Difference between Fixed Area & Variable Area FlowMeters

With orifice plates flowmeters:

• As the rate of flow increases, so does the differential pressure.

• By measuring this pressure difference it is possible to calculate the flowrate through the flowmeter.

• The pass area (for example, the size of the hole in the orifice plate) remains constant.

With any type of variable area flowmeter:

• The differential pressure remains almost constant as the flowrate varies.

• Flowrate is determine from the position of the float.

• The pass area (the area between the float and the tube) through which the flow passes increases with increasing flow.

Source : spiraxsarco