Because the float responds to gravity, the meter must be mounted in a vertical position, thus posing application limits on horizontal piping. General purpose models with ball floats have strong sensitivity to viscosity, so precision correlation for various fluids is not possible. Piston Type Flowmeter Piston type variable area flowmeters address several of these limitations. To minimize viscosity sensitivity, a sharp-edged orifice is used between the piston and a tapered metering cone. Flow through the meter creates a pressure differential proportional to the rate of flow which moves the piston along the tapered metering cone.
Different methods exist to transfer piston position into some form of readout.
One method incorporates a ring type permanent magnet into the piston assembly. An external indication ring is magnetically coupled to the piston and indicates the flow rate on a scale. Because spring return, piston type meters do not rely on gravity to balance float position against fluid flow, they are not limited to a vertical mounting piston. As a result, they can be installed directly into vertical and horizontal lines. Some units can monitor flow in a downward gravity feed line with an optional inverted flow scale.