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.