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8 TECHNIQUES OF WATER-RESOURCES INVESTIGATIONS Figure 10.-optical current meter. by Smoot and Novak (1968). A brief descrip- tion of the checks of the condition of a meter and the care and cleaning of it during daily field use is presented in the next few paragraphs. Before and after each discharge measure- ment, examine the meter cups or vanes, pivot and bearing, and shaft for damage, wear, or faulty alinement. Before using the meter, check its balance if on a hanger, check the alinement of the rotor axis with a hanger or wading rod, and adjust the conductor wire to prevent inter- ference with meter balance and rotor spin. Clean and oil meters daily when in use. If measurements are made in water carrying noticeable suspended sediment, clean the meter immediately after each measurement. Surfaces to be cleaned and oiled are the pivot bearing, pentagear teeth and shaft, cylindrical shaft bearing, and thrust bearing at the cap. Mter oiling, spin the rotor to make certain it operates freely. If the rotor stops abruptly, find the cause and correct the trouble before using the meter. On notes for each measurement , record the duration of spin. Obvious decrease in spin duration indicates need for attention to the bearings. The pivot needs replacement more often than other meter parts. Examine the pivot after each measurement. Replace a fractured, rough, or worn pivot. Keep the pivot and pivot bearing separated except during measurements. Use the raising nut if provided, or, for pygmy meters, replace the pivot by the brass plug. Most minor repairs can be made in the field. Repair lLttempts, however, should be limited only to minor damages. This is particularly true of the rotor because minor dents in the bucket wheel or cups can have a large influence on the meter rating. Unless minor dents in the cups can be straightened out to "like new" condition, the entire rotor should be replaced with a new one. Badly sprung yokes, bent yoke stems, misalined bearings and tailpieces, should be reconditioned in shops equipped with the specialized facilities needed. /. 1. Rating of current meters In order to determine the velocity of the water from the revolutions of the rotor of a current meter, a relation must be established between the angular velocity of the rotor and the velocity of the water turning it. The estab- lishment of this relation, known as "rating the current meter," is done for the Geological Survey by the National Bureau of Standards. Because there is rigid control in the manu- facture of the small Price meter, virtually identical meters are produced and, for all practical purposes, their rating equations are identical. Therefore there is no need to calibrate the meters individually. Instead, a standard rating is established by calibrating a large number of meters that have been constructed according to Survey specifications and this rating is then supplied with each meter. Identi- calness of meters is insured by supplying the dies and fixtures for the construction of small Price meters to the manufacturer who makes the successful bid. Meters which have been rated by means of rod suspension, and then by means of cable suspension using Columbus-type weights and hangers, have not shown significant differences in rating. Therefore, no suspension coefficients are needed, and none should be used, if weights and hangers are properly used. The current-meter rating station operated by the National Bureau of Standards in Wasbing- ton, D.C., has a sheltered reinforced concrete basin 400 feet long, 6 feet wide, and 6 feet deep. An electrically driven car rides on rails extend- ing the length of the basin. The car carries the current meter at a constant rate through the still water in the basin. Although the rate of travel can be accurately adjusted by means of a hydraulic regulating gear, the average velocity