EQUIPMENT--Policy to Ensure the Accurate Performance of Current Meters
In Reply Refer To: June 2, 1989
WGS-Mail Stop 415
OFFICE OF SURFACE WATER TECHNICAL MEMORANDUM NO. 89.07
SUBJECT: EQUIPMENT--Policy to Ensure the Accurate Performance
of Current Meters
The purpose of this memorandum is to establish a policy that
assigns responsibility for the care and maintenance of current
meters and to define the purpose and use of the timed spin test.
It is important that each hydrographer be aware of the
requirements for testing and care of current meters and follow
them closely. The accuracy of the discharge measurements and the
stage-discharge rating depends on it.
The current meters used by the U.S. Geological Survey are usually
supplied with a standard rating. The use of standard ratings
facilitates the field repair of meters and reduces the cost of the
meter. The standard rating is made possible by manufacturing the
meter and each part of the meter to a very tight specification to
assure that all parts are truly interchangeable. The standard
rating is developed by towing the meter at a known constant speed
in a tank of still water (in the hydraulics laboratory at the John
C. Stennis Space Center). An average rating is derived for a
group of meters which then becomes the standard rating. Since the
meters used to develop the standard rating are new, the successful
use of the standard rating depends on maintaining a current meter
in "as-new" condition.
The ultimate responsibility for the good condition and accuracy of
a current meter rests with the hydrographer who uses it. His or
her conscientiousness and professionalism are the only guarantees
that a meter is in proper operating condition when a discharge
measurement is made. A timed spin test made a few minutes before
a measurement does not ensure that the meter will not become
damaged or fouled during the measurement.
An important source of current-meter inaccuracy is bent cups (see
Smoot and Carter, 1968). A timed spin test is unlikely to reveal
that a meter with bent cups will give inaccurate results. Perhaps
the polymer rotor will eventually solve this problem, because they
break instead of bending. For a metal-cup rotor, however, a
thorough visual inspection is necessary.
It is the responsibility of the field hydrographer to determine if
the current-meter rotor is moving freely and does not stop
abruptly. A full spin test would demonstrate this, but a much
quicker observation of the rotor as it comes to rest will do as
well. The responsibility of the field person does not end here,
however. He or she should monitor the meter for wear, damage, and
fouling. This monitoring process should be somewhat continuous in
the field--before, during, and after measurements. The procedures
for protecting, cleaning, and lubricating the meter as described
in the applicable Techniques of Water-Resources Investigations and
other manuals should be followed conscientiously (see Smoot and
Novak, 1968).
These procedures will almost certainly guarantee the acceptable
performance of a meter. The full timed spin test should be an
office procedure between field trips, when performance of a meter
is suspect, and before and after repairs. It will not take the
place of continual care and inspection of the meter. Every field
person eventually comes to know this truism, hence the rather poor
adherence to stated or implied policy that timed spin tests must
precede and follow each measurement.
Timed spin tests are indicative of wear or damage to the bearing
surfaces. To record spin tests, a current meter log will be kept
on file in the appropriate office as designated in the District
quality-assurance plan. This log will become part of the archived
data of the Water Resources Division. The format of the log is
left to each District, but must include the name of the meter
user, checker, repairs, and the date and results of timed tests.
A suggested log format is attached, which will fit on a discharge-
measurement-sized card. Logs will be inspected at the periodic
Office of Surface Water reviews of District surface-water
activities. Repairs and other remarks also should be entered
along with the names of the persons making the entry. In addition
to spin tests between field trips, after repairs, and when a meter
is suspect, the Office of Surface Water recommends that periodic
meter inspections be made by the data chief, lead technician, or
someone other than the user.
The hydrographer should fill in the blanks on the measurement
front sheet to indicate that he or she has inspected the meter's
condition as described above. Notations such as "OK" or "free"
are acceptable. If the field person does make a timed spin test,
it should be noted on the measurement note sheet.
The following minimum acceptable spin test time replaces previous
guidance on this subject:
All types of Pygmy Meters 0:45 seconds
All types of AA Meters 2:00 minutes
These are minimum spin times for field use. Meters in good
condition will perform substantially better.
REFERENCES
Smoot, George F. and Carter, R. W., 1968, Are individual current-
meter ratings necessary?: American Society of Civil
Engineers Journal, v. 94, no. HYZ
Smoot, George F., and Novak, Charles E., 1968, Calibration and
maintenance of vertical-axis type current meters: U.S.
Geological Survey Techniques Water-Resources Investigations,
book 8, chap. B2, 23 p.
Ernest F. Hubbard
Acting Chief, Office of Surface
Water
Attachment
WRD Distribution: A, B, S, FO, PO
_____________________________________________
CURRENT-METER LOG
Meter Type _________________ Number __________
Meter Spin
Date User Checker Time Remarks
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