In this meter, a weighted float or plummet contained in an upright tapered tube, is lifted to the position of equilibrium between the downward force of the plummet and the upward force of the fluid in addition to the buoyancy effect of the fluid flowing past the float through the annular orifice. The flow rate can be read by observing the position of the float.
Piston Type Meter.
In this meter, a piston is accurately fitted inside a sleeve and is lifted by fluid pressure until sufficient post area in the sleeve is uncovered to permit the passage of the flow. The flow is indicated by the position of the piston.
Fig. shows the types of Variable area flow meter (a) Rotameter and (b) Piston Type meter.