CHAPTER 10 - CURRENT METERS

10. General Procedures and Precautions

Accuracy of measurement can be maintained by observing the following precautions for Price meters (including the pygmy meter modification of the Price meter):

• The meter should be spin tested before and after completing measurements to assure that the meter has no error-causing damage. With the shaft in a vertical position and the cups protected from air currents, the cups should be given a quick turn to start them spinning. If the meter is in proper adjustment and the bearings are free from foreign particles, the cups should come to rest in not less than 3 minutes. If the length of spin is only about 12 minutes, but the cup wheel comes to rest gradually, all flows except those of very low velocities may be measured. If the length of spin is only about 1 minute but the cup wheel comes to rest gradually, the meter may still be used to measure velocities above 1 ft/s. If the length of spin is less than 1 minute, the meter should be reconditioned. Under laboratory controlled conditions, rotation should continue for about 4 minutes. The manner in which rotation ceases will help indicate the condition of the meter and should be observed.
• The cross section of the stream should be divided vertically into 20 or more segments. Very small streams and sections with smooth, firm boundaries are exceptions, and a smaller number of stream cross-section segments would be sufficient. A single vertical reading is used if the distance between verticals is less than 1 ft. Horizontal divisions are generally selected so not more than 10 percent, and preferably not more than 5 percent, of the discharge will occur between any two adjacent verticals.
• The stopwatch should be checked frequently and kept in good condition.
• For low and irregular velocities, the period of observation should be lengthened to obtain a more accurate average count.
• The current meter should be withdrawn from the water between velocity readings to make sure that rotation is not being impeded by debris or any other cause.
• The meter should be allowed at least 10 to 20 seconds to attain rotation speed before counting commences.
• The total operation of the meter at each elevation of a vertical should consist of at least two consecutive periods of at least 40 seconds. If significant differences are apparent in each period, more readings should be taken.
• Measurements while wading should be done facing the bank, standing just downstream from the tag line, and at least 18 in to the side of the meter.