[Code of Federal Regulations]
[Title 40, Volume 5]
[Revised as of July 1, 2007]
From the U.S. Government Printing Office via GPO Access
[CITE: 40CFR53.23]
[Page 25-39]
TITLE 40--PROTECTION OF ENVIRONMENT
CHAPTER I--ENVIRONMENTAL PROTECTION AGENCY (CONTINUED)
PART 53_AMBIENT AIR MONITORING REFERENCE AND EQUIVALENT METHODS--Table of
Subpart B_Procedures for Testing Performance Characteristics of
Sec. 53.23 Test procedures.
(a) Range--(1) Technical definition. Nominal minimum and maximum
concentrations which a method is capable of measuring.
Note: The nominal range is specified at the lower and upper range
limits in concentration units, for example, 0-0.5 p/m.
(2) Test procedure. Submit a suitable calibration curve, as
specified in Sec. 53.21(b), showing the test analyzer's response over
at least 95 percent of the required range.
Note: A single calibration curve will normally suffice.
(b) Noise--(1) Technical definition. Spontaneous, short duration
deviations in output, about the mean output, which are not caused by
input concentration changes. Noise is determined as the standard
deviation about the mean and is expressed in concentration units.
(2) Test procedure. (i) Allow sufficient time for the test analyzer
to warm up and stabilize. Determine at two concentrations, first using
zero air and then a pollutant test gas concentration as indicated below.
The noise specification in table B-1 shall apply to both of these tests.
(ii) Connect an integrating-type digital meter (DM) suitable for the
test analyzer's output and accurate to three significant digits, to
measure the analyzer's output signal.
Note: Use of a chart recorder in addition to the DM is optional.
(iii) Measure zero air for 60 minutes. During this 60-minute
interval, record twenty-five (25) readings at 2-minute intervals. (See
Figure B-2 in appendix A.)
(iv) Convert each DM reading to concentration units (p/m) by
reference to the test analyzer's calibration curve as determined in
Sec. 53.21(b). Label the converted DM readings r1,
r2, r3 . . . ri . . .
r25.
(v) Calculate the standard deviation, S, as follows:
where i indicates the i-th DM reading in ppm.
(vi) Let S at 0 ppm be identified as So; compare
So to the noise specification given in table B-1.
(vii) Repeat steps (iii) through (vi) of this section using a
pollutant test atmosphere concentration of 805
percent of the upper range limit (URL) instead of zero gas, and let S at
80 percent of the URL be identified as S80.
Compare S80 to the noise specification given in
table B-1.
(viii) Both S0 and S80 must be less
than or equal to the specification for noise to pass the test for the
noise parameter.
(c) Lower detectable limit--(1) Technical definition. The minimum
pollutant concentration which produces a signal of twice the noise
level.
(2) Test procedure. (i) Allow sufficient time for the test analyzer
to warm up and stabilize. Measure zero air and record the stable reading
in ppm as BZ. (See Figure B-3 in appendix A.)
(ii) Generate and measure a pollutant test atmosphere concentration
equal to the value for the lower detectable limit specified in table B-
1.
Note: If necessary, the test atmosphere concentration may be
generated or verified at a higher concentration, then accurately
[[Page 26]]
diluted with zero air to the final required concentration.
(iii) Record the test analyzer's stable indicated reading, in ppm,
as BL.
(iv) Determine the Lower Detectable Limit (LDL) as LDL =
BL-BZ. Compare this LDL value with the noise
level, S0, determined in Sec. 53.23(b), for 0 concentration
test atmosphere. LDL must be equal to or higher than 2S0 to
pass this test.
(d) Interference equivalent--(1) Technical definition. Positive or
negative response caused by a substance other than the one being
measured.
(2) Test procedure. The test analyzer shall be tested for all
substances likely to cause a detectable response. The test analyzer
shall be challenged, in turn, with each interfering agent specified in
table B-3. In the event that there are substances likely to cause a
significant interference which have not been specified in table B-3,
these substances shall be tested at a concentration substantially higher
than that normally found in the ambient air. The interference may be
either positive or negative, depending on whether the test analyzer's
response is increased or decreased by the presence of the interferent.
Interference equivalents shall be determined by mixing each interferent,
one at a time, with the pollutant at the concentrations specified in
table B-3, and comparing the test analyzer's response to the response
caused by the pollutant alone. Known gas-phase reactions that might
occur between an interferent and the pollutant are designated by
footnote 3 in table B-3. In these cases, the interference equivalent
shall be determined in the absence of the pollutant.
(i) Allow sufficient time for warm-up and stabilization of the test
analyzer.
(ii) For a candidate method using a prefilter or scrubber based upon
a chemical reaction to derive part of its specificity, and which
requires periodic service or maintenance, the test analyzer shall be
``conditioned'' prior to each interference test as follows:
[[Page 27]]
Table B-3--Interferant Test Concentration,\1\ Parts Per Million
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Hydrochloric Hydrogen Sulfur Nitrogen Nitric Carbon M- Water Carbon
Pollutant Analyzer type \2\ acid Ammonia sulfide dioxide dioxide oxide dioxide Ethylene Ozone xylene vapor monoxide Methane Ethane
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
SO2..................... Flame photometric (FPD)..... ............ ....... 0.1 \1\ 0.14 ........ ....... 750 ........ ....... ....... \3\ 50 ....... .......
20,000
SO2..................... Gas chromatography (FPD).... ............ ....... .1 \4\.14 ........ ....... 750 ........ ....... ....... \3\ 50 ....... .......
20,000
SO2..................... Spectrophotometric-wet 0.2 \3\ 0.1 .1 \4\.14 0.5 ....... 750 ........ 0.5 ....... ........ ........ ....... .......
chemical (pararosaniline
reaction).
SO2..................... Electrochemical............. .2 \3\.1 .1 \4\.14 .5 0.5 ........ 0.2 .5 ....... \3\ ........ ....... .......
20,000
SO2..................... Conductivity................ .2 \3\.1 ........ \4\.14 .5 ....... 750 ........ ....... ....... ........ ........ ....... .......
SO2..................... Spectrophotometric-gas phase ............ ....... ........ \4\.14 .5 .5 ........ ........ .5 0.2 ........ ........ ....... .......
O3...................... Chemiluminescent............ ............ ....... \3\.1 ........ ........ ....... 750 ........ \4\.08 ....... \3\ ........ ....... .......
20,000
O3...................... Electrochemical............. ............ \3\.1 ........ .5 .5 ....... ........ ........ \4\.08 ....... \3\ ........ ....... .......
20,000
O3...................... Spectrophotometric-wet ............ \3\.1 ........ .5 .5 \3\.5 ........ ........ \4\.08 ....... ........ ........ ....... .......
chemical (potassium iodide
reaction).
O3...................... Spectrophotometric-gas phase ............ ....... ........ .5 .5 \3\.5 ........ ........ \4\.08 ....... ........ ........ ....... .......
CO...................... Infrared.................... ............ ....... ........ ........ ........ ....... 750 ........ ....... ....... 20,000 \4\ 10 ....... .......
CO...................... Gas chromatography with ............ ....... ........ ........ ........ ....... ........ ........ ....... ....... 20,000 \4\ 10 ....... 0.5
flame ionization detector.
CO...................... Electrochemical............. ............ ....... ........ ........ ........ .5 ........ .2 ....... ....... 20,000 \4\ 10 ....... .......
CO...................... Catalytic combustion-thermal ............ .1 ........ ........ ........ ....... 750 .2 ....... ....... 20,000 \4\ 10 5.0 .5
detection.
CO...................... IR fluorescence............. ............ ....... ........ ........ ........ ....... 750 ........ ....... ....... 20,000 \4\ 10 ....... .5
CO...................... Mercury replacement UV ............ ....... ........ ........ ........ ....... ........ .2 ....... ....... ........ \4\ 10 ....... .5
photometric.
NO2..................... Chemiluminescent............ ............ \3\.1 ........ .5 \4\.1 .5 ........ ........ ....... ....... 20,000 ........ ....... .......
NO2..................... Spectrophotometric-wet ............ ....... ........ .5 \4\.1 .5 750 ........ .5 ....... ........ ........ ....... .......
chemical (azo-dye reaction).
NO2..................... Electrochemical............. 0.2 \3\.1 ........ .5 \4\.1 .5 750 ........ .5 ....... 20,000 50 ....... .......
NO2..................... Spectrophotometric-gas phase ............ \3\.1 ........ .5 \4\.1 .5 ........ ........ .5 ....... 20,000 50 ....... .......
------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
\1\ Concentrations of interferant listed must be prepared and controlled to 10 percent of the state value.
\2\ Analyzer types not listed will be considered by the administrator as special cases.
\3\ Do not mix with pollutant.
\4\ Concentration of pollutant used for test. These pollutant concentrations must be prepared to 10 percent of the stated value.
[[Page 28]]
(A) Service or perform the indicated maintenance on the scrubber or
prefilter as directed in the manual referred to in Sec. 53.4(b)(3).
(B) Before testing for each interferent, allow the test analyzer to
sample through the scrubber a test atmosphere containing the interferent
at a concentration equal to the value specified in table B-3. Sampling
shall be at the normal flow rate and shall be continued for 6 continuous
hours prior to testing.
(iii) Generate three test atmosphere streams as follows:
(A) Test atmosphere P: Pollutant concentration.
(B) Test atmosphere I: Interference concentration.
(C) Test atmosphere Z: Zero air.
(iv) Adjust the individual flow rates and the pollutant or
interferent generators for the three test atmospheres as follows:
(A) The flow rates of test atmospheres I and Z shall be identical.
(B) The concentration of pollutant in test atmosphere P shall be
adjusted such that when P is mixed (diluted) with either test atmosphere
I or Z, the resulting concentration of pollutant shall be as specified
in table B-3.
(C) The concentration of interferent in test atmosphere I shall be
adjusted such that when I is mixed (diluted) with test atmosphere P, the
resulting concentration of interferent shall be equal to the value
specified in table B-3.
(D) To minimize concentration errors due to flow rate differences
between I and Z, it is recommended that, when possible, the flow rate of
P be from 10 to 20 times larger than the flow rates of I and Z.
(v) Mix test atmospheres P and Z by passing the total flow of both
atmospheres through a mixing flask.
(vi) Sample and measure the mixture of test atmospheres P and Z with
the test analyzer. Allow for a stable reading, and record the reading,
in concentration units, as R (see Figure B-3).
(vii) Mix test atmospheres P and I by passing the total flow of both
atmospheres through a mixing flask.
(viii) Sample and measure this mixture. Record the stable reading,
in concentration units, as RI.
(ix) Calculate the interference equivalent (IE) as:
IE = RI-R
IE must be equal to or less than the specification given in table B-1
for each interferent to pass the test.
(x) Follow steps (iii) through (ix) of this section, in turn, to
determine the interference equivalent for each interferent.
(xi) For those interferents which cannot be mixed with the
pollutant, as indicated by footnote (3) in table B-3, adjust the
concentration of test atmosphere I to the specified value without being
mixed or diluted by the pollutant test atmosphere. Determine IE as
follows:
(A) Sample and measure test atmosphere Z (zero air). Allow for a
stable reading and record the reading, in concentration units, as R.
(B) Sample and measure the interferent test atmosphere I. If the
test analyzer is not capable of negative readings, adjust the analyzer
(not the recorder) to give an offset zero. Record the stable reading in
concentration units as RI, extrapolating the calibration curve, if
necessary, to represent negative readings.
(C) Calculate IE=RI-R. IE must be equal to or less than
the specification in table B-1 to pass the test.
(xii) Sum the absolute value of all the individual interference
equivalents. This sum must be equal to or less than the total
interferent specification given in table B-1 to pass the test.
(e) Zero drift, span drift, lag time, rise time, fall time, and
precision--(1) Technical definitions--(i) Zero drift: The change in
response to zero pollutant concentration, over 12- and 24-hour periods
of continuous unadjusted operation.
(ii) Span drift: The percent change in response to an up-scale
pollutant concentration over a 24-hour period of continuous unadjusted
operation.
(iii) Lag time: The time interval between a step change in input
concentration and the first observable corresponding change in response.
(iv) Rise time: The time interval between initial response and 95
percent of final response after a step increase in input concentration.
[[Page 29]]
(v) Fall time: The time interval between initial response and 95
percent of final response after a step decrease in input concentration.
(vi) Precision: Variation about the mean of repeated measurements of
the same pollutant concentration, expressed as one standard deviation
about the mean.
(2) Tests for these performance parameters shall be accomplished
over a period of seven (7) or more days. During this time, the line
voltage supplied to the test analyzer and the ambient temperature
surrounding the analyzer shall be varied from day to day. One test
result for each performance parameter shall be obtained each test day,
for seven (7) or fifteen (15) test days as necessary. The tests are
performed sequentially in a single procedure.
(3) The 24-hour test day may begin at any clock hour. The first 12
hours out of each test day are required for testing 12-hour zero drift.
Tests for the other parameters shall be conducted during the remaining
12 hours.
(4) Table B-4 specifies the line voltage and room temperature to be
used for each test day. The line voltage and temperature shall be
changed to the specified values at the start of each test day (i.e., at
the start of the 12-hour zero test). Initial adjustments (day zero)
shall be made at a line voltage of 115 volts (rms) and a room
temperature of 25 [deg]C.
(5) The tests shall be conducted in blocks consisting of 3 test days
each until 7 or 15 test results have been obtained. (The final block may
contain fewer than three test days.) If a test is interrupted by an
occurrence other than a malfunction of the test analyzer, only the block
during which the interruption occurred shall be repeated.
(6) During each block, manual adjustments to the electronics, gas,
or reagent flows or periodic maintenance shall not be permitted.
Automatic adjustments which the test analyzer performs by itself are
permitted at any time.
(7) At least 4 hours prior to the start of the first test day of
each block, the test analyzer may be adjusted and/or serviced according
to the periodic maintenance procedures specified in the manual referred
to in Sec. 53.4(b)(3). If a new block is to immediately follow a
previous block, such adjustments or servicing may be done immediately
after completion of the day's tests for the last day of the previous
block and at the voltage and temperature specified for that day, but
only on test days 3, 6, 9, and 12.
Note: If necessary, the beginning of the test days succeeding such
maintenance or adjustment may be delayed as necessary to complete the
service or adjustment operation.
(8) All response readings to be recorded shall first be converted to
concentration units according to the calibration curve. Whenever a test
atmosphere is to be measured but a stable reading is not required, the
test atmosphere shall be measured long enough to cause a change in
response of at least 10% of full scale. Identify all readings and other
pertinent data on the strip chart. (See Figure B-1 illustrating the
pattern of the required readings.)
Table B-4--Line Voltage and Room Temperature Test Conditions
------------------------------------------------------------------------
Line Room
Test day voltage,\1\ temperature,\2\ Comments
rms [deg]C
------------------------------------------------------------------------
0................... 115 25 Initial set-up and
adjustments.
1................... 125 20
2................... 105 20
3................... 125 30 Adjustments and/or
periodic
maintenance
permitted at end of
tests.
4................... 105 30
5................... 125 20
6................... 105 20 Adjustments and/or
periodic
maintenance
permitted at end of
tests.
7................... 125 30 Examine test results
to ascertain if
further testing is
required.
8................... 105 30
9................... 125 20 Adjustments and/or
periodic
maintenance
permitted at end of
tests.
10.................. 105 20
11.................. 125 30
12.................. 105 30 Adjustments and/or
periodic
maintenance
permitted at end of
tests.
[[Page 30]]
13.................. 125 20
14.................. 105 20
15.................. 125 30
------------------------------------------------------------------------
\1\ Voltage specified shall be controlled to 1
volt.
\2\ Temperature specified shall be controlled to 1
[deg]C.
[[Page 31]]
[GRAPHIC] [TIFF OMITTED] TC01JY92.000
(9) Test procedure. (i) Arrange to generate pollutant test
atmospheres as follows:
------------------------------------------------------------------------
Pollutant concentration
Test atmosphere (percent)
------------------------------------------------------------------------
A0........................................ Zero air.
A20....................................... 205 of
the upper range limit.
A30....................................... 305 of
the upper range limit.
[[Page 32]]
A80....................................... 805 of
the upper range limit.
A90....................................... 905 of
the upper range limit.
------------------------------------------------------------------------
Test atmospheres A0, A20, and
A80 shall be consistent during the tests and from
day to day.
(ii) For steps (xxv) through (xxxi) of this section, a chart speed
of at least 10 centimeters per hour shall be used. The actual chart
speed, chart speed changes, and time checks shall be clearly marked on
the chart.
(iii) Allow sufficient time for test analyzer to warm up and
stabilize at a line voltage of 115 volts and a room temperature of 25
[deg]C. Recalibrate, if necessary, and adjust the zero baseline to 5
percent of chart. No further adjustments shall be made to the analyzer
until the end of the tests on the third day.
(iv) Measure test atmosphere A0 until a stable reading is
obtained, and record this reading (in ppm) as Z'n, where n =
0 (see Figure B-4 in appendix A).
(v) Measure test atmosphere A20. Allow for a
stable reading and record it as M'n, where n = 0.
(vi) Measure test atmosphere A80. Allow for a
stable reading and record it as S'n, where n = 0.
(vii) The above readings for Z'0, M'0, and
S'0 should be taken at least four (4) hours prior to the
beginning of test day 1.
(viii) At the beginning of each test day, adjust the line voltage
and room temperature to the values given in table B-4.
(ix) Measure test atmosphere A0 continuously for at least
twelve (12) continuous hours during each test day.
(x) After the 12-hour zero drift test (step ix), sample test
atmosphere A0. A stable reading is not required.
(xi) Measure test atmosphere A20 and record the stable
reading (in ppm) as P1. (See Figure B-4 in appendix A.)
(xii) Sample test atmosphere A30; a stable
reading is not required.
(xiii) Measure test atmosphere A20 and record
the stable reading as P2.
(xiv) Sample test atmosphere A0; a stable reading is not
required.
(xv) Measure test atmosphere A20 and record
the stable reading as P3.
(xvi) Sample test atmosphere A30; a stable reading is not
required.
(xvii) Measure test atmosphere A20 and record the stable
reading as P4.
(xviii) Sample test atmosphere A0; a stable reading is
not required.
(xix) Measure test atmosphere A20 and record
the stable reading as P5.
(xx) Sample test atmosphere A30; a stable
reading is not required.
(xxi) Measure test atmosphere A20 and record
the stable reading as P6.
(xxii) Measure test atmosphere A30 and record
the stable reading as P7.
(xxiii) Sample test atmosphere A90; a stable
reading is not required.
(xxiv) Measure test atmosphere A80 and record the stable
reading as P8. Increase chart speed to at least 10
centimeters per hour.
(xxv) Measure test atmosphere A0. Record the stable
reading as L1.
(xxvi) Quickly switch the test analyzer to measure test atmosphere
A80 and mark the recorder chart to show the exact time when
the switch occurred.
(xxvii) Measure test atmosphere A90 and record
the stable reading as P80.
(xxviii) Sample test atmosphere A90; a stable
reading is not required.
(xxix) Measure test atmosphere A80 and record the stable
reading as P10.
(xxx) Measure test atmosphere A0 and record the stable
reading as L2.
(xxxi) Measure test atmosphere A80 and record the stable
reading as P11.
(xxxii) Sample test atmosphere A90; a stable
reading is not required.
(xxxiii) Measure test atmosphere A80 and record the
stable reading as P12.
(xxxiv) Repeat steps (viii) through (xxxiii) of this section, each
test day.
(xxxv) If zero and span adjustments are made after the readings are
taken on test days 3, 6, 9, or 12, complete all adjustments; then
measure test atmospheres A0, A80, and
A20. Allow for a stable reading on each, and
record the readings as Z'nS'n, and Mn
respectively, where n = the test day number.
(10) Determine the results of each day's tests as follows. Mark the
recorder chart to show readings and determinations.
(i) Zero drift. (A) 12-hour. Examine the strip chart pertaining to
the 12-
[[Page 33]]
hour continuous zero air test. Determine the minimum (Cmin.) and maximum
(Cmax.) readings (in p/m) during this period of 12 consecutive hours,
extrapolating the calibration curve to negative concentration units if
necessary. Determine the 12-hour zero drift (12ZD) as 12ZD = C\max.\-
C\min.\. (See Figure B-5 in appendix A.)
(B) Calculate the 24-hour zero drift (24ZD) for the n-th test day as
24ZDn = Zn-Zn-1, or 24ZDn =
Zn-Z'n-1 if zero adjustment was made on the
previous day, where Zn = \1/2\(L1+L2)
for L1 and L2 taken on the n-th test day.
(C) Compare 12ZD and 24ZD to the zero drift specification in table
B-1. Both 12ZD and 24ZD must be equal to or less than the specified
value to pass the test for zero drift.
(ii) Span drift. (A) Span drift at 20 percent of URL (MSD)
[GRAPHIC] [TIFF OMITTED] TC09NO91.000
[GRAPHIC] [TIFF OMITTED] TC09NO91.001
If span adjustment was made on the previous day, where
[GRAPHIC] [TIFF OMITTED] TC09NO91.002
n indicates the n-th test day, and i indicates the i-th reading on the
n-th day.
(B) Span drift at 80 percent of URL (USD):
[GRAPHIC] [TIFF OMITTED] TC09NO91.003
or
[GRAPHIC] [TIFF OMITTED] TC09NO91.004
If span adjustment was made on the previous day, where
[GRAPHIC] [TIFF OMITTED] TC09NO91.005
n indicates the n-th test day, and i indicates the i-th reading on the
n-th test day.
(C) Both USD and MSD must be equal to or less than the respective
specifications given in table B-1 to pass the test for span draft.
(iii) Lag time. Determine, from the strip chart, the elapsed time in
minutes between the mark made in step (xxvi) and the first observable
(two times the noise level) response. This time must be equal to or less
than the time specified in table B-1 to pass the test for lag time.
(iv) Rise time. Calculate 95 percent of reading P9 and
determine from the recorder chart, the elapsed time between the first
observable (two times noise level) response and a response equal to 95
percent of the P9 reading. This time must be equal to or less
than the rise time specified in table B-1 to pass the test for rise
time.
(v) Fall time. Calculate five percent of (P10-
L2) and determine, from the strip chart, the elapsed time in
minutes between the first observable decrease in response following
reading P10 and a response equal to five percent of
(P10-L2). This time must be equal to or less than
the fall time specification in table B-1 to pass the test for fall time.
(vi) Precision. Calculate precision (P20 and
P80) for each day's test as follows:
(A)
[GRAPHIC] [TIFF OMITTED] TC09NO91.006
(B)
[GRAPHIC] [TIFF OMITTED] TC09NO91.007
(C) Both P20 and P80
must be equal to or less than the specification given in table B-1 to
pass the test for precision.
[40 FR 7049, Feb. 18, 1975, as amended at 41 FR 52694, Dec. 1, 1976]
[[Page 34]]
Appendix A to Subpart B of Part 53--Optional Forms for Reporting Test
Results
Table B-5--Symbols and Abbreviations
BL............................ Analyzer reading at specified LDL
concentration.
Bz............................ Analyzer reading at 0 concentration for
LDL test.
DM............................ Digital meter.
Cmax.......................... Maximum analyzer reading during 12ZD
test.
Cmin.......................... Minimum analyzer reading during 12ZD
test.
i............................. Subscript indicating the i-th quantity
in a series.
IE............................ Interference equivalent.
L1............................ First analyzer zero reading for 24ZD
test.
L2............................ Second analyzer zero reading for 24ZD
test.
Mn............................ Average of P1 . . . P6 for the n-th test
day.
M'n........................... Adjusted span reading at 20 percent of
URL on the n-th test day.
MSD........................... Span drift at 20 percent of URL.
n............................. Subscript indicating the test day
number.
P............................. Analyzer reading for precision test.
Pi............................ The i-th analyzer reading for precision
test.
P20........................... Precision at 20 percent of URL.
P80........................... Precision at 80 percent of URL.
R............................. Analyzer reading of pollutant alone for
IE test.
RI............................ Analyzer reading with interferent added
for IE test.
ri............................ The i-th DM reading for noise test.
S............................. Standard deviation of noise readings.
S0............................ Noise value (S) measured at 0
concentration.
S80........................... Noise value (S) measured at 80 percent
of URL.
Sn............................ Average of P7 . . . P12 for the n-th
test day.
S'n........................... Adjusted span reading at 80 percent of
URL on the n-th test day.
URL........................... Upper range limit.
USD........................... Span drift at 80 percent o
Z............................. Average of L1 and L2.
Zn............................ Average of L1 and L2 on the n-th test
day.
Z'n........................... Adjusted zero reading on the n-th test
day.
ZD............................ Zero drift.
12ZD.......................... 12-hour zero drift.
24ZD.......................... 24-hour zero drift.
[[Page 35]]
[[Page 36]]
[[Page 37]]
[[Page 38]]
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[GRAPHIC] [TIFF OMITTED] TC09NO91.031
[40 FR 7049, Feb. 18, 1975, as amended at 40 FR 18169, Apr. 25, 1975]