisfactory comparison of container-closure systems may be
done by several methods, i.e., literature reference to
permeation properties of different container materials;
performance of moisture permeation testing; or comparing the
properties of the original container-closure system to a new
system by stress testing. (Stress testing refers to testing
the product after storage under exaggerated conditions. This
will usually involve high temperature and high humidity.)
It is also current policy to allow firms to repackage solid
dosage units from plastic containers into glass containers
because glass has been shown to be a superior moisture and gas
barrier. This policy does not apply to liquid drugs because of
pH problems resulting from the alkaline nature of glass.
Policies relating to the expiration dating of unit dose
repackaged drugs may be found in Compliance Policy Guide
7132b.11. This also does not apply to repacking from bulk
containers.
7. Container Sizes to be Tested
When the same product is marketed in more than one size, e.g.,
bottles containing 100 tablets and bottles containing 1,000
tablets, or bottles containing 4 oz of syrup and bottles
containing 16 oz of syrup, it can be demonstrated, by
comparing the ratio of the surface area of the container to
the internal volume, that smaller containers have a higher
ratio than larger containers. This indicates that the smallest
marketed container is the most critical in terms of the
container properties contributing to product degradation.
Thus, moisture or oxygen permeation through a 4 oz bottle is
more critical than through a 16 oz bottle of similar
construction. For this reason, when studying stability of the
product marketed in several sizes of similar containers,
testing of the smallest container size is imperative to be in
compliance with CGMPs. While we recommend that all other
container sizes be subjected to stability testing, the fact
that some may not is not necessarily a violation of CGMPs.
8. Preservatives
Products formulated to contain preservatives to inhibit
microbial growth should be monitored throughout their shelf
life to assure the effectiveness of the preservative system.
Once a minimally effective level of preservative is
established, chemical testing for the preservative(s) may be
performed. The preservative system should be monitored at the
same stability testing times as other ingredients are
monitored.
9. Bulk Drug Substances (Bulk Pharmaceutical Chemicals)
While expiration dating is not required specifically for bulk
drugs in the CGMP regulations, it is feasible and valuable to
expect the manufacturer of bulk drug substances to assure that
their product is stable for the intended period of use.
A stability testing program for bulk drug substances should
contain, at the minimum, the following features:
a. The program shall be in writing.
b. The program should include samples from at least one
commercial-size batch; thereafter, one batch each year
should be entered into the program.
c. Samples should be stored in containers that approximate the
market containers; if it is not practical to do so, samples
may be stored in other similar containers, provided that
data show that such containers will yield results
comparable to those obtained with market containers.
d. Samples should be stored at room temperature; an additional
sample stored at elevated temperatures or under other
stress conditions may be used if it is appropriate to do
so.
10. Sterility Testing
Products manufactured as sterile must maintain that quality
throughout the labeled expiration dating period as long as the
product is unopened and stored according to labeled
instructions. The ability of the product to retain its sterile
condition is a function of the container-closure system. When
qualifying the container-closure system, sterility testing
should be performed initially and at the end of the expiration
dating period. Once any particular container-closure system
can be demonstrated to maintain sterility throughout the
expiration dating period, it is unnecessary to revalidate its
ability to maintain sterility for other ingredients that may
be placed into the same container-closure system.
Products sterilized in glass ampuls need not be subjected to
sterility testing as part of the stability testing program.
DEPT. OF HEALTH AND HUMAN SERVICES Date: 3/14/86 Number: 42 PUBLIC HEALTH SERVICE Related Program Areas: FOOD AND DRUG ADMINISTRATION Medical Devices *ORA/ORO/DEIO/IB*
ITG SUBJECT: TIN WHISKERS - PROBLEMS, CAUSES, AND SOLUTIONS
Recently, a little-known phenomenon called tin whiskering caused the
recall of several models of a pacemaker. This incident revealed tin
whiskers to be a general threat to all users and manufacturers of
medical devices that incorporate electronic circuitry. To prevent
future problems, field personnel will need to educate themselves and
manufacturers. This guide is intended to help in this endeavor by
describing the problems, causes, and solutions associated with tin
whiskers.
TIN WHISKERS
Tin whiskers are metal filaments which grow from tin. They are
extremely thin, 1-2uM typically, and grow as straight, kinked, or
spiraled single crystals of tin. They can reach a length of 9mm (3/8")
and carry 10mA of current before burning up. The electrical resistance
of a tin whisker 3mm (1/8") long is about 50 ohms. Because of
their
current carrying ability and low electrical resistance, whiskers are
a
threat to electronic circuits.
The ability of tin whiskers to cause electronic circuit problems was
established in 1951. Many sudden failures and intermittent problems
were associated with tin whiskers because of their ability to short
closely spaced electronic circuits. Whiskers were found to grow across
circuit connections and, because of their thin, brittle nature, would
break free and lodge across circuits. Investigation into preventive
measures was started, but solutions developed slowly due to the
complex nature of tin whisker growth.
The exact cause of tin whisker growth is still not fully understood.
It
is known that a whisker grows from its base and that the tin around
the
base does not thin as the whisker grows. It seems that the energy for
growth comes from microstrains present in the tin or from externally
applied pressure. Tin atoms appear to diffuse along screw dislocations
within the tin and are pushed outwards by stresses. Growth rate varies
tremendously, and it may be unsteady. Whiskers can fully develop in
minutes or take decades to form. Spurts of growth may occur.
The growth of tin whiskers is not directly related to the surrounding
medium. Whiskers will grow in sealed components, under high vacuum,
and
in low or high humidity. Temperature has some effect on the rate of
growth, and the thickness of tin deposits affects whisker density.
An
obvious factor affecting whisker growth is pressure. High-compression
pressure from bolts or screws will always produce whiskers in tin
deposits.
It was 1974, two decades after the problem was recognized, that
scientifically valid methods were established for controlling whisker
growth. Two methods are currently used. The most common is to avoid
using tin. Other metals or alloys of tin are used instead with solder
(tin/lead) being the most popular. The other method is known as "reflow."
After the tin is in place, the tin coated part is heated to a
temperature above tin's melting point. This heating releases any stress
that exists within the tin deposit.
The FDA became interested in tin whiskers as the result of pacemaker
failures. A group of pacemakers from a single manufacturer were found
to have a high rate of failure due to tin whiskers growing from the
tin-plated case of the pacemaker crystal component. An electrical
bridge between the crystal and its case disabled the crystal component,
resulting in the total loss of pacemaker output. The FDA issued a Class
I recall for the affected devices and initiated a follow-up
investigation.
Examination of the manufacturing process revealed that the
manufacturer's specification for the crystal component should have
prevented tin-whisker growth. The crystal component specification
called for gold, nickel, or solder plating. Any one of these case
coatings would have prevented the tin whisker problem. The
manufacturer, however, failed to test the crystal components for proper
material composition. It relied on its vendor to deliver proper
components. Unfortunately, a bad batch of crystal components resulted
in 80 percent of the affected devices having tin-plated crystal
components.
PROBLEM PREVENTION
Testing is the key to preventing tin whiskers. Testing for material
composition and/or material structure should be part of any critical
device manufacturing.
Testing must be performed independent of the vendor. As the pacemaker
case illustrates, a manufacturer cannot absolutely rely on the vendor
to meet composition specifications. Errors will occur which make
independent testing of critical components mandatory.
Manufacturers must test for the appropriate problem. Knowledge of tin
whiskers is relatively new in industry and problems of understanding
will arise. In the follow-up investigation, a manufacturer questioned
by FDA confused tin dendritic growth with whisker growth. Both of these
phenomena occur in tin but they are very different. Dendrites result
from a voltage induced plating phenomenon which requires both voltage
and high humidity. Whiskers grow spontaneously. The manufacturer
erroneously claimed that his pacemakers were resistant to tin whiskers
because his circuitry was protected from voltage and high humidity.
He
even used dendrite as a synonym for whisker. Obviously, this
manufacturer was confused about the causes and prevention of tin
whiskers.
Some knowledge of tin whiskers, its causes and consequences, should
be
available at the manufacturing site. Asking about component composition
testing will quickly reveal a manufacture's understanding of the tin
whisker problem. If a manufacturer specifies whisker resistant parts
and uses component composition testing to verify his specifications,
then tin whiskers will not cause problems.
CONCLUSIONS
Untreated tin coating should never be used in conjunction with
electronic circuitry.
Device manufacturers should verify the material composition and/or
the
material structure of all critical electronic components independent
of
the component supplier.
The investigator can help prevent tin whisker problems by enquiring
into material testing of critical electronic components. However,
knowledge of tin whiskers is new in the industry. The investigator
may
first have to educate manufacturers about tin whisker problems before
looking into material testing.
Manufacturers should attempt to control the problem for the most part
by specification with subsequent assurance of vendor conformance to
the
specification. Every manufacturer of electronic devices would not be
expected to do material analyses of all incoming components. It would,
however, be expected of every pacemaker manufacturer.
REFERENCES:
1) "Eliminate Whisker Growth on Contacts by Using a Tin Alloy
Plate," R.P. Diehl & N.A. Cifaldi, Insulation/Circuits, pp.
37-39, Apr. 1976.
2) "How to Avoid Metallic Growth Problems on Electronic Hardware,"
Tech Report IPC-TR-476, The Institute for Interconnecting &
Packaging Electronic Circuits (IPC), 1977.
3) "Identification of Fused Tin Coatings on Integrated Circuit
Device Leads," W.G. Bader, Plating & Surface Finishing, pp.
56-57, Aug. 1977.
4) "Spontaneous Growth of Whiskers on Tin Coatings: 20 Years of
Observation," S.C. Britton, Trans. Inst. of Metal Finishing,
Vol 52, pp. 95-102, Apr. 1974.
5) "Tin Whiskers: Causes & Remedies," N.A.J. Sabbagh
&
H.J. McQueen, Metal Finishing, pp. 27-31, Mar. 1975.
DEPT. OF HEALTH AND HUMAN SERVICES Date: 4/18/86 Number: 43 PUBLIC HEALTH SERVICE Related Program Areas: FOOD AND DRUG ADMINISTRATION Drugs, Biologics, Diagnostics *ORA/ORO/DEIO/IB*
ITG SUBJECT: LYOPHILIZATION OF PARENTERALS
Recent inspections have disclosed potency and sterility problems
associated with the manufacture and control of lyophilized products.
In
order to provide guidance and information to investigators, some
industry procedures and deficiencies associated with lyophilized
products are identified in this ITG.
It is recognized that there is complex technology associated with the
manufacture and control of a lyophilized pharmaceutical dosage form.
Some of the important aspects of these operations include: the
formulation of solutions; filling of vials and validation of the
filling operation; sterilization and engineering aspects of the
lyophilizer; and testing of the end product. This discussion will
address some of the problems associated with the manufacture and
control of a lyophilized dosage form.
Product Type/Formulation
Products are manufactured in the lyophilized form due to their
instability when in solution. Many of the antibiotics, such as some
of
the semi-synthetic penicillins, cephalosporins, and also some of the
salts of erythromycin, doxycycline and chloramphenicol are made by
the
lyophilization process. Because they are antibiotics, low bioburden
of
these formulations would be expected at the time of batching. However,
some of the other dosage forms that are lyophilized, such as
hydrocortisone sodium succinate and methylprednisolone sodium succinate
have no antibacterial effect when in solution. For these types of
products, bioburden should be minimal and the bioburden should be
determined prior to sterilization of these bulk solutions prior to
filling. Obviously, the batching or compounding of these bulk solutions
should be controlled in order to prevent any potential increase in
microbiological levels that may occur up to the time that the bulk
solutions are filtered (sterilized). The concern with any
microbiological level is the possible increase in endotoxins that may
develop. Good practice for the compounding of lyophilized products
would also include batching in a controlled environment and in sealed
tanks, particularly if the solution is to be stored prior to
sterilization.
Filling
The filling of vials that are to be lyophilized has some problems that
are somewhat unique. The stopper is placed on top of the vial and is
ultimately seated in the lyophilizer. Because different stoppering
equipment is required, stoppers are sometimes placed on vials by
operators rather than by machine. Obviously, there is a much greater
chance of contamination in this type of operation because of the
involvement of people. Validation of these types of filling operations
should include media fills and the sampling of critical surfaces and
air during active filling (dynamic conditions).
Once vials are partially stoppered, they are transported to the
lyophilizer. Any transfer and handling, such as loading of the
lyophilizer, should take place under primary barriers, such as laminar
flow hoods under which the vials were filled. Validation of this
handling should also include the use of filled vials of media.
A major concern with the filling operation is assurance of fill
volumes. Obviously, a low fill would represent a subpotency in the
vial. Unlike a powder or liquid fill, a low fill would not be readily
apparent after lyophilization. Because many of the types of products
are antibiotics, subpotency in a vial can present a potentially very
serious situation. Good practice and a good quality assurance program
would include the frequent monitoring of the volume of fill such as
every 15 minutes. Also, there should be provisions to isolate
particular sections of the filling operations when low or high fills
are encountered.
Lyophilizer Sterilization
The sterilization of the lyophilizer is one of the most frequently
encountered problems. Some of the older lyophilizers cannot tolerate
steam under pressure and sterilization is marginal at best. These
lyophilizers can only have their inside surfaces wiped with a chemical
agent that may be a sterilant. Unfortunately, piping such as that for
the administration of inert gas (usually nitrogen) and sterile air
for
backfill or vacuum break are often inaccessible to such surface
"sterilization" or treatment. It would seem very difficult
for a
manufacturer to be able to demonstrate satisfactory validation of
sterilization of a lyophilizer by chemical "treatment."
Another method of sterilization that has been practiced is the use
of
gaseous ethylene oxide. As with any ethylene oxide treatment,
humidification is necessary. Providing a method for introducing the
sterile moisture with uniformity may be difficult.
A generally recognized acceptable method of sterilizing the lyophilizer
is through the use of moist steam under pressure. Sterilization
procedures should parallel that of an autoclave and a typical system
should include:
a) Two independent temperature sensing systems. One would be used to
control and record temperatures of the cycle as with sterilizers,
and the other would be in the cold spot of the chamber.
b) Provisions for sterilizing the inert gas or air supply lines.
c) Provisions for sterilizing and assuring the integrity of vent
filters or filters used to sterilize the inert gas and air.
d) Provisions for sterilizing the shelf support rods unless they are
only exposed to the inside of the chamber after stoppering.
Generally, lyophilizers should be sterilized after each cycle
because of the potential for contamination of the shelf support
rods.
Finished Product Testing
Typical finished product testing that should be reviewed includes dose
uniformity testing, sterility testing, and any required stability
testing of aged batches of reconstituted solutions.
a) Dose Uniformity
When the lyophilized product does not include an excipient or other
additive and only represents the active ingredient, weight
variation can be employed as the means to test for dose uniformity.
However, when excipients or other additives are present, weight
variation may be applied provided there is correlation with the
sample weight and potency results.
b) Sterility Testing
Although products may be labeled for reconstitution with
Bacteriostatic Water for Injection, Sterile Water for Injection
(WFI) should be used to reconstitute products. Because of the
potential toxicities associated with Bacteriostatic Water for
Injection, many hospitals only utilize WFI. Bacteriostatic Water
for Injection may kill some of the vegetative cells if present as
contaminants.
c) Stability
Generally lyophilized products have short expiration dates.
Stability data should be reviewed and the justification of the
expiration date should be based on batches with the higher moisture
content. Additionally, stability data should include provisions for
the assay of aged samples and subsequent reconstitution of these
aged samples for the maximum amount of time specified in the
labeling. Additionally, this stability testing should include least
concentrated as well as the most concentrated reconstituted
solutions.
References:
Remington's Pharmaceutical Sciences, 17th Edition, Mack Publishing
Co.,
Easton, PA, 1985.
Morgan, SL and Spotts, MR; "How to Select a Pharmaceutical
Freeze-Drying System"; Pharmaceutical Technology, volume 3, No.
11, pp.
94-101, 114; 1979.
Beisswenger, H.L.; "Manufacturing Economies and Product Improvement
Through Advanced Freeze Drying Techniques"; Reprint of talk to
Annual
Meeting of Parenteral Drug Assn.; New York City, Nov. 13, 1968.
Flamberg, DW; "Manufacturing Considerations in the Lyophilization
of
Parenteral Products"; Pharmaceutical Manufacturing, Volume 3,
No. 3,
pp. 29-31, 48; 1986.
DEPT. OF HEALTH AND HUMAN SERVICES Date: 5/15/86
Number: 44
PUBLIC HEALTH SERVICE
FOOD AND DRUG ADMINISTRATION Related Program Areas:
*ORA/ORO/DEIO/IB* ALL
ITG SUBJECT: RADIATION PROTECTION TERMINOLOGY
The recent incident at the Soviet Union's Chernobyl nuclear power plant
has heightened concerns for radioactive contamination in FDA regulated
products. The potential hazard will undoubtedly necessitate increased
sampling of both imported and domestic food products such as produce,
milk, etc. A list of technical terms has been compiled to serve as
a
quick reference for the terminology associated with radiation
protection. If you have any concerns and questions on radiation
protection, contact the Regional Radiological Health Representatives
(RRHR).
alpha particle ( ) A charged particle emitted from the nucleus
of an atom having a mass and charge equal
in magnitude to a helium nucleus. It cannot
penetrate the outer layer of the skin and
represents very little external hazard.
becquerel (Bq) A unit of measurement of activity. See the
Conversion Table.
beta particle (ß) A charged particle emitted from the nucleus
of an atom, with a mass and charge equal in
magnitude to that of the electron. It has a
penetrating range of a few feet in air and
can be easily stopped by a thin sheet of
metal or plastic.
counter Geiger-Muller (G-M) counter
A radiation detection and measuring
instrument consisting of a gas-filled tube
and associated electronic circuits.
Scintillation counter
A radiation measuring instrument consisting
of phosphor, photomultiplier tube, and
associated electronic circuits for counting
light emissions produced in the phosphor by
ionizing radiation. It is a more sensitive
device than the G-M counter.
curie (Ci) The special unit of activity. One curie
equals 3.7 x 10 10 nuclear transformations
per second. This unit reflects the
intensity of the radioactive source.
Some common fractions are:
Millicurie: One thousandth (10 -3) of a
curie (mCi)
Microcurie: One millionth (10 -6) of a
curie (µCi)
Nanocurie: One billionth (10 -9) of a
curie (nCi)
Picocurie: One millionth of a microcurie
(10 -12) (pCi)
dose, absorbed The energy imparted to matter by ionizing
radiation per unit mass of irradiated
material. The unit of absorbed dose is the
rad.
dose equivalent (DE) The quantity that expresses all radiation
on a common scale for calculating the
effective absorbed dose. It is defined as
the product of the absorbed dose in rads
and certain modifying factors. The unit of
DE is the rem.
dosimeter A portable instrument for measuring and
registering total accumulated gamma ray
exposure. Two common types are the
"self-reading" pocket size dosimeters and
the TLD (thermal luminescent dosimeter)
requiring laboratory processing. Dosimeter
readings are normally in roentgens or
milliroentgens.
exposure A measure of the ionization produced in the
air by X or gamma radiation. The special
unit of exposure is the roentgen.
gamma ray ( ) High-energy, short wavelength
electromagnetic radiation, emitted from the
nucleus. Gamma rays are similar to X-rays,
but are usually more energetic. It travels
at the speed of light and is the most
penetrating type of radiation.
gray (Gy) A unit of absorbed dose. See the
Conversion Table.
half-life, radioactive Time required for a radioactive substance
to lose 50% of its activity by decay. Each
radionuclide has a unique half-life.
Radionuclide Half-life (approximate)
barium (Ba) 140 13 days
cesium (Ce) 134 2 years
cesium (Ce) 137 30 years
cobalt (Co) 60 5 years
iodine (I) 131 8 days
strontium (Sr) 90 30 years
Protective Action Guides Refer to Federal Register, Oct. 22, 1982.
for human food (FDA)
rad (radiation The unit of absorbed dose equal to 100 ergs
absorbed dose) per gram or 0.01 joule per kg.
radiation, background Radiation arising from radioactive material
in the environment. Background radiation
due to cosmic rays and naturalr
adioactivity is always present. The
estimated total dose a person receives from
natural radiation in the U.S. is 100
mrem/year.
radiation, external Radiation from a source outside the body.
radiation, internal Radiation from a source within the body (as
a result of the deposition of
radionuclides in body tissue).
radiation, ionizing Any electromagnetic or particulate
radiation capable of producing ions,
directly or indirectly, in its passage
through matter.
relative biological The RBE is a factor used to compare the
effectiveness (RBE) biological effectiveness of absorbed
radiation doses (i.e., rads) due to
different types of ionizing radiation. The
RBE is the ratio of rem to rad.
rem (roentgen A special unit of dose equivalent. The dose
equivalent man) equivalent in rems is numerically equal to
the absorbed dose in rads multiplied by
certain modifying factors. With X or gamma
rays, the factor is close to one, so I rem
is equal to 1 rad.
roentgen (R) The special unit of exposure. One roentgen
equals 2.58 x 10 -4 coulomb per Kg. of
air.
sievert (Sv) A unit of dose equivalent. See the
Conversion Table.
X-rays Penetrating electromagnetic radiation whose
wave lengths are shorter than those of
visible light. In nuclear reactions, it is
customary to refer to photons originating
in a nucleus as gamma rays, and those
originating in the extranuclear part of the
atom as X-rays. These rays are sometimes
called roentgen rays, after their
discoverer, W. K. Roentgen.
CONVERSION TABLE
----------------
quantity international int'l old special conversion
unit & symbol system unit & symbol factor
used in Europe units used in U.S.
and Canada
-------- ------------- ------ ------------ ----------------
activity becquerel s -1 curie (Ci) 1 Bq ?? 27 pCi
(Bq)
exposure - CKg -1 roentgen (R) 1 Ckg -1 ?? 3876 R
absorbed gray (Gy) Jkg -1 rad (rad) 1 Gy = 100 rad
dose
dose sievert Jkg -1 rem (rem) 1 Sv = 100 rem
equivalent (Sv)
------------------------------
Acknowledgment
We wish to acknowledge gratefully the technical information provided
by
Dr. Donald L. Thompson and Mr. Gail D. Schmidt, Office of Health
Physics.
References
The Effects on Populations of Exposure to Low Levels of Ionizing
Radiation: 1980, Committee on the Biological Effects of Ionizing
Radiations, Division of Medical Sciences, Assembly of Life Sciences,
National Research Council, National Academy Press, Washington, D.C.,
1980.
Glossary of Terms: Nuclear Power and Radiation, U.S. Nuclear Regulatory
Commission, Washington, D.C. 20555, June 1981.
Handbook of Forensic Science, FBI, March 1984.
Federal Register, Vol. 47,47073-83. Oct. 22, 1982.
DEPT. OF HEALTH AND HUMAN SERVICES Date: 2/20/87 Number: 45 PUBLIC HEALTH SERVICE FOOD AND DRUG ADMINISTRATION Related Program Areas: *ORA/ORO/DEIO/IB* All Programs
ITG SUBJECT: CIRCULAR TEMPERATURE RECORDING CHART MEASUREMENTS
INTRODUCTION
Circular temperature recording charts are used in a broad variety of
firms from food canneries to medical device manufacturers to drug
manufacturers. Investigators currently use draftman's dividers to
measure chord (straight line, point-to-point) instead of arc (curve)
distances to check process time. There is no easy way to measure arcs
in investigational work. Firms also use dividers or stamped metal
gauges to do the same chord measurements. When an investigator wants
to
set dividers to a 25 minute chord, for instance, he/she normally checks
the setting by spanning 2 (on charts with 10 minute markings) or 3
(on
charts with 15 minute markings) estimated 25 minute chords to come
up
with an even marked time (50 and 75 minutes on 10 and 15 minute marked
charts, respectively). He/she repeats the process until successful.
Once the dividers come up on the even markings, the investigator
assumes the dividers are properly set. However, estimating odd times
such as 13, 19, or 23 minutes is difficult using this method.
In addition to the previous problem, a one minute time difference is
only about 0.014 inch on a typical Taylor OP524 chart. Introducing
a
0.014 inch error by making 15 spans or misstamping a gauge is easy
to
envision.
Another problem is the extreme difficulty in extrapolating times from
known chord lengths. An investigator cannot calculate a 17 minute chord
distance accurately, given either longer (20, 30 minutes) or shorter
(10, 15 minutes) chord distances, because of the simple fact that two
chords representing shorter times (i.e., 10 minutes each) have a
combined length longer than a 20 minute chord.
These chord problems are compounded when a firm uses different process
temperatures, handles multiple products and/or containers, or uses
different temperature recording charts.
COMPUTER-CALCULATED CHORDS
The geometry for determining the arc or chord length of a circle is
shown in Appendix 1. A computer can be used to generate reference
tables of chord lengths related to times for any circular recording
chart at any temperature. An investigator only needs to know: (1) the
number of hours on the chart, and (2) the diameter of the circle for
the particular temperature desired. Geometric theory is based on the
fact that a radius bisecting a chord creates two equal right triangles.
APPLICATION
It is suggested that personnel modify the attached computer programs
which were written in Microsoft BASIC. Program Listing 1, written for
a
Tandy 200 laptop portable computer, is compatible on most
microcomputers. Program Listing 2 was written for a TRS-80 Model 1
computer. Both programs should run on IBM-PCs unchanged (except
possibly for adding spaces where required by GWBASIC). The programs
can
be adapted to any district computer by knowledgeable personnel. Test
the resultant programs by using values from a 24 hour Taylor OP600
chart. The diameter of a circle for 240 degrees F is 7.929 inches.
Results should be 0.017299 for 1 minute, 0.034596 for 2 minutes,
0.051895 for 3 minutes, 0.172970 for 10 minutes, and 2.052180 for 120
minutes. Program Listing 2 can be used to print reference tables while
Program Listing 1 is primarily a demonstration program. The most useful
application of the programs would be to develop reference tables and
maintain them in EI files. Once a blank chart is collected and expected
process temperatures are run through Program Listing 2, the resultant
tables (similar to Table 1 attached) can be kept in EI files and
photocopies can be taken to inspection sites. Another possible
application is to take laptop computers to inspection sites. A computer
used in conjunction with a dot matrix printer can also generate chord
gauges on paper similar to the metal gauges made in cannery machine
shops.
Ouestions about the geometric theory or computer programming can be
addressed to DEIO/IB, (301)443-3340.
Appendix 1. Geometric Theory (Diagram not available)
1. An isosceles triangle is formed by connecting a chord (C) with the
center of a circle. Call (A) the angle in the center of the circle.
2. A line through the center of the circle, perpendicular to the
chord, bisects the chord and its arc.
3. Since the bisecting line is perpendicular to the chord, each half
of the original triangle is a right triangle.
4. Each right triangle formed above has a small angle equal to A/2
Let
A/2 = a.
5. Therefore Sin a = C/2 divided by the radius (R) or, Sin a = C/2R
=
C/D where D is the diameter of the circle.
6. Solving for the chord, C = D Sin a. The chord represents the time
which the investigator measures with dividers.
7. We want to make chord values equal to minutes. It is simplest to
standardize on the fact that there are 360 degrees in a circle. If
we assign one minute equal one degree to angle A and its chords,
there are 720 angle a's (degrees) in the circle. Thus, we create a
formula where a (minutes) = a (degrees) divided by 2, or more
apprpriately t (minutes) = a (degrees)/2.
8. We must also consider a factor to correct different chart values
to
minutes because while there are 360 minutes on a 6 hour chart,
there are 720 minutes on a 12 hour chart and 1440 minutes on a 24
hour chart. From these figures, factor F = 6/H where H is the
number of hours on the chart.
9. Combining the above two formulas, t (minutes) = a (degrees) 1/2.
Placing this in step 6 above, we get chord = diameter Sin (3t/H) or
C = D Sin (3t/H). This is the formula used in Program Listings 1
and 2.
Program Listing 1
1 REM DARRELL LEE, SALT LAKE CITY RESIDENT POST FTS 588-5285
10 DIM C(360): K = .0174533: CLS : REM DIMENSIONS VARIABLES, INSERTS
RADIAN
CORRECTION FACTOR (K)
100 INPUT "Enter chart name and number"; CH$
110 INPUT "Enter no. hours on chart"; H
120 F = H / 3: REM F IS CORRECTION FACTOR
130 INPUT "Enter target temperature for chord lengths"; TE
140 PRINT "Enter diameter of"; TE; : INPUT " degree
circle in inches or
millimeters"; D
150 IF D > 15 THEN M$ = "millimeter" ELSE M$ = "inch"
160 INPUT "Is this correct (y/n)"; A$
170 IF LEFT$(A$, 1) = "y" OR LEFT$(A$, 1) = "Y"
THEN 200 ELSE 100
200 PRINT "Table of times with corresponding chord lengths for
a(n) "; CH$;
" chart at "; TE; " degrees (a"; D; M$; "
diameter circle)."
210 PRINT : PRINT "Minutes ";
220 IF M$ = "millimeter" THEN S$ = "s" ELSE S$
= "es"
230 PRINT M$; S$: PRINT
240 FOR T = 1 TO 120 STEP 4
250 FOR TT = T TO T + 3
260 C(TT) = D * SIN(K * TT / F)
270 NEXT TT
280 PRINT T; C(T): PRINT T + 1; C(T + 1): PRINT T + 2; C(T + 2): PRINT
T + 3; C(T + 3)
290 NEXT T
Program Listing 2
10 ' REPORT CHART PLANNER
20 ' RUNS ON RADIO SHACK TRS-80 COMPUTER. MODIFY FOR OTHER COMPUTERS
NOT
USING MICRSOFT BASIC.
30 CLEAR 2000
35 LPRINT CHR$(3): REM PRINTER COMMAND RESETS PAGE
40 DIM C(360): K = .0174533: A$ = "### ###.###### ### ###.######
### ###.
###### ### ###.###### ### ###.######"
50 ' K = RADIAN CORRECTION FACTOR. NOT USED IF COMPUTER CALUCULATES
SINE
USING DEGREES.
60 CLS
70 FOR I = 1 TO 8: PRINT : NEXT I
80 PRINT "********** RETORT CHART PLANNER **********"
90 FOR I = 1 TO 500: NEXT: FOR I = 1 TO 6: PRINT : NEXT I
100 PRINT " ***** DARRELL LEE *****"
110 PRINT " * 2073 E. GYRFALCON DR. *"
120 PRINT " * SANDY. UT 84092 *"
130 PRINT " * SALT LAKE CITY RESIDENT POST *"
140 PRINT " * ROOM 1033 ADMINISTRATION BLDG. *"
150 PRINT " * 1745 WEST 1700 SOUTH *"
160 PRINT " * SALT LAKE CITY. UT 84104 *"
170 PRINT " *******************************************"
180 PRINT : PRINT
190 FOR I = 1 TO 1000: NEXT I
200 INPUT "ENTER RETORT CHART MANUFACTURER'S NUMBER (IE. TAYLOR
OP 524)"; O$
210 INPUT "ENTER NUMBER OF HOURS ON RETORT CHART (6,12,24)";
H
220 F = H / 3
260 INPUT "ENTER TARGET TEMPERATURE FOR CHORD LENGTHS"; TE
270 PRINT "ENTER DIAMETER OF"; TE; : INPUT "DEGREE CIRCLE
IN INCHES OR
MILLIMETERS"; D
280 IF D > 15 THEN M$ = "MM. ": GOTO 300
290 M$ = "INCH "
300 LPRINT "TABLE OF TIMES WITH CORRESPONDING CHORD LENGTHS FOR
A(N)
": LPRINT
310 LPRINT O$:' CHART AT";TE; "DEGREES (A";D;M$;"DIAMETER
CIRCLE)":LPRINT
320 LPRINT "FOR ADDITIONAL CUSTOM MADE DATA TABLES CONTACT DARRELL
LEE AT
SALT LAKE CITY RESIDENT POST.": LPRINT : LPRINT : LPRINT
330 FOR T = 1 TO 120 STEP 4
340 FOR TT = T TO T + 3
350 C(TT) = D * SIN(K * TT / F): NEXT TT
360 LPRINT USINGGA$; T, C(T), T + 1, C(T + 1), T + 2, C(T + 2), T +
3,
C(T + 3)
370 NEXT T
380 INPUT "DO YOU WISH TO PRINT ANOTHER TABLE"; Q$
390 IF LEFT$(Q$, 1) = "Y" THEN 30 ELSE END
Table 1. Chart Example
TABLE OF TIMES WITH CORRESPONDING CHORD LENGTHS FOR A(N) TAYLOR OP600
CHART AT 240 DEGREES (A 7.929 INCH DIAMETER CIRCLE) FOR ADDITIONAL
CUSTOM MADE DATA TABLES CONTACT DARRELL LEE AT SALT LAKE CITY RESIDENT
POST
Time(min) Chord(in) Time(min) Chord(in) Time(min) Chord(in) Time(min)
Chord(in)
_________ _________ _________ _________ _________ _________ _________
_________
1 0.017299 2 0.034596 3 0.051895 4 0.069193
5 0.086491 6 0.103787 7 0.121084 8 0.138580
9 0.155675 10 0.172970 11 0.190264 12 0.207557
13 0.224849 14 0.242140 15 0.259429 16 0.276718
17 0.294006 18 0.311292 19 0.328575 20 0.345858
21 0.363140 22 0.380418 23 0.397696 24 0.414972
25 0.432245 26 0.449517 27 0.466787 28 0.484054
29 0.501319 30 0.518582 31 0.535842 32 0.553099
33 0.570354 34 0.587607 35 0.604856 36 0.622102
37 0.639346 38 0.656587 39 0.673824 40 0.691058
41 0.708289 42 0.725516 43 0.742741 44 0.759961
45 0.777178 46 0.794391 47 0.811601 48 0.828807
49 0.846008 50 0.863206 51 0.880400 52 0.897588
53 0.914774 54 0.931955 55 0.949130 56 0.966302
57 0.983470 58 1.000630 59 1.017790 60 1.034940
61 1.052090 62 1.069230 63 1.086370 64 1.103500
65 1.120630 66 1.137750 67 1.154870 68 1.171980
69 1.189090 70 1.206190 71 1.223280 72 1.240370
73 1.257450 74 1.274530 75 1.291600 76 1.308660
77 1.325720 78 1.342770 79 1.359820 80 1.366860
81 1.393890 82 1.410910 83 1.417930 84 1.444950
85 1.461950 86 1.478950 67 1.49540 88 1.512930
89 1.529900 90 1.546870 91 1.563830 92 1.580790
93 1.597740 94 1.614680 95 1.631610 96 1.648530
97 1.665450 98 1.682360 99 1.699260 100 1.716150
101 1.733040 102 1.749910 103 1.766780 104 1.783640
105 1.800490 106 1.817330 107 1.834160 108 1.850990
109 1.867810 110 1.884610 111 1.901410 112 1.918200
113 1.934980 114 1.951750 115 1.968510 116 1.985260
117 2.002010 118 2.018740 119 2.035460 120 2.052180
NOTE: Darrell Lee is now a National Expert in Computers and is located
in the
SAN-DO office.
DEPT. OF HEALTH AND HUMAN SERVICES Date: 12/31/86 Number: 46
PUBLIC HEALTH SERVICE
FOOD AND DRUG ADMINISTRATION Related Program Areas: Drugs,
*ORA/ORO/DEIO/IB* Biologics, Medical Devices
ITG SUBJECT: WATER FOR PHARMACEUTICAL USE
PURPOSE
This ITG will cover the different types of water used in the
manufacture of drug products.
THE 8 TYPES OF WATER ARE:
1. Non-potable
2. Potable (drinkable) water
3. USP purified water
4. USP water for injection (WFI)
5. USP sterile water for injection
6. USP sterile water for inhalation
7. USP bacteriostatic water for injection
8. USP sterile water for irrigation
The USP designation means that the water is the subject of an official
monograph in the current US PHARMACOPEIA with various specifications
for each type. The latter 4 waters are "finished" products
that are
packaged and labeled as such and need not be of concern during an
inspection outside of plants which actually produce these products.
The USP purified water and the USP WFI on the other hand are components
or "ingredient materials" as they are termed by the USP,
intended to be
used in the production of drug products.
But what about potable water as a component? Is it required to undergo
routine sampling and testing before use in production? According to
the
preamble to the Current Good Manufacturing Practice regulations
(CGMPs), no acceptance testing is required for potable water unless
it
is obtained from sources that do not control water quality to
Environmental Protection Agency (EPA) standards. It is important to
know that potable water may not be used to prepare USP dosage form
drug
products or for laboratory reagents to test solutions. However, potable
water may be used to manufacture drug substances (also known as bulk
drugs or bulk pharmaceutical chemicals).
During your inspection, determine the source of the water used for
wet
granulations or for any aqueous liquid preparations as well as for
the
laboratory. It should be of USP purified water quality both chemically
and microbiologically.
Is non-potable water a concern during drug inspections? It may be
present in a plant in the boiler feed water, cooling water for the
air
conditioning or the fire-sprinkler systems. Look carefully for any
cross-connections to the potable water supply. Non-potable water supply
lines should be clearly marked as such, especially when adjacent to
potable water supply connections.
WATER PRODUCTION SOURCES
The USP defines acceptable means of producing the various types of
component waters. USP WFI may be made only by distillation or reverse
osmosis.
Potable water is obtained primarily from municipal water systems but
may also be drawn from wells, rivers, or ponds.
SOURCES OF WATER CONTAMINATION
Piping system defects may cause contamination of clean incoming water.
Because of this possibility, point-of-use sampling is indicated, that
is, drawing the water sample after it has passed through the piping
system.
Microbial contamination of oral liquid and topical drug products
continues to be a significant problem, and is usually rooted in the
use
of contaminated water. Because of the potential health risks involved
with the use of contaminated water, particular attention should be
paid
to deionized (DI) water systems, especially at small, less
sophisticated manufacturers.
To minimize this contamination, the USP notes that water systems for
pharmaceutical manufacturing should have "corrective facilities."
By
this they mean access to the system for sanitization or introduction
of
steam, chlorinators, storage at elevated temperatures, filtration,
etc.
Inquire about these during your inspection.
Seasonal variations in temperature and growth of flora may also cause
fluctuations in microbial content of source water. Monitoring should
be
frequent enough to cover these variations.
IN-PLANT WATER TREATMENT SYSTEMS
Sand bed filters with or without chlorination equipment are common
in
larger plants. However, these may be centrally located and the water
piped to the pharmaceutical manufacturing site. The operations of
these systems should be validated along with any subsequent treatment.
If storage tanks are used, determine the capacity, the rate of use,
the
frequency of flushing and sanitizing the internal surfaces.
While depth or membrane type filters are often used in water systems,
final filtration as the sole treatment for water purification is
generally not acceptable. However, filtration could be acceptable,
for
example, when used for reducing microbial/particulate loads in potable
water used as an ingredient in chemical manufacturing where water need
not be sterile.
Chlorination of potable water is an effective treatment if minimum
levels of 0.2mg/liter of free chlorine are attained. Be aware however,
that any carbon or charcoal filters in the system will remove this
protective chlorine and thus eliminate any inhibitory effect on
microbial growth after this point.
USP WFI is usually produced in a continuously circulating system
maintained at an elevated temperature. The high temperature, maintained
uniformly throughout the system by constant circulation, prevents
significant microbial growth. A temperature of 80^oC is commonly used
and is acceptable. Somewhat lower temperatures may also be acceptable,
provided the firm has adequate data to demonstrate that a lower
temperature works as intended. If WFI is held at ambient temperature
rather than recirculation at elevated temperature, it must be dumped
or
diverted to non-WFI use 24 hours after being produced.
GENERAL COMMENT
Although there are no absolute microbial standards for water (other
than water intended to be sterile), the CGMP regulations require that
appropriate specifications be established and monitored. The
specification must take into account the intended use of the water;
i.e., water used to formulate a product should contain no organisms
capable of growing in the product. Action or alert limits must be based
upon validation data and must be set low enough to signal significant
changes from normal operating conditions.
REFERENCES
FDA Current Good Manufacturing Practice regulations, Federal Register,
Vol.43, No. 190 - Sept. 29, 1978, I. General Comments and Subpart
C, para. 211.48.
Water Programs, Environmental Protection Agency, National Interim
Primary Drinking Water Regulations, Dec. 16, 1985, 40 Code of
Federal Regulations, Part 141, para. 141.14 and 141.21.
United States Pharmacopeia XXI, Water for Pharmaceutical Purposes,
section 1231 and Official Monographs-various types of water, 1985.
FDA LETTER TO THE PHARMACEUTICAL INDUSTRY Re: Validation and Control
of
Deionized Water Systems, - Daniel L. Michels, Bureau of Drugs,
Aug. 1981.
FDA Inspection Technical Guide, Number 36, Reverse Osmosis, Oct. 1980.
FDA Inspection Technical Guide, Number 40, Bacterial
Endotoxins/Pyrogens, March 1985.
Protection of Water Treatment Systems series, PMA Deionized Water
Committee, PHARMACEUTICAL TECHNOLOGY - May, Sept. and Oct., 1983;
Sept. 1984, and Nov. 1985.
Parenteral Drug Association, Design Concepts for the Validation of
a
Water for Injection System, Technical Report No. 4, 1983.
Monitoring and Validation of High Purity Water Systems with the LAL
test for pyrogens, T.J. Novistsky, Pharmaceutical Engineering,
March-April, 1984.
DEPT. OF HEALTH AND HUMAN SERVICES Date: 4/30/87 Number: 47 PUBLIC HEALTH SERVICE FOOD AND DRUG ADMINISTRATION Related Program Areas: Drugs *ORA/ORO/DEIO/IB* and Medical Devices
ITG SUBJECT: MEASUREMENT OF RELATIVE HUMIDITY IN THE EO PROCESS
The importance and criticality of the moisture content or relative
humidity (RH) of the environment within the enclosed vessel of the
Ethylene Oxide (EO) sterilization process has been well established
by
the drug and device industries. Appropriate instrumentation systems
are
necessary to sense, control, and record this parameter. Of these, the
key element of the system and the most difficult to establish and
maintain in terms of accuracy is the moisture sensor. This ITG will
briefly describe basic types of sensors together with some limitations.
TYPES OF SENSORS/INSTRUMENTS
The simplest form of RH measurement and one with which most are
familiar, is the traditional Mechanical Hygrometer. It is based on
the
principle that certain materials will expand or contract as a function
of the amount of moisture absorbed. Materials such as paper, wood,
bone, plant leaves, textiles, animal tissue and plastics have been
historically used. The most common materials, however, have been human
or horse hair. Direct readout is usually obtained on a dial indicator
interconnected to the material via mechanical linkage. It is also
obvious that such an instrument would be difficult for remote reading
in the enclosed space of a sterilization chamber and for all
practically could not be used. It is also obvious that precision would
not be a relied-upon quality. Response is slow for both increasing
and
decreasing humidity and temperature compensation cannot be made
(calibration applies only to a specific temperature). Also, the aging
process of the material is not uniform, making calibration extremely
difficult. It is doubtful that the investigator will ever encounter
the use of such an instrument to measure the RH in an EO chamber.
Electric Hygrometer - This type utilizes a sensor that is a hygroscopic
film. An instantaneous change of electrical resistance or capacitance
can be measured as the result of small changes in absorbed moisture
by
the film. The hygroscopic film is sometimes replaced by a wire grid
wrapped around a substrate. The wire grid is usually coated with a
hygroscopic salt solution (lithium chloride is common) which absorbs
water vapor. Resistance of the wire grid corresponds to a calibrated
RH
value. A calibration curve is usually furnished for each unit by the
manufacturer. In physical appearance for EO chamber use, the sensor
most likely will resemble a metallic or plastic cylinder approximately
3/4 inch diameter by 2 inch long with perforations on the side and
electrical contact pins or connector on an end. The response to
changing RH is rapid and accuracy tolerances can be as low as ± 1.5%
RH. This type of sensor usually is not reliable after multiple
exposures to EO. A maintenance program to clean, rejuvenate, and
recalibrate periodically is necessary for the user. The investigator
should also be alerted to the fact that humidity sensitive materials
are also temperature dependent. This means that the sensor must be
calibrated over the entire temperature range of operation.
Dew-Point Hygrometer - This is commonly known as the chilled mirror
technique wherein the optical qualities of a mirror are measured as
it
is cooled by the sampled gas to cause condensation. The amount of
condensation is related to the dew-point temperature which together
with a dry bulb temperature and a standard chart can be interpolated
to
reveal RH. This method can be quite accurate (± 2% RH). One
manufacturer utilizes microprocessor technology to offer simultaneous
measurement of not only dew-point but temperature and RH as well, with
a digital readout display. Accuracy in determining RH has been claimed
as low as ± 0.5%. Maintenance required for this type of sensor would
be primarily the cleaning of the mirror and sampled gas line.
Gas Liquid Chromatography (GLC) - This technique utilizes sampling
of
the gas mixtures in the contained vessel. The areas under the curves
generated by the GLC are measured manually or automatically with a
microprocessor to determine concentrations of sterilant gases, air,
and moisture contents (RH). This is probably the most accurate means
to
measure RH. The critical factor is proper maintenance of the instrument
and periodic calibration. The sampling ports must be clear and free
of
any debris and the sampling lines must be heated and insulated to
prevent condensation. Not everyone understands the principles of GC;
therefore, a qualified person, usually a chemist should be responsible
for adjustments or maintenance of the equipment. This method also lends
itself quite well to process control systems.
CONCLUSION
Other forms of sensors and instruments are available for measuring
RH.
Most are variations of the basic types described but they offer much
less in practicality for the EO sterilizer. Some firms choose to use
an
empirical method for determining RH. Based upon the laws of
thermodynamics for an ideal gas, RH, which is a ratio, can be expressed
as a percentage of the mole fraction of water vapor in a space
(chamber) to the mole fraction of water vapor in the space at
saturation at a particular temperature. In the case of the EO chamber,
this could be expressed in pressures for practical engineering
applications by the equation:
% RH = P (Change in absolute pressure) psia x 100
------
P sv (Saturation vapor pressure) psia
Where, P is the change in absolute pressure of the chamber
resulting from steam injection after the pre-evacuation portion of
the
cycle and P sv is the saturation vapor pressure taken from published
tables of saturated steam and temperature. (See the attached excerpt
of
such a table). As an example, steam is injected into the EO chamber
to
increase the pressure by 3.0 in. Hg. (1.47 psia). Sterilization
temperature is 130 F and from the attached table of saturation vapor
pressure at a temperature, P sv = 2.2230 psia. The equation then
becomes:
RH = 1.47
-------- x 100 = 66%
2.2230
The use of the empirical method is adequate providing that all
instrumentation to measure both pressures and temperature are
adequately calibrated and maintained. Validation/qualification of the
process should verify the accuracy of the calculations. Some firms
utilize this method as a means for back-up or for cross-checking to
determine faulty sensors. The investigator is invited to also utilize
this method for a rapid check of the RH parameter of the EO process
during the Establishment Inspection.
References
1. The American Society of Mechanical Engineers; ASME Steam Tables;
1967.
2. Robertson, J.H., Townsend, M.W., Allen, P.M., Devisser, A., and
Enzinger, R.M.; Validation of Ethylene Oxide Sterilization Cycles;
PDA Spring Meeting; Chicago, IL; March 25, 1977.
3. Quin, F.C.; Humidity/Moisture Considerations; American Instrument
Co., Silver Spring, MD; Reprint No. 490.
Attachment to ITG #47
Attachment to ITG #47 _____________________
Saturated Steam: Temperature Table
Chamber
Temp. Psv
___________________________________________________________________________________________________________________________________
Abs Press. Specific Volume Enthalpy Entropy
Temp Lb per Sat. Sat. Sat. Sat. Sat. Sat. Temp
Fahr Sq In. Liquid Evap Vapor Liquid Evap Vapor Liquid Evap Vapor Fahr
t p v f v fg v g h f h fg h g S f S fg S g t
___________________________________________________________________________________________________________________________________
32.0* 0.08859 0.016022 3304.7 3304.7 -0.0179 1075.5 1075.5 0.0000 2.1873
2.1873 32.0*
34.0 0.09600 0.016021 3061.9 3061.9 1.996 1074.4 1076.4 0.0041 2.1762
2.1802 34.0
36.0 0.10395 0.016020 2839.0 2839.0 4.008 1073.2 1077.2 0.0081 2.1651
2.1732 36.0
38.0 0.11249 0.016019 2634.1 2634.2 6.018 1072.1 1078.1 0.0122 2.1541
2.1663 38.0
40.0 0.12163 0.016019 2445.8 2445.8 8.027 1071.0 1079.0 0.0162 2.1432
2.1594 40.0
42.0 0.13143 0.016019 2272.4 2272.4 10.035 1069.8 1079.9 0.0202 2.1325
2.1527 42.0
44.0 0.14192 0.016019 2112.8 2112.8 12.041 1068.7 1080.7 0.0242 2.1217
2.1459 44.0
46.0 0.15314 0.016020 1965.7 1965.7 14.047 1067.6 1081.6 0.0282 2.1111
2.1393 46.0
48.0 0.16514 0.016021 1830.0 1830.0 16.051 1066.4 1082.5 0.0321 2.1006
2.1327 48.0
50.0 0.17796 0.016023 1704.8 1704.8 18.054 1065.3 1083.4 0.0361 2.0901
2.1262 50.0
52.0 0.19165 0.016024 1589.2 1589.2 20.057 1064.2 1084.2 0.0400 2.0798
2.1197 52.0
54.0 0.20625 0.016026 1482.4 1482.4 22.058 1063.1 1085.1 0.0439 2.0695
2.1134 54.0
56.0 0.22183 0.016028 1383.6 1383.6 24.059 1061.9 1086.0 0.0478 2.0593
2.1070 56.0
58.0 0.23843 0.016031 1292.2 1292.2 26.060 1060.8 1086.9 0.0516 2.0491
2.1008 58.0
60.0 0.25611 0.016033 1207.6 1207.6 28.060 1059.7 1087.7 0.0555 2.0391
2.0946 60.0
62.0 0.27494 0.016036 1129.2 1129.2 30.059 1058.5 1088.6 0.0593 2.0291
2.0885 62.0
64.0 0.29497 0.016039 1056.5 1056.5 32.058 1057.4 1089.5 0.0632 2.0192
2.0824 64.0
66.0 0.31626 0.016043 989.0 989.1 34.056 1056.3 1090.4 0.0670 2.0094
2.0764 66.0
68.0 0.33889 0.016046 926.5 926.5 36.054 1055.2 1091.2 0.0708 1.9996
2.0704 68.0
70.0 0.36292 0.016050 868.3 868.4 38.052 1054.0 1092.1 0.0745 1.9900
2.0645 70.0
72.0 0.38844 0.016054 814.3 814.3 40.049 1052.9 1093.0 0.0783 1.9804
2.0587 72.0
74.0 0.41550 0.016058 764.1 764.1 42.046 1051.8 1093.8 0.0821 1.9708
2.0529 74.0
76.0 0.44420 0.016063 717.4 717.4 44.043 1050.7 1094.7 0.0858 1.9614
2.0472 76.0
78.0 0.47461 0.016067 673.8 673.9 46.040 1049.5 1095.6 0.0895 1.9520
2.0415 78.0
80.0 0.50683 0.016072 633.3 633.3 48.037 1048.4 1096.4 0.0932 1.9426
2.0959 80.0
82.0 0.54093 0.016077 595.5 595.5 50.033 1047.3 1097.3 0.0969 1.9334
2.0303 82.0
84.0 0.57702 0.016082 560.3 560.3 52.029 1046.1 1098.2 0.1006 1.9242
2.0248 84.0
86.0 0.61518 0.016087 227.5 527.5 54.026 1045.0 1099.0 0.1043 1.9151
2.0193 86.0
88.0 0.65551 0.016093 496.8 496.8 56.022 1043.9 1099.9 0.1079 1.9060
2.0139 88.0
90.0 0.69813 0.016099 468.1 468.1 58.018 1042.7 1100.8 0.1115 1.8970
2.0086 90.0
92.0 0.74313 0.016105 441.3 441.3 60.014 1041.6 1101.6 0.1152 1.8881
2.0033 92.0
94.0 0.79062 0.016111 416.3 416.3 62.010 1040.5 1102.5 0.1188 1.8792
1.9980 94.0
96.0 0.84072 0.016117 392.8 392.9 64.006 1039.3 1103.3 0.1224 1.8704
1.9928 96.0
98.0 0.89356 0.016123 370.9 370.9 66.003 1038.2 1104.2 0.1260 1.8617
1.9876 98.0
100.0 0.94924 0.016130 350.4 350.4 67.999 1037.1 1105.1 0.1295 1.8530
1.9825 100.0
102.0 1.00789 0.016137 331.1 331.1 69.995 1035.9 1105.9 0.1331 1.8444
1.9775 102.0
104.0 1.06965 0.016144 313.1 313.1 71.992 1034.8 1106.8 0.1366 1.8358
1.9725 104.0
106.0 1.1347 0.016151 296.16 296.18 73.99 1033.6 1107.6 0.1402 1.8273
1.9675 106.0
108.0 1.2030 0.016158 280.28 280.30 75.98 1032.5 1108.5 0.1437 1.8188
1.9626 108.0
110.0 1.2750 0.016165 265.37 265.39 77.98 1031.4 1109.3 0.1472 1.8105
1.9577 110.0
112.0 1.3505 0.016173 251.37 251.38 79.98 1030.2 1110.2 0.1507 1.8021
1.9528 112.0
114.0 1.4299 0.016180 238.21 238.22 81.97 1029.1 1111.0 0.1542 1.7938
1.9480 114.0
116.0 1.5133 0.016188 225.84 225.85 83.97 1027.9 1111.9 0.1577 1.7856
1.9433 116.0
118.0 1.6009 0.016196 214.20 214.21 85.97 1026.8 1112.7 0.1611 1.7774
1.9386 118.0
120.0 1.6927 0.016204 203.25 203.26 87.97 1025.6 1113.6 0.1646 1.7693
1.9339 120.0
122.0 1.7891 0.016213 192.94 192.95 89.96 1024.5 1114.4 0.1680 1.7613
1.9293 122.0
124.0 1.8901 0.016221 183.23 183.24 91.96 1023.3 1115.3 0.1715 1.7533
1.9247 124.0
126.0 1.9959 0.016229 174.08 174.09 93.96 1022.2 1116.1 0.1749 1.7453
1.9202 126.0
128.0 2.1068 0.016238 165.45 165.47 95.96 1021.0 1117.0 0.1783 1.7374
1.9157 128.0
130.0 2.2230 0.016247 157.32 157.33 97.96 1019.8 1117.8 0.1817 1.7295
1.9112 130.0
132.0 2.3445 0.016256 149.64 149.66 99.95 1018.7 1118.6 0.1851 1.7217
1.9068 132.0
134.0 2.4717 0.016265 142.40 142.41 101.95 1017.5 1119.5 0.1884 1.7140
1.9024 134.0
136.0 2.6047 0.016274 135.55 135.57 103.95 1016.4 1120.3 0.1918 1.7063
1.8980 136.0
138.0 2.7438 0.016284 129.09 129.11 105.95 1015.2 1121.1 0.1951 1.6986
1.8937 138.0
140.0 2.8892 0.016293 122.98 123.00 107.95 1014.0 1122.0 0.1985 1.6910
1.8895 140.0
142.0 3.0411 0.016303 117.21 117.22 109.95 1012.9 1122.8 0.2018 1.6534
1.8852 142.0
144.0 3.1997 0.016312 111.74 111.76 111.95 1011.7 1123.6 0.2051 1.6759
1.8810 144.0
146.0 3.3653 0.016322 106.58 106.59 113.95 1010.5 1124.5 0.2084 1.6684
1.8769 146.0
148.0 3.5381 0.016332 101.68 101.70 115.95 1009.3 1125.3 0.2117 1.6610
1.8727 148.0
150.0 3.7184 0.016343 97.05 97.07 117.95 1008.2 1126.1 0.2150 1.6536
1.8686 150.0
152.0 3.9065 0.016353 92.66 92.68 119.95 1007.0 1126.9 0.2183 1.6463
1.8646 152.0
154.0 4.1025 0.016363 88.50 88.52 121.95 1005.8 1127.7 0.2216 1.6390
1.8606 154.0
156.0 4.3068 0.016374 84.56 84.57 123.95 1004.6 1128.6 0.2248 1.6318
1.8566 156.0
158.0 4.5197 0.016384 80.82 80.83 125.96 1003.4 1129.4 0.2281 1.6245
1.8526 158.0
160.0 4.7414 0.016395 77.27 77.29 127.96 1002.2 1130.2 0.2313 1.6174
1.8487 160.0
162.0 4.9722 0.016406 73.90 73.92 129.96 1001.0 1131.0 0.2345 1.6103
1.8448 162.0
164.0 5.2124 0.016417 70.70 70.72 131.96 999.8 1131.8 0.2377 1.6032
1.8409 164.0
166.0 5.4623 0.016428 67.67 67.68 133.97 998.6 1132.6 0.2409 1.5961
1.8371 166.0
168.0 5.7223 0.016440 64.78 64.80 135.97 997.4 1133.4 0.2441 1.5892
1.8333 168.0
170.0 5.9926 0.016451 62.04 62.06 137.97 996.2 1134.2 0.2473 1.5822
1.8295 170.0
172.0 6.2736 0.016463 59.43 59.45 139.98 995.0 1135.0 0.2505 1.5753
1.8258 172.0
174.0 6.5656 0.016474 56.95 56.97 141.98 993.8 1135.8 0.2537 1.5684
1.8221 174.0
176.0 6.8690 0.016486 54.59 54.61 143.99 992.6 1136.6 0.2568 1.5616
1.8184 176.0
178.0 7.1840 0.016498 52.35 52.36 145.99 991.4 1137.4 0.2600 1.5548
1.8147 178.0
DEPT. OF HEALTH AND HUMAN SERVICES Date: 12/31/86 Number: 48 PUBLIC HEALTH SERVICE FOOD AND DRUG ADMINISTRATION Related Program Areas: *ORA/ORO/DEIO/IB* All Programs
ITG SUBJECT: MICROBIOLOGICAL CONTAMINATION OF EQUIPMENT GASKETS
WITH PRODUCT CONTACT
Background/Discussion
A firm was inspected as part of a district dairy initiative. Original
samples of finished product filled by a dedicated filling line revealed
that the product was contaminated with pathogenic E. Coli. A follow-up
inspection focusing on that particular filling line revealed the
presence of that same bacteria in an equipment joint. As a result of
that inspection, the firm conducted an extensive clean-up operation
and initiated an equipment clean-up procedure that would, under
ordinary conditions, appear adequate. However, subsequent testing by
a
private laboratory revealed that the equipment was still contaminated
by this organism, even after extensive equipment cleaning. Further
investigation by the firm, equipment manufacturer, and the consulting
laboratory, identified the equipment's gaskets as the source of the
contamination. When the gaskets were replaced, the organism could no
longer be detected.
The importance of proper gasket inspection and replacement, as well
as
sanitization, was a critical factor in this firm's product quality.
The
use of worn or cracked gaskets may pose a more significant potential
contamination source than would appear on the surface.
A deterioration in the gasket also may not be evident without close
examination. The presence of minor cracks in the gasket surface would
allow the invasion of bacteria. As a gasket continues to deteriorate,
these cracks enlarge and become more of a problem. Gaskets are often
under pressure when in use which could widen these cracks allowing
finished product to come in contact with organisms in the gasket
cracks.
Although these gaskets are removed and sanitized when they are no
longer under pressure, the cracks may narrow and reduce the exposure
potential of sanitizing fluids.
Although not a conclusively proven fact, the information developed
during these inspections would indicate equipment gaskets for product
contact surfaces to be a potential source of significant problems and
particular attention should be paid them during bacteriological
inspections.
Tips to Investigators
Investigators conducting inspections of equipment used to process
bacteriologically susceptible products (foods, drugs, invitro
diagnostic reagents and cosmetics) should pay particular attention
to
the gaskets in the equipment. Questions to ask should include:
1. Does a physical examination show any evidence of cracking,
pitting, or etching of the surface? Such areas will not be
sanitizable.
2. How often does the firm inspect and/or replace gaskets? The
firm's written procedure should take into account the physical
stresses placed on the gasket material as well as any periodic
chemical stress which may cause etching of the surfaces.
3. How often does the firm dismantle, clean and sanitize the
gaskets? The CIP cleanability of a gasketed joint is dubious at
best. If the gasket was installed even slightly off center, a
resulting pocket on the side of the gasket will probably not be
subjected to any CIP solutions used.
4. How are the gaskets handled when they are installed initially
or after periodic inspection/cleaning? Properly cleaning and
sanitizing a gasket is useless if the individual reassembling
the equipment re-contaminates the gasket during the process.
CIP sanitizing after recontaminating a previously sanitized
gasket is useless since, as stated in item 3 above, even
slightly off center gaskets cannot be adequately CIP cleaned.
5. Is the gasket material approved for use and does it meet
specifications.
Conclusion
If the investigator has reason to believe that the firm is not properly handling their gaskets, finished product samples are warranted. Because of the dilution factor involving production lines used to manufacturer large quantities of liquid or semi-liquid products, the investigator should consider sampling the gasket itself. This can best be done by swabbing the gaskets (product contact surfaces only). Aseptic technique is necessary. Immersion of the gasket in a transfer solution or in media itself would not be acceptable since the outer edge of the gasket was probably not a product contact surface. Collection of the gasket in a dry state would be better than nothing, but the firm can always argue that any positive result was from bacteria on non-product contact surfaces (outer edge of the gasket). Collecting the gasket may not be possible if the firm has no replacement. In addition, if a collected gasket is positive, we are also faced with the fact that we may have removed the source of contamination. Consequently any regulatory action, other than that against the product manufactured with the contaminated gasket, may be jeopardized. DEPT. OF HEALTH AND HUMAN SERVICES Date: 5/30/87 Number: 49 PUBLIC HEALTH SERVICE FOOD AND DRUG ADMINISTRATION Related Program Areas: *ORA/ORO/DEIO/IB* Foods
ITG SUBJECT: STOCK ROTOMAT
GENERAL
The ROTOMAT, a discontinuous or batch type, end-over-end agitating
retort was developed by Hermann Stock Maschinenfabrik, GmbH; New
Munster, West Germany in close collaboration with the Hamburg firm
Mittelhauser-Watter. These units are distributed in the United States
by Stock America, Inc.; Milwaukee, WI. The basic concept behind
development of the sterilizer was to avoid burning the product around
the inner sides of the container and to speed up heat transfer to the
coldest point by rotating the baskets holding the containers in a
high-pressure horizontal retort, submerged in a sterilizing medium
of
water, pre-heated in a storage tank and circulated in the system
utilizing a high-pressure centrifugal pump.
Some of the benefits claimed for the ROTOMAT are:
1. It saves up to 80% of the normal retorting time in a still retort
through a combination of:
a) High temperature - short time.
b) End-over-end rotation.
c) Continuous forced circulation of high temperature water in
the working drum.
2. Water and energy consumption is greatly reduced because the water
used as a sterilizing medium is pumped back to the unit's storage
drum and then reused.
3. The unit operates completely automatically, according to one of
several modes, among them, electric switch, punch card and
microprocessor or computer control.
Stock manufactures a variety of rotary over-pressure retorts for the
thermal processing of foods, of which the ROTOMAT is one version. In
addition, static (still) versions of these retorts are available. The
rotary models include the ROTOMAT and JUMBOMAT (full water immersion
as a heating medium) and the ROTOVAP (water spray or cascade as a
heating medium). The AUTOMAT is the still version of the ROTOMAT, the
AUTOVAP is the still version of the ROTOVAP. Capacities in these units
range from 1 to 5 cages or baskets. Container capacity in the JUMBOMAT
is three times that of the standard four cage model due to the size
of
the baskets. The basic unit in the full-immersion models, however,
is
organized as a two-drum (over-under) system with a top storage vessel
or drum used to pre-heat water used as the sterilizing medium; and
a
lower or working (sterilizing) drum where the baskets are agitated,
and
the sterilizing water circulated by being drawn out through a suction
manifold in the bottom of the drum, pumped through a heat exchange
system (direct steam injection), then discharged through a distribution
manifold located along the top of the drum. The organization of the
ROTOMAT is detailed in Figure #1 (Figure [FIGURE]) as follows:
Water is preheated in the top storage vessel (1), generally to a
temperature not lower than 260-275 F. In some cases, depending upon
the scheduled process, the temperature can be in the range of
290-300 F. A "rule of thumb" is to heat the water in the
storage drum
to a temperature of at least 10 F higher than processing temperature.
At the beginning of the sterilizing cycle, water is dropped to the
lower processing or working drum (2). A counter-pressure which was
generated during the heating of the upper drum, assists in moving the
water to the lower drum. At the time the processing water is dropped
to
the working drum, the baskets or cages begin to be rotated end-over-end
by a variable speed drive motor which turns the rotary device or reel
(3). The reel can be rotated in either direction or can be oscillated.
Rotation can be programmed within a range of 6-46 RPM.
The process of moving the preheated sterilizing water to the working
drum involves dropping it from the upper storage drum, to the positive
displacement pump located at the rear of the unit (4). The pump moves
the water through the circulation line (5) where steam is injected
(6)
to insure maintenance of the programmed temperature; then to a
distribution manifold (7) located along the top of the working drum.
Water flows down over the crates and is drawn out through a suction
manifold (8) located along the bottom of the working drum and sent
back
through the circulation line again. This completes the circulation
system for the sterilizing water.
At the beginning of the cooling cycle, the hot sterilizing water is
pumped back up to the storage drum where it is re-heated to the proper
drop temperature for the next cycle. At the same time the cooling water
pump (9) starts and pumps cold water into the circulation line. The
cold water mixes with the hot water during the initial stages of the
cooling cycle. The temperature of the sterilizing water is maintained
at approximately 212-215 F by the time it is pumped back to the
storage drum. Thus, re-heating the upper drum takes less time after
the
initial sterilizing cycle. During the cooling cycle, and the subsequent
draining phase, the reel continues to rotate.
All automatic valves used in the ROTOMAT are electro-activated ball
valves. The steam control valves for heating the upper drum, and
maintaining the proper temperature in the working drum, have five
second actuators. A tacho generator is used to control the reel speed
with a higher voltage resulting in higher RPM. The ratio is
approximately 1.1 to 1.2. For example, 15 volt AC represents
approximately 12 RPM. The back plate of the rotary device is flexible,
so even if the two front rollers on which the reel rests wear out,
the
reel will still turn. The guide (transfer) rollers along the side of
the reel bear the weight of the cages so they tend to wear out sooner.
Two types of cages are used with the ROTOMAT - one slotted, one
perforated. The slotted cage has at least 50% total open space. The
newer, perforated cage has approximately 60% open space. Two types
of
spacer mats have been used, as layer dividers and on top of cages in
varying numbers (depending on the size of container being processed)
to
provide contact between the top locking plate of the reel and the top
layer of containers in a cage or basket. The older design consisted
of
square perforations with approximate 1 1/8" sides, in 18 rows
of 20
squares each for a total open area of approximately 61%. The newer
design employs square perforations with 3/4" sides in 21 rows
of 24
squares each for a total open area of approximately 35%. The newer
design also employes "ribs," along the periphery of the perforations,
which are channeled on the bottom to provide for proper water
circulation.
The ROTOMAT Model RN comes equipped with a 2-track ink strip chart
recording device. Recordings are made for temperature and pressure.
Reel speed (in RPM) is monitored by a needle-type indicator. The Models
RE and and RSE come equipped with a three-pen spark-erosion (the
tracing is actually burned into the paper) strip chart recording
device. Recordings are made for pressure, RPM, and temperature, (all
in
the working drum). Two gauges located on the upper portion of the
recording device indicate the pressure and temperature in the upper
storage drum. Units sold in the U.S. have been modified to have an
attached Taylor recording device either in lieu of, or in addition
to,
the spark-erosion strip chart, which did not always conform to 21 CFR
Part 113.40 re: chart temperature graduations. Generally speaking,
if
one function is controlled (i.e. temperature) a twelve hour Taylor
chart is employed. If more than one function (i.e. temperature and
pressure), a 24-hour chart is used. The Rotomatronic S,
(computer-controlled version) does not routinely come equipped with
the
spark-erosion strip chart recorder, relying instead on the Taylor
recording thermometer. Both the spark-erosion strip chart and Taylor
recorders serve only to record, not to control the temperature in the
working drum.
Steam control in all models of the ROTOMAT is effected by a PT-100
resistance temperature device (RTD) located either just below the
circulation line, or in the circulation line itself, on the electronic
switch program model and the card reader model reader, respectively.
The Rotomatronic S has the PT-100 located in the right front side (when
facing the unit) of the working drum in proximity to a MIG thermometer.
The ROTOMAT originally did not come equipped with an MIG; however,
it
was subsequently added to all models in the U.S. to meet requirements
of 21 CFR Part 113.40.
PROGRAM CONTROL
The sequencing of the sterilization and cooling cycles in the ROTOMAT
is completely automatic (except for the Model RG) and controlled in
one
of five different modes (called "models"):
1. Model RG - A manual switch-operated system with different colored
illuminated buttons (light-buttons) to operate all valves, except
those for temperature and counter-pressure control, which are
operated automatically. The RG is similar to the RN except there is
no recording device. The only RG models sold in the U.S. have been
for research only.
2. Model RN - An electro-mechanical program control system. The
processing program is selected by the manually pre-setting various
switches, and runs automatically. Process recording is by a 2-track
ink strip chart recorder (see Figure 2). (Figure [FIGURE])
Recordings are made for temperature and pressure. No model RN are
currently in use in the U.S.
3. Model RE - Electronic pre-programmed logic control (PLC). The
pre-set instruments consist of a series of dials/selector switches
which are manually set on the control panel. The various steps in
the process are indicated by various colored illuminated buttons
(light-buttons). The process is recorded (temperature, pressure and
RPM) by spark-erosion on a metal foil strip chart recorder (see
Figure 3). (Figure [FIGURE]) It should be noted that in the past,
STOCK vendored the models RN & RE as a single control version both
employing the electro-mechanical control system and spark-erosion
strip chart recorder. If vendored in the U.S., a Taylor circular
recording chart for recording temperature would also be supplied to
supplement the 3-pen spark erosion recorder. No model RE's are
currently in use in the U.S.
4. Model RSE - Electronic control by punch card, or the "card-reader"
system. The program is established according to a program sheet.
Once the program has been written on the sheet, corresponding
squares are punched out on the program card for the respective
products. Once the card is placed into the proper slot in the
control panel for the unit, the holes in the card come in contact
with limit switches, closing these switches and causing the machine
to advance through a processing cycle from heating the upper drum
to drain (see Figure 4). (Figure [FIGURE])
5. Rotomatronic - Micro processor or computer controlled system (see
Figure 5). All steps in the processing cycle are programmed into
the computer using function and alpha-numeric keyboards, both
employing pressure-sensitive keys. The computer has two basic
software packages, consisting of chips. One is a pre-designed
system for the unit to sequence through each step in the processing
cycle, e.g., Heating the Upper Drum, Sterilization I, Sterilization
II, Cool I, etc. This is basically a PROM (programmable read-only
memory) which is permanently burned on and cannot be changed.
Therefore the sequence of operation itself cannot be changed by
anyone at the cannery. The other package is the actual input values
for the target temperatures for heating the upper drum, scheduled
RPM, process time and temperatures, etc. These values can be
changed by accessing the computer through an access code. The
programmer is directed via instructions illuminated on a LED screen
on the control panel, while the program values are being written.
Prior to starting a cycle, the retort operator enters certain
values, such as operator number, batch number, product number
(which determines the time, temperature, pressure and RPM of the
process) and initial temperature. This information is necessary to
begin heating the upper drum. During a processing cycle, the LED
screen will inform the retort operator what the programmed values
and actual values are, as the machine progresses through each step
in the cycle. At the end of the cycle, a computer-generated
processing record (CGPR) is printed out, listing the times,
temperatures, reel speed and other critical factors. In addition,
any alarm conditions, such as low temperature noted during the
process, are also printed out. Generally no audible alarm is
sounded, nor any other visible indication given, should one of
these conditions occur. At the end of the cycle, the retort
operator has the option of reviewing each step on the LED screen,
and having another copy of the CGPR printed. In some instances, the
firms may maintain a hand-written processing record to complement
the CGPR.
There are currently alarm functions in the software for
temperature, pressure and RPM. According to Stock, a new software
package has been prepared that will also alarm in the event a low
water level is reached, even though this has not been determined to
be critical from a thermal processing stand point. There will also
be a fifth alarm function which will be left "open" in the
package.
That is, any individual authorized to program the machine will be
able to program a fifth alarm condition of his or her choosing.
In addition, another software change (in chips) has been prepared
which will give a printout of temperature, RPM or pressure every 30
seconds, should there be a deviation from scheduled parameters.
SEQUENCE OF OPERATION
After pre-heating the water in the upper drum to the programmed
temperature, the basic sequence of operation in the ROTOMAT involves
a
one or two stage come-up (depending on the model); a hold phase at
processing temperature; a two-stage cool and a drain phase. The
sequence is as follows (see Figure 6): (Figure [FIGURE])
At the time the sterilizing cycle begins, the connection valve (1)
opens and the circulation pump (2) is activated. Water pre-heated to
the programmed temperature flows through the connection line to the
pump which moves it through the circulation line (3) where steam is
injected via the heating valve (4) in decreasing amounts until process
temperature is reached. At that time the heating valve functions as
a
proportional controller, maintaining the programmed holding
temperature. At the same time the pump is activated, the reel begins
to rotate the cages or baskets and the vent valve (5) opens to remove
air from the working drum. This vent valve can remain open from 1/2
to
2 minutes, (this is programmed on the basis of time only, not on water
level or any other function) although the normal time frame is
approximately one minute. Air is removed from the working drum by the
super heated, pressurized water from the storage drum entering
atmospheric conditions in the working drum and flashing to steam. This
pressurizes the working drum and forces the air out the vent line.
Water is pumped through the circulation line, through the throttle
valve (6) to the distribution manifold (7) along the top of the working
drum. The distribution manifold has approximate one inch ports opening
to the lower drum. Water is drawn out through a suction manifold (also
called the "circulation channel") along the bottom of the
working drum.
The manifold has square, approximate one inch ports, each covered with
wire mesh. It takes approximately two to three minutes to fill the
lower working drum to the desired level. ROTOMAT in the U.S. are
generally equipped with a water level sight gauge. Conversations with
Stock America and another processing specialist familiar with the
operation of the ROTOMAT indicate that ensuring a water level above
the
top of the cages is not critical from a heat distribution standpoint,
since over-pressure is provided by steam.
On the Models RG (semi-automatic), RN/RE (electric switch and PLC)
and
RSE (card-reader), the time from the drop of pre-heated water to the
circulation pump, until processing temperature is reached, is
designated as "Sterilization I." On the Rotomatronic, the
come-up phase
has been split into "Sterilization I" - from the beginning
of the drop
of pre-heated water to the close of the vent valve - and "Sterilization
II" - from the closing of the vent valve until processing temperature
is reached. It should be remarked that a minimum come-up time (CUT)
has
been designated a critical factor for certain products and this will
have been programmed into the machine. This CUT may be 7-11 minutes
depending upon the initial temperature stipulated by heat penetration
studies. The reason for dividing the CUT into two phases in the
Rotomatronic, is that use of a computer allows for more exact control.
The hold phase at processing temperature has been designated
"Sterilization II" for the Models RG, RN/RE and RSE; and
"Sterilization
III" for the Rotomatronic. At the beginning of the hold phase
in the
Rotomatronic, the operator is alarmed to read - and input into the
computer - the MIG and recording thermometer readings and to time the
reel speed. Input of these values has no effect on the programmed
process time; however, the readings are listed on the CGPR. Reel speed
is checked on the Rotomatronic by the operator observing the number
of
times a light on the control panel, which indicates the baskets are
in
the horizontal position, blinks on and off in a minute.
Pressure in the working drum (normally in the range of 2.0-3.0 bars
or
30-44 PSI) is indicated by a dial gauge on the door in all models.
It
is also continuously recorded on both the spark-erosion type and Taylor
recording devices. Pressure in the working drum has not been declared
to be a factor critical to the scheduled process.
At the end of the scheduled process time, the machine sequences into
the first cooling phase ("Cool I"), then into the second
("Cool II").
Total cooling time is approximately 20 minutes. The reel continues
to
turn at the scheduled speed during these two phases. Following Cool
II,
the machine sequences into the last phase - the drain. The reel speed
normally is slowed at this point. Recovery of the sterilizing water
in
the upper storage drum is completed by the end of Cool I and the
connection valve (1) closes at this time.
PRESSURE CONTROL
Proper pressure in the ROTOMAT is achieved by steam. Air is not used
unless specifically requested by a processor. In this case, air would
be supplied through the "auxiliary line" (15) shown in Figure
#6. Stock
generally prefers to establish and maintain over-pressure with steam
rather than air.
During the "Heating Upper Drum" phase, water is heated initially
to
212 F at which time steam (10) is programmed to be injected through
a 2 1/2 inch line controlled by an automatic valve (11) to provide
an
over pressure of 2-2.5 bars (29.5-37.0 PSI). If the pressure rises
above the programmed value, it is released via the small vent valve
(12). Once the superheated water is dropped through the connection
line, the connection valve (1) remains open until the beginning of
Cool
II. This allows steam and non-condensable gases accumulating in
the head space of the working drum to rise through the connection line
and accumulate in the head space of the storage drum, from which they
would be released via the small vent valve. This provides for the
maintenance of a proper pressure balance between the upper storage
and
lower working drums during the sterilization phases.
A particular pressure is also programmed for the cooling phases. This
is controlled via the pressure drop regulating valve (13) and the vent
and pressure drop regulation valve (14). If the pressure gets too high
during the cooling phase, steam is vented out through these valves
and
more water is admitted. If the pressure gets too low, the steam is
injected to maintain the product over-pressure.
HEAT DISTRIBUTION AND HEAT PENETRATION
Adequate heat distribution in the ROTOMAT is dependent upon such
factors as the proper functioning of the centrifugal pump, rotation
of
the reel and the number of cages in the retort. The number of cages
also effects the CUT to the holding (processing) temperature as does
the initial temperature, with IT having, perhaps, the greater effect.
In addition, the open area provided by divider plates or spacers, as
well as the cages themselves, can have an effect on adequate heat
distribution. Any change to a more restrictive-to-flow design in any
of the above must be validated by new heat distribution studies.
The lack of rotation has been shown to cause a temperature
stratification in some instances with cooler temperatures being in
the
lower part of the working drum.
With respect to the proper function of the pump, all ROTOMAT models
have visual indicators which signal if the pump is not working
properly. That is, whether or not it is on. There are currently no
provisions for monitoring the flow rate (e.g., by a flow meter) which
would indicate if the pump is delivering the desired flow. According
to
Stock, the circulation pump has a maximum capacity of approximately
400 gallons per minute. During Sterilization I, the pumping rate is
variable. During Sterilization II (for the RG, RN/RE and RSE) and
Sterilization III (for the Rotomatronic), the pumping rate is
approximately 260 gallons per minute. Stock has stated that no
significant effect has been noticed on heat distribution down to a
rate
of 100 gallons per minute. Below that, it could be questionable
although a determination has been made that the rate would probably
have to get down to practically zero to have a noticeable effect,
providing the reel is rotated.
However, according to Stock, any case of the circulating pump going
completely down, would have to be treated as a process deviation. Any
evaluation for public health significance would involve reproducing
such a condition while conducting a heat distribution study.
With respect to the wearing out of the pump impellers, and any possible
effect this might have on heat distribution, Stock has seen little
effect. Data from 6 to 7 year old studies where pump impellers were
quite worn (glass pack operation) indicated no problem.
Because of the various factors which can affect heat distribution and
heat penetration in the ROTOMAT, an agitating process should never
be
adopted by a processor until comprehensive heat distribution and heat
penetration studies have established the point of least sterilizing
valve in the container and in a specific location in the basket. With
respect to location of the "cold spot" or point of least
lethality in
the container, variations in fill, viscosity, and solid-to-liquid
ratios can have significant effects. With respect to the location of
a
particular can in the basket, consideration must be given to the
variable rotational path of the container, depending upon whatever
it
is in the center of the basket or adjacent to the basket wall. In many
instances, the most critical aspects of heat penetration are minimum
head space and rotation of the reel. Even a slight (1/16") change
in
head space can have a significant effect on heat penetration, when
other factors are held constant.
REFERENCES
Cathers, H., December 22, 1986. Personal Communication.
Cathers, H., 1985. Remarks at Seminar on Operation of the ROTOMAT.
FDA/Center for Food Safety and Applied Nutrition, Washington, DC.
Eisner, M., 1972. Introduction into the Technique and Technology of
Rotary Sterilization Verlay Gunter Hempel - Bramschmsig, Federal
Republic of Germany.
Lopez, A., 1981. A Complete Course in Canning. Book I - Basic
Information on Canning. 11th Edition. The Canning Trade, Baltimore.
Stock Bulletin, 1986. Stock AUTOMAT/ROTOMAT/JUMBOMAT Full inversion
overpressure retorts for static or rotary sterilization PN 7/01 86/500
d/GB/-M/ Payasus Werbeagentur Hamburg.
Stock Bulletin, 1985. Stock AUTOVAP/ROTOMAT. Spray Circulation
overpressure retorts with direct or indirect heating and cooling. For
static or rotary sterilization PNG/10.85/5000D/GB/-M/Peyasus
Werbeagentur Hamburg.
Stock Bulletin, undated. Sequence of Operation Stock America, Inc.
Milwaukee, Wisconsin.
Tosca, G., August 21, 1986. Personal Communication.
Illustrations used with permission of Stock America, Inc. (Figure
[FIGURE])
Heating Storage Drum (Figure [FIGURE])
DEPT. OF HEALTH AND HUMAN SERVICES Date: 10/23/87 Number: 50
PUBLIC HEALTH SERVICE
FOOD AND DRUG ADMINISTRATION Related Program Areas:
*ORA/ORO/DEIO/IB* Medical Devices,
Radiological Health
ITG SUBJECT: CAPACITOR
The purpose of this ITG is to acquaint the investigator with the
capacitor. Only the basics will be discussed, since it is beyond the
scope of this ITG to go into great detail. It is stressed that there
is
no single capacitor that out performs all others, as each capacitor
is
designed to perform a specific task. This ITG will explain capacitor
operation theory, the various types of capacitors, physical and
electrical specifications of capacitors, the failure modes of the
various types, design considerations, and environmental effects.
Theory
Electrically, "capacitance" is present between any two adjacent
conductors. A capacitor consists of two conductors, usually parallel
metal plates, separated by a dielectric material or vacuum so as to
store a large electrical charge in a small volume. Depending on
proposed application the dielectric can be air, gas, paper, organic
film, mica, glass, or ceramic. The operation of a capacitor is similar
to blowing up a balloon and releasing the air from it. Imagine blowing
up a balloon, pinching the air nozzle for a few seconds, and then
releasing the air nozzle so that air may flow out. Similarly, a
capacitor is charged (blown up) to some voltage (air pressure) by an
AC
or DC voltage source (air blower). Once the voltage source is removed
the capacitor will hold the voltage for some time (pinching the air
nozzle) and then it will begin to rid itself of the electricity
(releasing the air nozzle). The rate at which the capacitor discharges
is dependent on how much resistance the discharging current meets.
The
more resistance you have the slower the current will discharge from
the
capacitor. Thinking in terms of balloons, we can say that the tighter
you pinch the air nozzle (resistance) the slower the air will flow
out
(current discharge). If a big piece of metal is put across the two
capacitor terminals, the capacitor will instantaneously discharge and
sparks will occur. This is due to the sudden flow of discharge current
thru a neglible resistance. This phenomenon is similar to popping a
balloon where the unresisted flow of air through the pinhole is so
great that the balloon explodes.
The basic equations governing the operation of a capacitor are:
(1) Capacitance (C) = Charge (Q) = ke A
----------- --o-
Voltage (V) d
Where C is in units of farads (f), Q is in coulombs (C), and V is in
volts (V). A capacitor possesses one farad of capacitance if its
potential is raised one volt when it receives a charge of one coulomb.
On the right hand side of the equation, k is the dielectric constant
(no units), e o is the premittivity of air (8.85 x 10 -1 2 f/cm), A
is
the area of one of the capacitor plates (cm 2), and d is the separation
distance between the two plates (cm). Capacitance is most commonly
expressed in 10 6 subdivisions called microfarads (uf).
(2) Energy (J) = 1/2 Capacitance (c) x Voltage 2 (V) = QV
--
2
where J is in units of watt-seconds or Joules.
Equation (1) shows that capacitance can be increased in several ways;
by decreasing the voltage, obtaining a dielectric with higher k,
increasing the capacitor plate area, or decreasing the distance between
the capacitor plates. Equation (2) shows that the energy experiences
its largest increase if the voltage is increased.
Capacitors are mainly used as energy storage devices; that is, they
store electrical energy until the energy is required to enter the
circuit which is using the capacitor. Capacitors are now widely used
for keeping DC current from entering a part of a circuit (blocking),
ridding a circuit of unwanted noise or distortion (filtering),
combining desired frequencies to resonate in a circuit (coupling),
and
excluding certain frequencies from resonating in a circuit (bypassing).
Types
Capacitors generally come in two types; fixed and variable. Fixed
capacitors are manufactured to possess a specific capacitance which
cannot be changed and variable capacitors are manufactured to allow
capacitance to be varied over a wide range.
Capacitors are also classified into two generic categories;
electrostatic and electrolytic. Electrostatic capacitors are filled
with dielectrics composed of a gas, liquid, solid, or combination of
these. Electrolytic capacitors are characterized by a very thin
metallic oxide dielectric film formed on the surface of one or more
electrodes.
A. Fixed Capacitors
Ceramic Capacitors - These are a unique family of capacitors with
dielectric constants ranging from 6-10,000. They can be easily
manufactured to desired physical and electrical characteristics by
applying ceramic chemistry. Ceramic capacitors are so widely used
that they come in three classes. Class I ceramics are used for
resonant circuits and high-frequency bypass and coupling. These
capacitors have a wider temperature range compared to Class II and
Class III capacitors. Class II ceramics are used where
miniaturization is required for bypassing at radio frequencies,
filtering, and interstage coupling. Class III ceramics are used
where low-voltage coupling and bypassing in transistor circuits are
necessary.
Vacuum Capacitors - These capacitors have the lowest possible
dielectric constant and are limited to capacitances of 10 3 pf
(10- 3 uf), can range up to 50 kv (50x10 3 volts), and can carry
huge currents up to 100 amperes. Vacuum capacitors, are extremely
useful because their lifetime, barring any particle contamination
in the vacuum chamber, is indefinite.
Mica Capacitors - These capacitors find their use in such
applications as high-frequency filtering, bypassing, blocking,
buffering, coupling, and fixed tuning.
Metalized Paper and Film Dielectric Capacitors - The use of this
class of capacitors is ideal where great amounts of heat will be
present in a circuit. These capacitors possess a unique property
called self-healing whereby they eliminate momentary short circuits
induced in their dielectrics caused by surrounding circuit
elements. Once the capacitor becomes too hot, the localized heat
generated is sufficient to vaporize the thin electrode in the area
of the possible breakdown. The ability to self-heal permits these
capacitors to have higher voltage ratings for a given thickness.
Radio Frequency Interference (RFI) Capacitors - RFI capacitors are
ideal for suppressing unwanted noise from electronic circuits. This
minimizes the amount of noise passing from one stage of the circuit
to another, thus improving overall circuit performance.
Film Capacitors - These capacitors are widely used where circuits
will experience exposure to moisture. Their resistance to moisture
penetration is, by far, superior. Film capacitors are applied in
circuits requiring blocking, buffering, bypassing, coupling,
tuning, and timing.
Electrolytic Capacitors - Electrolytic capacitors are very
different from those previously mentioned in that electrolytics are
usually polarized. This means that the polarity of the applied
voltage must match the polarity of the capacitor or intense heating
will occur and the capacitor will burn out. Electrolytics meet
design needs for low-frequency filtering, long-term timing,
coupling and decoupling, and certain bypass applications requiring
high capacitance values and small volumes.
Other capacitors commonly used as fixed capacitors are air, glass,
and paper types. These are the earliest capacitors to be used and
they still find usage in general purpose cases.
B. Variable Capacitors
Variable capacitors, also called trimmers, are invaluable in the
design of electronic equipment. Variable capacitors are generally
employed to provide a range of capacitance and are commonly used in
applications where exact capacitance values cannot be obtained
using normal design procedures. These capacitors are usually
constructed such that varying the capacitance is accomplished by
adjusting the metal plates in the capacitor. Screws on these
capacitors increase or decrease effective plate area thereby
causing an increase or decrease in capacitance. (Inspection of
Equation (1) shows this.) The most widely used trimmers are
ceramic, glass, air, plastic and mica.
C. Special Capacitors
Feed-through Capacitors - These capacitors are used in cases where
conventional capacitors are not effective for filtering at high
radio frequencies. Feed-through capacitors are three terminal
devices that do not exhibit the series-resonant characteristic of
the conventional capacitor. This enables them to suppress
radio-frequency interference over a wide range of frequencies and
they are especially valuable in filtering power-supply and
control-circuit wiring in shielded high-frequency equipment.
High-energy Storage Capacitors - These capacitors are constructed
with oil-impregnated paper and/or film dielectrics. Their primary
use is for pulse forming networks which employ voltages greater
than 1000 volts. For slightly lower voltages special electrolytic
capacitors can be used. Commutation Capacitors - These are
constructed from oil-impregnated paper and film dielectrics. They
are mainly used in triggering circuits since they are characterized
by fast rise times (time it takes capacitor to rise from 10% to 90%
of its maximum voltage) and high current transients and peak
voltages associated with switching.
Packaging - Capacitors come in a wide variety of packaging styles.
The most common styles are molded, glass-encased, chip, potted,
coated, and Dual-In-Line Packaging (DIP). Molded capacitors are
rectangular-chip capacitors that can be molded into radial or
axial-lead rectangular packages or axial-lead cylindrical packages.
Glass-encased capacitors can be single or multi-layered chips with
axial leads attached sealed into a glass tube. These look a lot
like molded capacitors. Chip capacitors are thin, flat rectangular
capacitors without leads or body encasement so that they may be put
into microelectronic circuits. Potted capacitors, in many ways, are
synonymous with molded capacitors. The only difference is that
potted capacitors are oven cured. Coated capacitors, more commonly
known as dipped capacitors, come in rectangular and disk styles
with radial leads and are dipped in liquid resin. Coated capacitors
find great usage where exact dimensions can be compromised. DIP
capacitors are single or multi-layered capacitors processed into
integrated-circuit-type packages. Mica chips come in button styles.
This package is composed of a stack of silvered-mica disks
connected in parallel.
Figures 1, 2, and 3 (Figures [FIGURE]) show a few of the various
types and packaging styles of capacitors. Figure 1A shows dipped-
radial-lead capacitors (top row) and molded-axial-lead capacitors
(bottom group); Figure 1B (Figure [FIGURE]) shows glass-encased-
axial-lead capacitors (A), chip capacitors (B & C), molded-radial-
lead capacitors (D), molded-axial-lead capacitors, and dipped-radial-
lead capacitors (F); Figure 1C (Figure [FIGURE]) shows the various
styles of feed-through capacitors; and Figure 1D (Figure [FIGURE])
shows dipped-radial-lead capacitors (top and bottom left), molded-
axial-lead capacitors (bottom right group), button capacitors (Middle-
middle group), and fixed terminal capacitors (top middle and top
right). Figure 2A-C (Figure [FIGURE]) shows various types of
trimmer capacitors. Figure 3 (Figure [FIGURE]) shows (a) mica;
(b) glass; (c) ceramic; (d) general-purpose ceramic; (e) solid
electrolyte tantalum; (f) foil tantalum; (g) feed-through button
mica and ceramic; (h) general-purpose plastic film; and (i) general-
purpose paper.
Physical and Electrical Specifications
There are numerous criteria which the designer uses to choose the
capacitor that will best perform a specific task. Listed here are some
of the most important specifications used in evaluating capacitor
performance.
Dissipation Factor (DF) - This is a measure of loss in a capacitor.
Sometimes this is interchanged with a measure of loss called the power
factor (PF). Losses in large AC coil and paper capacitors are DF's
while losses in most capacitors used in DC or low-level AC capacitors
are PF's. Ideally current should lead voltage by 90 in a capacitor
but due to manufacturing processes the current leads the voltage by
some angle A. The DF = tan(90 -A) and PF = sin(90 -A). The lower the
DF, the better the capacitor.
Equivalent Series Resistance (ESR) - In capacitors, this is defined
as
the AC resistance (R) of a capacitor expressing loss at a given
frequency (f). The ESR is related to the PF by the relation:
R = PF x 10 6 in units of ohms.
---
2 fc
Insulation Resistance (IR) - This is the resistance across the
terminals of a capacitor. IR is inversely proportional to capacitance
and temperature so as capacitance (or temperature) increases the IR
will decrease.
Dielectric Strength - This corresponds to the maximum voltage that
a
dielectric material can withstand without rupturing. Electrostatic
capacitors are often specified by their dielectric withstanding voltage
(DWV) and this is synonymous with dielectric strength. Dielectric
strength is usually specified in volts per mil at constant temperature.
Dielectric Absorption - This is the property of an imperfect dielectric
where all electric charges within the body of the material caused by
an
electric field are not returned to that field. Dielectric absorption
is
measured by determining the "reappearing voltage" which appears
across
a capacitor at some point in time after the capacitor has been fully
discharged under short circuit conditions. It is expressed as the ratio
of reappearing voltage to charging voltage.
Volumetric Efficiency - This is achieved by getting the most
capacitance out of the smallest volume possible. The volume is a
function of dielectric material used and the method of construction.
Capacitors with high volumetric efficiency are the most applicable
in
most of the new integrated-circuit electronic-equipment designs.
Temperature Coefficient (TC) - TC is the change in capacitance per
degree change in temperature. It may be positive, negative, or even
zero and is expressed in parts per million per degree Celsius
(ppm/ 0C). The equation that determines the TC is:
TC = C1-C 2 x 10 6
------
(T 1-T 2)C 1
where C 1 and C 2 are the initial and final capacitances and T 1 and
T 2 are the initial and final temperatures.
Voltage Ratings - There are two types of voltage ratings to consider
when evaluating capacitor performance; DC and surge voltage and AC
voltage. In the case of DC and surge voltage ratings, the thickness
of
the dielectric determines the maximum surge and DC voltages that may
be
applied. AC voltage ratings are usually specified for ceramic
capacitors. This rating corresponds to the AC voltage required to make
the sum of the given DC voltage and AC voltage less than the rated
DC
voltage.
In addition to these ratings there are certain types of electrolytic
capacitors in which the applied voltage is of primary concern.
Electrolytic capacitors are sensitive to the effects of voltage because
they are highly polarized devices. Even if the applied voltage is less
than the maximum voltage specified, the voltage drop across the ESR
of
the capacitor will shorten the capacitor's life expectancy through
an
accelerated effect of internal heating.
Current Ratings - Current ratings to consider are the leakage and
ripple currents. Leakage current is the stray DC current of relatively
small value which flows through the capacitor when voltage is applied
across the terminals. Ripple current is the AC component of a
unidirectional current. For electrolytic capacitors, there is also
a
maximum allowable charge and discharge current rating.
Frequency - Since there is an internal inductance in a capacitor there
will be a resonant frequency. Depending on capacitor type, this
frequency may or may not fall in a range that is a problem for the
designer. This problem would arise because the designer would want
the
capacitor to block or minimize DC current, and at resonance the
internal impedance is a minimum which causes maximum DC current.
Failure Modes
Electrolytic Capacitors - Most failures in electrolytic capacitors
result from two cases; either the breakdown of the dielectric film
due
to low IR or the leakage of the electrolyte due to high IR. Dielectric
breakdown is an electrochemical failure that is caused by improper
chemical composition of dielectric material used in their manufacture.
The addition of contaminants such as chloride is also a predominant
factor in dielectric breakdown. Electrolyte leakage is a mechanical
failure and is most commonly caused by insufficient compression seal,
leakage at the weld on the bottom of the cylinder (in axial-lead
devices), and leakage around the aluminum or tantalum terminals in
plastic (molded) headers or seals. Other failure modes exist in the
form of poor welds or pressure connections becoming open-circuits after
a short shelf life or operating life.
Ceramic Capacitors - Most failures in ceramic capacitors are caused
by
encasement materials used to protect the capacitor and lead assembly
from external environments. Other failures include electrical
degradation and intermittent failures. Electrical degradation is caused
by thermal expansion of encapsulants and moisture between the coating
and capacitor section. Intermittent or open failures are caused by
poor
soldering techniques and terminal design that result in loose or
detached leads.
Paper and Film Capacitors - Paper and film capacitors are subject to
the same failure modes as electrolytic capacitors with the exception
of
electrolyte leakage. Seal leakage is common in poorly made
oil-impregnated capacitors. Mechanical failures are caused by fracture
of the electrode tab at the point of attachment to the electrode or
to
the external lead. Rough edges on foil electrodes cause early
shorting, especially if the lower plate is thicker than the upper.
Design Considerations
The reliability of a capacitor is dependent upon the degree of success
achieved in housing the capacitor element in a mechanically and
environmentally secure enclosure. Capacitors with internal lead
construction must be mechanically and electrically sound before the
encasement is applied. Encapsulated dipped, or molded capacitors can
not withstand dynamic environments such as high levels of shock and
vibration. For mechanical integrity, metallurgical bonds and
reinforcing materials should be used.
When considering which capacitor best performs a specific circuit task
there are several options available. These options depend on the cost
of the capacitor and the capacitor's physical and electrical properties
with respect to the task it is about to perform. If precision is a
must, then it is advised that mica, glass, ceramic and film
(polystyrene) capacitors be employed. These capacitors possess
exceptional capacitance stability with respect to temperature, voltage,
frequency, and life. Circuits that will settle for semiprecision can
use paper/plastic film capacitors (with foil or metalized dielectric)
since they presently constitute a large portion of applications. If
precision is of no importance whatsoever, then general purpose
capacitors are recommended. These are the least expensive capacitors
and they have good performance ratings. Where suppression of
radio-frequency interference is required, RFI and feed-through
capacitors are the best equipped. For heavy currents (60-40 Hz power
supplies), paper or film dielectric capacitors should be used for
suppression, and ceramic and button-mica high-frequency style
capacitors are recommended for low currents. Ceramic chip capacitors
are highest on the list for use in microelectronic circuits. These
capacitors are electrically and physically the best suited for such
purposes. If a capacitor needs to be used as a transmitter, then it
is
advised that gas, vacuum, or ceramic capacitors be employed. These
capacitors possess the necessary high radio frequency (rf)
power-handling capability, high rf current and voltage rating, low
loss, low internal inductance, and very low ESR.
Environmental Effects
The effective operation of a capacitor is greatly dependent on the
physical environment that will be surrounding it. Out of these many
possible effects, those of primary concern with respect to medical
devices are temperature, humidity, dynamics, pressure, and radiation.
Temperature - The maximum operating ambient temperature surrounding
a
capacitor in an application is of critical importance. As the ambient
temperature changes, the dielectric constant and capacitance of most
capacitors change. The useful service life of a capacitor decreases
if
it is subjected to high temperatures for great amounts of time. As
the
temperature of the environment which surrounds the capacitor rises,
the
capacitor should receive less than the rated applied peak voltage.
On the other end of the spectrum, cold temperatures can present
problems as well. Electrolytic capacitors change their capacitance
immensely within a few degrees once they are exposed to temperatures
below 25 C. Aluminum electrolytics lose their capacitance at -55 C
and
tantalum loses about 20%. Equipment at low temperatures should be given
time for the capacitance to rise once the equipment has been powered
up.
Humidity (Moisture) - An important consideration in the application
of
a capacitor is making sure that no moisture penetrates the sealing
of
the capacitor case. The effects of humidity are parametric changes
(especially IR), reduced service life, and serious failure due to gross
moisture penetration. Most sensitive to moisture are the
paper-dielectric nonhermetically-sealed capacitors. Moisture can easily
penetrate into paper and can be trapped during manufacture, penetrate
the capacitor during service life, or penetrate the capacitor once
exposed to a moist environment.
Dynamic Environments - Dynamic environments can mechanically damage
or
destroy a capacitor. The main dynamic environments are in the form
of
shock, vibration, and acceleration. The movement of a capacitor
assembly inside a case can cause capacitance fluctuations, electrode
attachment failures, and dielectric and insulation failures. A
capacitor's susceptibility to dynamic environments is dependent on
its
physical construction; the larger the complex elements in the
capacitor, the lower the frequency of response of the elements.
Barometric Pressure - The pressure dictates the altitude at which a
hermetically sealed capacitor can safely operate. This altitude is
dependent on the design of the end-seal case-wall, the voltage at which
the capacitor will be operated, and the type of impregnant used in
the
dielectric material. As the altitude increases, the dielectric strength
across the end-seal will decrease. If the altitude is increased with
barometric pressure reduced, then the pressure inside the capacitor
will increase the mechanical stress on the case and seal until failure
occurs.
Radiation - Radiation particles can degrade the electrical performance
of capacitors. The principle cause of radiation-induced capacitor
defects is dimensional changes in the interelectrode spacing. This
change is due to gas evolution and swelling. Changes due to radiation
are more pronounced in organic-dielectric capacitors. Capacitors using
organic materials like polystyrene, polyethylene terephthalate, and
polyethylene are less satisfactory in a radiation environment by nearly
a factor of ten than those capacitors employing inorganic dielectrics.
The electrolytic capacitors (aluminum and tantalum) are capable of
extended radiation exposure with tantalum being more radiation
resistant. Another defect from radiation occurs when the dielectric
in
the capacitor experiences a noticeable increase in its conductivity
in
an ionizing-radiation environment. This results in the very dangerous
discharging of a charged capacitor.
References
1. Chute, George M., Electronics in Industry. New York: McGraw-Hill
Book Company, 1971.
2. Fink, Donald G., ed., Electronics Engineers Handbook. New York:
McGraw-Hill Book Company, 1975.
3. Fink, Donald G., ed., Standard Handbook for Electrical Engineers.
New York: McGraw-Hill Book Company, 1960.
4. Fugiel, Max, Modern Microelectronics. New York: Research and
Education Association, 1972.
5. Harper, Charles A., ed., Handbook of Components for Electronics.
New York: McGraw-Hill Book Company, 1977.
[FIGURE])
DEPT. OF HEALTH AND HUMAN SERVICES Date: 11/10/87 Number: 51
PUBLIC HEALTH SERVICE
FOOD AND DRUG ADMINISTRATION Related Program Areas:
*ORA/ORO/DEIO/IB* Medical Devices
Radiological Health
ITG SUBJECT: ELECTRONIC RELAYS
This ITG is intended to introduce the investigator to the electronic
relay. Since this is only an introduction, a discussion of the simpler
relays (EMR's, dry-reeds, mercury-wetted, and SSR's) will be given;
as
an indepth discussion of the more complicated models may destroy the
primary focus of this ITG. Included will be explanations of relay
theory, environmental effects, design, and failure.
THEORY
Relays are electrically controlled devices that open or close
electrical contacts to effect the operation of other devices in the
same or another electric circuit. This opening and closing of the relay
contacts is not an instantaneous action; as a tiny amount of time,
nearly 0.5 to 50 microseconds, is required in order for action to take
place. The relay's most basic components are its coil, armature, and
contacts. When the relay is put into some given circuit, the current
from that circuit induces a magnetic field in the relay coil. The
magnetic field in the coil then affects the armature in such a fashion
that it causes the contacts to make or break the part of the circuit
to
which the relay output terminals are connected.
The relay performs by means of a series of sequential events involving
both energization and deenergization. Beginning when the relay is off,
if the voltage or current is increased, then the relay will start to
move through its inactive (non-pickup) region where no switching takes
place. Next, as the current or voltage is still increasing, the relay
enters a region where it is both inactive and active (non-pickup and
pickup). Here, the relay uncontrollably cuts on and off and is said
to
be experiencing "bounce." Then the relay reaches the active
(pick-up)
region and begins to fully operate. The relay is now energized and
is
in the "operated state." Once the voltage or current begins
to
continually decrease, the relay starts to move back through its active
region. The relay is now trying to hold its present state (contacts
open or closed). Then the relay approaches the region where it is both
holding action and inactive (dropout). This state of relay operation
is
the parallel to the active/inactive mode when current or voltage is
increased. Finally, the relay reaches the inactive region and becomes
inoperable. The relay is now deenergized and is in the "restored
state." Although the process of energizing and deenergizing is
descriptively long, it must be restated that the actual process is
faster than the blink of an eye.
RELAY CONTACTS
Since the primary purpose of the relay is to "make" (closing
of the
contacts) or "break" (opening of the contacts) circuits,
a discussion
of the relay contacts is necessary.
The contacts of the relay must be large enough so that no deterioration
from destructive melting occurs; yet they must not be too large or
else
the current density will fall below a critical level and hinder
successful operation. The best contacting occurs when there is
sufficient electrical pressure (voltage) and current, along with
sufficient mechanical pressure on the contacts to cause fusing of the
contact surfaces on each operation.
Contacts can be damaged on both closure and opening. Contact closure
damage is usually due to current surges, because contact forces at
this instant are light, permitting contact sliding and bouncing to
take
place. This is not good because the load current is often many times
greater than the steady-state value at this instant. A microscopic
weld
or "bridge" will often form at the point of contact closure.
In DC
circuits this bridge usually ruptures asymmetrically at the next
contact opening, resulting in metal transfer. In AC circuits, there
is
usually a net loss of contact material, and the metal vapor that
condenses in the vicinity of the actual contact area is normally black
and is mistaken for carbon.
Contact damage due to opening comes in two forms; DC and AC. In the
DC
case, transients are more than certain to exist upon contact opening.
When the circuit to a DC inductive load is opened, most of the energy
stored in the load must be dissipated as arcing at the contacts unless
some other means of energy absorption is provided. Some of the load
energy is dissipated as heat in the load resistance, in eddy-current
losses in its magnetic circuit, and in the distributed capacitance
of
the coil winding. AC loads are treated differently because a stable
arc
be terminated when the current passes through zero and reverses at
the
end of the first half-cycle following contact separation. Under
moderate arcing conditions, contact life may be greatly increased by
shunting the load with a resistor-capacitor-diode combination whose
time constant is equal to that of the load.
Several cautions should be observed in order to insure successful
operation of the relay contacts. Relays operating near sensitive
circuits may cause trouble in electronic equipment from arcs generated
as the contacts function. Some type of suppression must be applied
as
electrical protection and interference protection. Another thing to
be
aware of is the transient voltages developed when the contacts open
the
load circuit. These voltages may exceed the dielectric withstanding
voltage between the contacts and another part of the relay. In some
circuits these voltages may be high enough to cause breakdown of
another circuit component. These transients often cause interference
in
adjacent or associated circuits. Elimination of high voltage transients
greatly improve system reliability, as well as speed of response and
consistency. As a final caution, careful attention should be paid to
contact protection. Proper protection can increase life expectancy
as
much as three orders of magnitude.
RELAY SPECIFICATIONS
Listed below are some common relay specifications that the investigator
should know.
Contact Bounce - This consists of the uncontrolled opening and closing
of contacts due to forces within the relay.
Contact Chatter - This is the uncontrolled opening and closing of
contacts due to external forces (e.g., shock/vibration).
Contact Rating - This is given as the electrical load on the contacts
in terms of closing surge current, steady-state voltage and current,
and induced breaking voltage.
Coil Winding Polarity - Unless conventional types of relays designed
for low-voltage (50 volts) circuits are used in short-lived equipment,
it is best to connect negative potential to the outer coil terminal(s).
The relay can then be controlled by switching grounded positive
potential to the inside terminals of the relay winding(s). This
minimizes electrolysis and adds years of life to relay coils.
Contact Spring Polarity - The same potential should be connected to
all
movable springs. This lessens the chance of accidental short
circuiting, which can destroy relay contacts in an unguarded instant.
Life - An electromagnetic/electromechanical relay's (EMR) cyclic life
may vary from less than one million operations to hundreds of millions.
Some special relays are capable of many billion operations. The static
life of EMR's is limited by physical or chemical deterioration of their
components. Other possible limitations are coil deterioration and
galvanic action between certain dissimilar metals. The design,
materials, and manufacturing processes of the relay are the ultimate
factors that determine static life.
The cyclic life of solid state relays (SSR's) is insignificant since
they are purely static devices. Their static life is limited by
physical or chemical changes affecting the intended function of their
junctions. The maximum junction temperature for SSR's limits the power
dissipated. This internally dissipated power is caused by the forward
voltage drop across the device and by the requirements of the device
drive (the relay power source). Above-rated voltage transients can
destroy or cause a device to go into an unwanted condition. The
environmental surroundings, design, application, and fabrication of
the
SSR determine static life.
TYPES
Relays come in a variety of types and classifications. As stated
earlier, the only types to be discussed are EMR's, dry-reeds, mercury-
wetted, and SSR's. Relays are classified by input, output, duty
rating, usage, and overall performance.
A. ELECTROMAGNETIC/ELECTROMECHANICAL RELAYS (EMR'S)
General-purpose - These relays are such that their design,
construction, operational characteristics, and ratings are
adaptable to a wide range of uses. They are usually constructed to
have clapper-type armature, leaf springs, button contacts and an
L-or U-shaped heelpiece (Figure 1) (Figure [FIGURE]). Their
operation consists of a coil pulling directly on an armature and
movable contacts attached to the armature. General-purpose relays
come in three duty ranges; light (two amperes of current or less),
medium (two to ten amperes), and heavy or power type (15 or more
amperes). They have a life expectancy of 100,000 operations for
their contacts and 10 million operations overall. General-purpose
relays find their most popular use in air-conditioning and heating,
household electrical appliances, control of low-wattage motors,
lighting controls, and elevator controls.
Power-type Relays - These look similar to general-purpose relays,
only they are larger and more rugged (Figure 2). (Figure [FIGURE])
Their contacts are suited for heavy currents and highly inductive
loads. Power-type relays are characterized by a contact current
rating of 20-25 amperes, the ability to best handle contact loads,
and easy repair. They are of little use in situations where varying
positions and shock or vibration are involved. Power relays are
specifically used for electric-motor control.
Telephone-type Relays - Their construction consists of an armature
with an end-mounted coil and spring pickup contacts mounted
parallel to the long axis of the relay coil (Figure 3). (Figure
[FIGURE]) Telephone-type relays are most used in business
machines, communications systems, computer input/output devices,
electronic data processing, laboratory test instruments, machine-
tool control logic, and production test equipment.
Resonant Reed Relays - These relays are designed to respond to a
given frequency of coil input current. Their operation involves an
electro-magnetic coil that, when energized, drives a vibrating
reed with a contact at its end. When the coil input frequency
corresponds to the resonant frequency of the reed, the reed will
vibrate and cause its contact to touch a stationary contact,
thereby closing a circuit once each electric cycle. At other
frequencies the reed does not respond. Unfortunately, their
contacts do not close with a firm positive closure and they
sometimes demonstrate undesired frequency drift due to temperature
extremes, tampering, shock, or vibration. Resonant reed relays are
used for applications where response to frequency is only desired,
such as communications, selective signaling, data transmission, and
telemetry.
Crystal Can Relays - This type of relay came about when
environmental conditions began to dictate that relays be
hermetically sealed, light-weight, and shock and
vibration-resistant (Figure 4) (Figure [FIGURE]). With crystal
can relays, relatively small contacts with fairly light pressures
can be operated. Also, contact ratings must be restricted to light
loads. These relays are small and adaptable to printed-circuit (PC)
boards and solid state circuitry. Their problem is that their inside
mechanisms are inaccessible during use for inspection of remaining
life and they are expensive.
Time-Delay Relays (TDR's) - TDR's basically consist of a
synchronous motor used for accurate long-time delay in opening and
closing contact. The most popular TDR's use a conventional relay
plus some required hybrid circuitry, plus an enclosure used to
combine all these elements into a unit (Figure 5) (Figure
[FIGURE]). Adjustable timing is done by altering pot setting by
means of a knob that can be turned externally, or slotted shaft
for screwdriver setting. With TDR's, all kinds of timing functions
can behandled; such as operate-time delay, release-time delay,
generation of a delay interval with reset, sequence timing with
repetition, pulse generation, and interval timing. The TDR's only
defect is that its repeat accuracy is poor.
"Permissire Make" Relays - In this type of relay, contact
switching
takes place when the energized coil provides sufficient force to
overpower a pretensioned spring that held the contacts in an
unoperated or normal position. When the biasing force is overcome
by sufficient armature pull, owing to coil energization, switching
of the contacts takes place. When the coil is deenergized, the
contact springs return to their unoperated position because the
biasing force of the restoring spring is now unopposed.
Latch-in Relays - These relays have contacts that lock in either
the energized or deenergized position until reset either manually
or electrically.
Differential Relays - These function when the voltage, current, or
power difference between its multiple windings reaches a
predetermined value.
Stepping Relays - Stepping relays operate by having their contacts
stepped to successive positions as the coil is energized in pulses.
They may be stepped in either direction.
B. DRY-REED RELAYS
Dry-reed relays are different from EMR's in that they require no
armature. They generate a flux that acts directly on the contacts
without employing any linkage. They are constructed so that two
normally separated, electrically conducting, and magnetic flux
conducting elements in a sealed glass envelope provide a portion of
the main flux path of the coil so that when the coil is energized,
these elements are attached to each other to form a closed contact
(Figure 6) (Figure [FIGURE]). Dry-reeds find most of their use
in business machines, communication systems, computer input/output
devices, electronic data processing, lab test instruments, and
production test equipment.
C. MERCURY-WETTED RELAYS
Mercury-Wetted Contact Relays - In these relays, the electrical
contacting is mercury to mercury. The contacting faces are renewed
by capillary action drawing a film of mercury over the surfaces of
the constant switching members as the movable contact member is
moved from one transfer position to the other. The mercury film is
drawn-up from a pool at the bottom of the capsule, between the
stationary members to provide bridging. No solid metal to solid
metal contacting takes place; so the contacts are actually renewed
on each operation. With mercury-wetted contact relays, wide ranges
of signal and power levels can be reliably switched without having
the nature of the load affect either contact life or performance.
One very important detail about these relays is that they must be
mounted right-side-up with an axis tilt less than 20 -30 from the
vertical. If the relay is inverted, the contacts will be flooded
from the mercury pool and may not perform properly for some time.
Also, since mercury is a primary part of this relay's operation;
low temperatures below - 38.8 C are a problem because mercury
solidifies at this temperature. Mercury-wetted contact relays are
ideal for pulsing highly inductive electro-magnets, such as rotary
stepping switches. They find their most common use in
air-conditioning and heating, business machines, communications,
computer input/output devices, electric power control, electronic
data processing, lab test instruments, and production test
equipment. Figure 7 (Figure [FIGURE]) shows a typical mercury-
wetted contact relay.
Heavy-Duty Power-Type Mercury Contact Relays - These relays were
developed from the necessity of ridding contact erosion from relays
handling heavy power loads. The continuous contact-renewal
properties of mercury accomplishes this task. The conduction in
power-type mercury contacts is through a pool of mercury, and the
two principal means for this process are: (1) The tilted mercury
tube (which causes the terminals to be bridged when the tube
containing the mercury is in one position and non-bridged or open
in the other position) and the mercury-displacement technique.
Here, a plunger is pulled down into the mercury pool so that a
bridge of conducting mercury extends from one terminal to the
other; thus closing a circuit over a dam that otherwise isolates
one terminal from the other. (2) When the coil is deenergized, the
plunger floats back up, the mercury returns to refill the pool, and
the circuit is opened. Figure 8 (Figure [FIGURE]) shows a typical
model.
D. SOLID STATE RELAYS (SSR'S)
Solid state relays are completely different from the three
previously mentioned types because they have no moving parts (Figure
9) (Figure [FIGURE]). An SSR is basically a semiconductor
switching device with input terminals isolated from the output
switch path. The output switch may be an FET (for low-level
switching) or a trial (for AC power switching, as is the case for
most of today's SSR's), and the input is usually a low-level DC
signal in the 3-32 Hz range. The SSR consists of a control, which
is equivalent to a coil, and a controlled output, the equivalent of
contacts.
Environment
For some relays any environment will do. The relay chosen should
just accommodate the proposed environment and should not be
overengineered. Some environments include extremes of temperature
and radiation/contamination, especially as encountered in airborne
service and space applications.
General Environments - In the case of EMR's, commercial atmospheres
are well tolerated in either an enclosed or unenclosed condition.
Extreme problems of atmosphere, particles, and moisture may require
hermetic sealing. For SSR's, packaging and small mass make then
immune to most environments, especially shock and vibration.
Temperature - For EMR's, the ability to withstand heat is limited
by the type of insulating materials used. Greater than maximum
allowed temperatures, if sustained, will produce a faster
deterioration and decomposition of most insulating materials. EMR
designs are available that can operate in maximum ambients of
125 C. The SSR's ability to withstand heat is limited by junction
temperature considerations. Above-rated, elevated-ambient
temperature surges usually have sufficient inertia to cause
irreversible changes in the relay if it is operating near maximum
capacity. Many SSR's can operate in temperatures of 125 C or
more, but their gate sensitivity and gain fall off below - 20 C.
Contamination - In EMR's, contamination of contacts is of most
concern. Results may vary from slightly increased contact
resistance to an electrically open condition. The relay coils are
susceptible to certain contaminants which will chemically
deteriorate the coil and results in electrical breakdown and
shorting. In SSR's, contamination is mostly encountered in
semiconductor pellets, resulting in a decrease in blocking voltage
and an increase in leakage current results.
Design Consideration
Design considerations for relays are not too complicated, but they
are vital nonetheless. Considerations of primary concern to the
designer include the relay contacts (as well as the attached
armature and springs), the type of input (AC/DC), and the load that
will be attached to the relay.
The dynamic characteristics of the armature and contact assembly
are primarily determined by the mass of the armature and depend
upon the magnet design and flux linkage. Contact and
restoring-force springs are attached or linked to the armature to
achieve the desired make and/or break characteristics. Primary
characteristics for these springs are modules of elasticity,
fatigue strength, conductivity, and corrosion resistance.
When choosing between AC or DC input relays, most designers prefer
the performance received from DC input relays. Although AC relays
offer economic advantages, DC relays are most often employed
because:
(1) DC relays have longer life. The contacts of AC relays flatten
prematurely due to wear from AC vibrations during their closing
& opening.
(2) DC relays have greater sensitivity. Since they don't chatter,
lighter energizing forces may be used than is the case for AC.
(3) DC coils have lower heat loss and can be made smaller.
(4) DC relays, especially if heavily loaded, can carry a wider
voltage range then AC.
(5) Timing is impossible when operating conventional relays with
AC.
If the only requirement is that the relay simply shall operate
when a switch to it is closed and release when that switch is
opened, then it doesn't matter whether the relays are powered by
AC or DC.
In terms of loads, the investigator should be aware of what is
expected from the relay when connected to such devices. For
communications equipment, the relay is expected to have long life,
reliability, freedom from too frequent maintenance, and favorable
environments. Telephone-types are ideal for these applications. In
computer I/O devices the relay must meet heavy-duty requirements
and have maximum life expectancy with utmost reliability. Quick
disconnect mounting, so that the unit in need of attention can be
instantly replaced, is also a requirement. Environment is no
problem. With electric power control, long life, reliability, and
freedom from frequent maintenance are the primary requirement.
Environment serves as no threat to these relays. For electronic
data processing, the only things to be consider are contacts that
accommodate a high variation in contact load and environment. In
lab test instruments, maximum reliability with good lite and
freedom from frequent maintenance are a must. Environment is of no
consequence. For production test equipment, a high order of
insulation and resistance dielectric withstanding voltage with low
contact resistance are requirement.
Failure
Most EMR failures are easily detected because of visual evidence of
failure. Failures usually occur in the contacts. Contact failure
comes in the form of film formation, wear erosion, gap erosion,
surface contamination, and cold welding. Film formation is the
effect of organic and inorganic corrosion, causing excessive
resistance, particularly at dry-crack conditions. Wear erosion
results from particles in the contact area which can cause bridging
between small contact gaps. Surface contamination results when dirt
and dust particles on the contact surface prevent the achievement
of low resistance between the contacts, and may actually cause an
open crack. Cold-welding is the self-adhering of clean contacts in
a dry environment. Some symptoms of contact failure are high
contact resistance, mechanical failure, coil opening or shorting,
and contact sticking, transferring or welding. Contact sticking and
high contact resistance may be intermittent and perceived as misses
instead of failures.
In SSR's, there is usually no visual evidence of failure unless it
is heat discoloration. SSR failures are characterized by permanent
shorts, the inability to block voltage, or leakage current reaching
failure proportions. General failure factors related to SSR's are:
exceeding of maximum voltage ratings; thermomechanical fatigue
caused by cyclic temperature surges; chemical reactions, such as
channeling; and physical changes, such as crystallization of
materials. SSR failures are quickened with prolonged temperature
increases. Since visual detection is so difficult, SSR failure
detection can become quite involved depending on the knowledge,
experience, and equipment required.
References
1. Chute, George Mr., Electronics in Industry. New York: McGraw-Hill
Book Company, 1971.
2. Fink, Donald G., ed., Electronics Engineers Handbook. New York:
McGraw-Hill Book Company, 1975.
3. Fink, Donald G., ed., Standard Handbook for Electronical
Engineers. New York: McGraw-Hill Book Company, 1960.
4. Harper, Charles A., ed., Handbook of Components for
Electronics. New York: McGraw-Hill Book Company, 1977.
[FIGURE])
(Figure [FIGURE])
DEPT. OF HEALTH AND HUMAN SERVICES Date: 11/10/87 Number: 52 PUBLIC HEALTH SERVICE FOOD AND DRUG ADMINISTRATION Related Program Areas: *ORA/ORO/DEIO/IB* GLP, Devices
ITG SUBJECT: VOICE RECOGNITION SYSTEMS
A voice recognition system is an electronic computer controlled device
which responds to the human voice. When hooked up to a computer
terminal, it enables an operator to enter data merely by voice commands
rather than having to manually enter the data using a keyboard.
You may encounter these voice recognition systems during your on-site
inspections, most frequently where large amounts of data must be
entered, generally for lists of numbers. These can be found during
Good
Laboratory Practice (GLP) inspections particularly for those aspects
relating to laboratory analysis of various animal body fluids.
Typically, the operator will be wearing a microphone/headset while
peering through a microscope counting various cells. With the voice
recognition system, the numerical information can be entered into the
computer merely by speaking the numbers aloud. The operator does not
even have to lift his head away from the microscope.
Given the current state-of-the-art, the voice recognition systems are
not universal, that is, they will not respond to just any voice. They
have to be custom tailored to each operator who will be utilizing the
system. In effect, the computer "memorizes" the way a particular
person
pronounces numbers and then will respond only to those numbers
pronounced in that way.
Most programs have a training mode where the computer is "taught"
to
recognize that particular person's voice in pronouncing numbers. The
software is written so that a numerical score can be granted to the
recognition quality for that particular person. This is done by a
validation procedure where the operator verifies the accuracy of the
output compared to the input.
One feature normally found with the voice recognition systems is an
audio repeat system where the synthesized computer voice repeats the
word over a loudspeaker after the operator has enunciated it. In this
way, the operators can keep their attention focused on the task at
hand
and yet make sure that the computer has "understood" the
number that
they were trying to enter. This aspect is quite important as generally
no other positive verification is available.
During your on-site investigations when you find a firm utilizing a
voice recognition system, you should try to observe the system while
it
is in use and determine the vocabulary limits for that particular
system. Often only a certain limited range of numbers can be put into
the system. You should also determine if such factors as speed of
articulation, pitch, volume, ambient noise or other possible factors
can corrupt data transcription. Operators may also be able to
deactivate computer voice feedback as an irritant. There should also
be
a written SOP for both the computer "training" phase as well
as the
validation, tailored to each operator.
In the future, as the computer memory capacity expands, we may see
additional use of these voice recognition systems.
