A [[flowmeter]]. The downhole instrument consists of an impeller, inside a protective cage, which is caused to rotate by the motion of borehole fluid past the blades. An alternating current, or the frequency of pulses, constitutes the [[signal]] sent to the surface and is related to the rate of fluid flow pass the impeller. A [[survey]] is made by moving the [[tool]] against the flow of fluid, with the flow, or maintained stationary in the hole. Its primary use is in monophasic flow streams.

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