From Flow Measurement

Variable area flowmeters were invented in the nineteenth century but were not
introduced for general industrial use until the 1930s. They became very popular
and widely used since they offered a low-cost alternative to the differential pressure
flowmeter-especially in smaller pipe sizes.

The original designs introduced were available for visual indication of flow rate
only. In the 1940s designs became available with secondary functions such as
transmission, alarm, recording, and totalizing. Variable area flowmeters are available
in pipe sizes from 1/8 inch (0.3 cm) to 8 inches (20 cm) and full scale flow
rates from less than one ml/minute to over 1000 gpm (3800 lpm) of water and
approximately 30 scc/minute to 2500 scfm (71 nm3/min) of air.

Since they are in-line devices, they have been designed for a wide range of line
pressure ratings, pipe connection types, and many different materials of construction.

Operating Principle

The variable area flowmeter is, in essence, a special type of differential pressure
flowmeter. On differential pressure flowmeters, the area of the opening is
fixed and the flow rate is measured as a function of differential pressure across the
opening. In the variable area flowmeter, the differential pressure across the opening
is...


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© 2001 ISA

Products & Services
Flow Controllers
Flow controllers monitor and maintain flow-rate variables, typically in process applications.
Gas Flow Meters
Gas flow meters are used for measuring the flow or quantity of a moving gas in an enclosed pipe or passage.
Liquid Flow Meters
Liquid flow meters are used to measure the volumetric flow rate or amount of a moving liquid.
Volumetric Gas Flow Switches
Volumetric gas flow switches are devices with a switch output used for measuring the flow or quantity of a moving gas in terms of a unit of volume per unit time, such as cubic feet per minute.
Volumetric Liquid Flow Switches
Volumetric liquid flow switches measure the flow or quantity of a moving fluid in terms of a unit of volume per time, such as liters per minute. These flow sensing devices have a switch output.

Topics of Interest

Most flow measurements are made by meters that are installed in-line such that all of the fluid passes through the meter body. Insertion (sampling) flow measurement is a different technique because...

Next time you want to measure flow, think of the rotameter first. This simple and economical means of flow control, with local readout, for a wide variety of gases and liquids of low-to-moderate...

With the many flowmeters available today, choosing the most appropriate one for a given application can be difficult. This article discusses six popular flowmeter technologies, in terms of the major...

A large number of many types of flowmeters are available today. The most commonly used is the variable differential pressure type, which includes "the orifice meter," an engineering expression for the...

The selection of six types of flowmeters was previously discussed here (previous article, pp 176- 182, in this book). These were: variable-area, mass, Coriolis, differential-pressure, turbine, and...