A [[flowmeter]] with retractable impeller blades which can be used below the bottom of tubing where the impeller blades open to almost full inside diameter of the casing.

The measurement made is related to the velocity of the fluid relative to the [[tool]], which in turn is related to the volumetric flow rates. In polyphasic flow, however, much higher threshold flow rates are needed for useful measurements. A continuous [[log]] can be recorded with or against the flow of fluid.