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APPENDIX B VELOCITY, DEPTH AND DISCHARGE MEASUREMENTS Velocity measurements are made to deters tion across a transect, and are also used in discharge. Depth measurements are also used discharge; and on rivers where surveying the tical, depth measurements can be substituted the cross-sectional profi 1 e.. EQUIPMENT nine the velocity distribu- the calculation of the in the calculation of the cross section is imprac- for rod readings to obtain Current Meters The most commonly used instrument for measuring the velocity is a current meter. The meter consists of a wheel which rotates in flowing water and a device for determining the number of revolutions. Figure B- 1 shows a line drawing of a Price AA current meter, which is fairly typical of all "vane-type" meters. As the bucket wheel (21) rotates, an electrical contact is closed on either a single-contact cam, or a penta gear (5). If a headset or counter is attached.to the single contact post (4), a signal is produced each time the bucket wheel completes a revolu- tion. If the headset is connected to the penta-contact post (5), a signal is produced once every five revolutions. The penta-contact is very useful in fast water. Figure B-1. Line drawing of a Price AA current meter. The velocity at the point of the current meter is measured by counting the number of signals (revolutions in a specified time inter- val). Thus, a standard piece of equipment accompanying the use of a current meter is a stopwatch. Each meter is calibrated by the supplier and an equation for the relationship between velocity and revolutions per unit time derived. For most Price meters, the meter is supplied with a rating table, such as the one shown in Figure 8-2, which shows the velocity for a given number of revolutions in a given time interval. From Figure B-2, 40 revolutions in 40 seconds equals a velocity of 2.17 91