PCWA-L 467.pdf - PCWA Middle Fork American River Project ...

94 MEASUREMENT OF STAGE AND DISCHARGE
should be reconditioned in shops equipped with the specialized
facilities needed.
There are only few details connected with care of the optical
current meter. The meter should be transported in a shock-proof carrying
case and the battery should be checked periodically. Field performance
of the tachometer should also be checked periodically.
Three steps are involved in the checking process. First, a cam speed is
measured by counting and timing mirror oscillations with a stopwatch,
and the corresponding dial reading of the tachometer is observed.
Next, a tachometer dial readout is computed from the measured
cam speed and the known scale factor of the tachometer diaL In
the final step the observed dial reading and the computed dial readout
are compared.
RATING OF CURRENT METERS
To determine the velocity of the water from the revolutions of the
rotor ofa conventional current meter, a relation must he established
between the angular velocity ofthe rotor and the velocity ofthe water
that spins the rotor. That relation is known as the rating of the
current meter. The rating is established by first towing the meter at a
constant velocity through a long water-filled trough, and then relating
the linear and rotational velocities of the current meter. The
following paragraphs describe the rating of meters by the U.s.
Geological Survey at the Hydrologic Instrumentation Facility in
Mississippi.
The rating trough used is a sheltered concrete tank 450 ft (137 m)
long, 12 ft (3.7 m) wide, and 12 ft (3.7 m) deep. An electrically driven
car rides on rails extending the length ofthe tank. The car carries the
current meter at a constant rate through the still water in the basin.
Although the rate oftravel can be accurately adjusted by means ofan
electronic regulating gear, the average velocity of the moving car is
determined for each run by making an independent measurement of
the distance it travels during the time that the revolutions of the
rotor are electrically counted. Eight pairs of runs are usually made
for each current meter. A pair of runs consists of two traverses of the
basin, one in each direction, at the same speed. Practical considerations
usually limit the ratings to velocities ranging from 0.1 to
about 15 ftls (0.03 to about 4.6 mls), although the car can be operated
at lower speeds. Unless a special request is made for a more extensive
rating, the lowest velocity used in the rating is about 0.2 ftls (0.06
mls), and the highest is about 8.0 ftls (2.5 mls).
For convenience in field use, the data from the current-meter ratings
are reproduced in tables, a sample ofwhich is shown in figure 51.