A velocimeter which is designed to measure fluid velocities in the [[casing]]. Usually this [[tool]] is capable of passing through production tubing to make fluid velocity measurements in the casing below. The [[tool]] is held in the center of the fluid column usually by spring [[centralizer|centralizers]] and moved at a constant rate of speed against (or with) the direction of flow. The spinner speed, a linear function of fluid velocity relative to the tool, is recorded continually versus depth.

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Primarily, this tool should be used in monophasic flow regimes; i.e., injection wells and high-flow-rate gas wells or oil wells.