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1.1 This test method covers the measurement of flow rate of water in open channels, streams, and closed conduits with a free water surface.

1.2 The test method covers the use of acoustic transmissions to measure the average water velocity along a line between one or more opposing sets of transducers—by the time difference or frequency difference techniques.

1.3 The values stated in SI units are to be regarded as the standard.

1.4 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use. Specific precautionary statements are given in Section 6.

5.1 This test method is used where high accuracy of velocity or continuous discharge measurement over a long period of time is required and other test methods of measurement are not feasible due to low velocities in the channel, variable stage-discharge relations, complex stage-discharge relations, or the presence of marine traffic. It has the additional advantages of requiring no moving parts, introducing no head loss, and providing virtually instantaneous readings (1 to 100 readings per second).

5.2 The test method may require a relatively large amount of site work and survey effort and is therefore most suitable for permanent or semi-permanent installations.