variable area DFM Variable Area Low cost Flow Meter
NFX Glass Tube Low cost Flow Meter
Uniflux flow meters Uniflux Flow Meters Variable Area Flow Meter

Rotameters, otherwise known as Variable Area flow meters are very simple in design, ‘a float in a tube’. 

More precisely, the flowmeter consists of an internally tapered plastic or glass tube with a scale engraved along its length.  Inside the tube sits a tapered float.  The density of the float must always be greater than the density of the metered medium, be it a liquid or a gas.  In most cases the Variable Area, or V/A meter, must be installed vertically and look a little like household mercury thermometer.

In a ‘no flow’ situation the float sits at the base of the tube blocking the inlet port.  As flow begins the float will be lifted by the force of the flowing medium.  The higher the flow the higher the float is pushed up the inside of the tube.  Flow rate can then be read by observing where the top edge of the float meets the graduated scale marked on the outside of the tube.

Rotameters do not offer high accuracy and a typical accuracy of 2 to 3% FSD is probably the best that can be achieved.  However, they are low cost devices and the perfect solution for many flow applications.  With only one moving part Rotameters are maintenance-free and provided they are well looked after will operate indefinitely without requiring any maintenance.  Accuracy may be an issue but repeatability is fine.  The flowmeter will indicate the same flow rate day after day for any given set of working conditions.

Whilst Rotameters are the simplest of devices they can be fitted with a number of accessories.  Perhaps the most common is a Limit Switch.  In such cases the float has a magnetic core and the Limit Switch usually takes the form of a bi-stable Reed Switch which can be slid up and down the tube and which is actuated as the float moves past it.  The flowmeter can also be fitted with a 4-20mA transmitter, ATEX or Safe Area, and an integral Needle Valve which can be used to regulate flow through the flowmeter.

When sizing Rotameters/Variable Area flow meters it is critical to know the density and viscosity of the liquid or the working pressure and name of the gas.  This is because these factors have an effect on the buoyancy of the float inside the flowmeter and will need to be taken into account when drawing the scale which runs up the outside of the tube of the flowmeter.  For this reason a V/A flowmeter can only be supplied for a given set of working conditions.  If viscosity and/or SG in the case of a liquid or working pressure or density in the case of a gas change from the information supplied at the time the flowmeter was ordered then accuracy will suffer as a result.

Metal Tube Variable Area flowmeters are used in cases of higher working pressures or temperature, harsh working environments or where higher flow rates are required.  Whereas a plastic or glass tube V/A meter would typically work at pressures of a few barg and up to 60 degs. C. a metal tube V/A flowmeter will operate up to 40 barg and at temperatures up to 150 degs. C..  The flowmeter has a circular scaled display with a magnetically coupled pointer which shows instantaneous flow rate.

Flowquip's Rotameters

Flowquip’s range of Rotameters include the Uniflux Series designed for low flows (1 cc/min to 4.4 l/min for water or 5cc/min to 100 l/min for air).  This glass tube flowmeter has an integral Needle Valve option.

The DFM range of low cost plastic tube V/A meters are suitable for 3 to 50000 l/hr for water and 0.2 to 1500 NCMH for air).  This flowmeter can be provided with Limit Switch, 4-20mA Transmitter and integral regulating valve.

The IMT range of metal tube V/A flowmeters are suitable for 5 l/min to 50 m3/hr for water and 0.15 to 1500 NCMH for air.  This flowmeter can be provided with Limit Switch or 4-20mA Transmitter

For more information on Flowquip's Rotameters, call us on 01422 829920 or email