Video:How to Measure the Flow Rate Using a Variable Area Flowmeter (3mins) - Summary available at bottom of page


Variable area flowmeters, sometimes called VAFs or rotameters, are simple devices that can be put in systems to measure the flow of fluid. They must be mounted vertically – if they are mounted horizontally, they will not work. VAFs simply rely on the water flow flowing around a weighted cone – the faster the velocity of the water, the higher it pushes the cone up the flowmeter. The flow is then measured using the scale on the VAF – you must use the top of the weighted cone as the indication of what measurement you should take from the scale. As the flow is increased, the weight will lift higher, and as the flow decreases, the weight will drop lower.

A VAF can have two solenoid switches, which supply it with either one switching point or two switching points. You can choose to have it switching when the flow goes up, or have it switching when the flow goes down, and you can link it in to your control system to either set alarms, to turn another pump on, or to adjust a valve automatically. As the top of the weight passes by the centre of the switch, it'll switch the system. The switches can be used to give feedback to a PLC control system, or to a speed controller on a pump, or used as an alarm to make sure that the flow doesn't get too high or too low.

When mounting flowmeters, do not put an elbow underneath - if you do, the flow will be very turbulent as it goes through the elbow and will cause the weight inside the VAF to wobble, as it will create a faster velocity on one side than the other, therefore providing an inaccurate reading. Ideally always have at least 5 or 6 times the diameter of the pipe in a straight length before your flowmeter - this gives the flow time to even out and provide a much more accurate reading on your flowmeter.