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