A variable-area flowmeter is one where the fluid must pass through a restriction whose area increases with flow rate. This stands in contrast to flowmeters such as orifice plates and venturi tubes where the cross-sectional area of the flow element remains fixed.
The variable-area flowmeter is also a head-type flow sensor, but it does not measure the pressure drop across a fixed orifice; instead, the pressure drop is held relatively constant, and the orifice area is varied to match the flow. In gravity-type variable-area flowmeters, increasing flow lifts the float, piston, or vane, and it is the weight of these flow elements that have to be balanced by the kinetic energy of the flowing stream. These units can operate only in a vertical position. When the lifting of the float, piston, or vane is resisted by a spring instead of gravity, the meter can be installed in any position and can be configured as a pipeline spool piece.
This advantage of piping convenience has to be weighed against the fact that a spring in the process stream can be a maintenance problem and may not yield as constant a force as gravity. All variable-area flowmeters can be provided with direct local indicators. In addition, most of them can also be furnished with pneumatic, electronic, digital, or fiber optic transmission or with microprocessors for intelligent and convenient operation. A few of the designs can also be used as self-controlled flowmeters or purge meters.
Types:
- Rotameter
- Wier and flume
List of Prominent Manufacturers: Aalborg Instruments & Controls, ABB, Brooks Instrument, Dwyer, Divatec, Effast, Elettrotec, FIP, GHM Group, Kobold, Krohne, M&C Techgroup, New-Flow, Omega, Parker, Process Control Devices, Siemens, Val.Co, Voegtlin, VP Instruments, Yokogawa
Instrumentation Basics 