Variable area flowmeters measure flow by allowing the flow stream to change the opening within the flowmeter by moving an internal part. When the flow increases, the fluid generates more force and moves the internal part farther.

Vane-style variable area flowmeters have a spring-opposed vane that moves in relation to the flow rate. Similarly, piston variable area flowmeters use a spring-opposed piston that moves in relation to the flow rate. These flowmeters can be connected to an indicator or transmitter. Vane and piston style variable area flowmeters have a direct mechanical linkage from the fluid chamber to the control box making the addition of alarm switches or transmitters very cost effective.

Vane and piston style variable area flowmeters inferentially measure the flow of liquids and gases, such as water, air, lubrication oils and chemicals. Be careful because using variable area flowmeters on dirty fluids, or fluids that can plug the flowmeter which render the flowmeter inoperable.

This flowmeter can be applied to relatively clean liquids and gases. Vane style variable area flowmeters are available in sizes up to approximately 8 inches with materials of construction that include stainless steel. This is considerably more flow that the typical variable area meters that are typically limited to a 3 inch diameter pipe.

Vane and piston variable area flowmeters are commonly used to provide local indication of liquid flows, such as for seal water or circulating lubrication oil in the mining, mineral processing, pulp and paper, petroleum, chemical, petrochemical, and waste water industries.

Do not apply variable area flowmeters to fluids that are dirty because these may render the flowmeter inoperable. Most variable are meters require vertical orientation but the vane or piston style can be mounted in any orientation.