PROGRAMS AND PLANS--Approval of Standard Rating for OAA Current Meter (Price AA Meter with Metal Bucket Wheel and Optical Head)

In Reply Refer To:
WGS-Mail Stop 415                        October 12, 1990


SUBJECT:  PROGRAMS AND PLANS--Approval of Standard Rating for OAA
                              Current Meter (Price AA Meter with
                              Metal Bucket Wheel and Optical Head)

This memorandum defines a standard rating for the OAA current
meter and releases the meter for general field use with the stan-
dard rating.  The OAA current meter has a Price AA metal bucket
wheel and yoke and a fiber-optic counting head.  A supply of
optical counting heads was procured in 1989 for retrofitting on
standard AA-meter yokes.  A random sample of 50 meters was
retrofitted with OAA heads, and calibration tests were performed
in February-May 1990 at the U.S. Geological Survey tow tank at the
Stennis Space Center, Mississippi.  The calibration data were
analyzed between June and August 1990.

The standard rating for the OAA meter is defined by the equation

V = 0.014 + 2.194 R     (R < 0.856)
    0.041 + 2.162 R     (R > 0.856)

where V is water velocity in feet per second (ft/s) and R is rotor
speed in revolutions per second.  Stall velocity is about 0.08
ft/s.  This rating is defined by calibration data on 50 meters at
12 standard rating velocities between 0.10 and 12 ft/s.

The calibration data show root-mean-square (RMS) deviations of
about 0.5 percent between individual measured velocities and the
standard rating for velocities greater than or equal to 0.5 ft/s.
RMS deviations are less than about 2 percent for velocities
between 0.15 and 0.5 ft/s and are about 5 percent at 0.10 ft/s.
These RMS deviations are about half as great as those commonly
accepted for AA meters with cat-whisker heads.

The RMS deviation represents the magnitude of error that would be
expected in a single velocity measurement using a randomly
selected meter.  It is a mean or average value.  About one-third
of the calibration data have absolute-value deviations larger than
the RMS value.  About one observation in 20 has an absolute devia-
tion greater than twice the RMS value.

Examination of repeatability data suggests that all velocity
measurements made with a particular meter during a single
discharge measurement will have highly correlated errors.  Thus,
the RMS errors quoted for single velocity measurement are applica-
ble also to mean velocities for discharge measurements.

Adoption of a standard rating for the OAA meter does not imply
that the standard rating should be accepted for an individual
meter that consistently deviates significantly from the standard
rating.  If an OAA meter appears to be giving questionable
results, it first should be visually inspected and checked for a
satisfactory spin test.  Spin time for a properly adjusted OAA
meter normally will be 4 1/2 to 5 minutes and under no circum-
stances should be less than 2 minutes.  The rotor should move
freely and come to a smooth gliding stop.  Bucket shape and rotor
and yoke alignment should be checked and minor adjustments made,
if necessary, by properly trained and qualified personnel.  If
major adjustments are needed or if the meter continues to give
questionable results, the complete meter and a memorandum explain-
ing the circumstances should be sent to the Hydrologic Instrument
Facility, Field Service and Supply Section, John C. Stennis Space
Center, Mississippi, for inspection and possible repair or indi-
vidual rating.  Copies of the memorandum should be directed to
Office of Surface Water (OSW) Hydraulic Lab, John C. Stennis Space
Center, Mississippi, and the Chief, OSW, Reston, Virginia.  Any
information thus obtained may be useful in assessing the accuracy
of OAA meters.

                               Charles W. Boning
                               Chief, Office of Surface Water