Variable Area Flowmeters
About variable area flowmeters
In this class of flowmeter, the
differential pressure across a variable aperture is used to adjust the
area of the aperture. The aperture area is then a measure of the flow
rate. This type of instrument normally only gives a visual indication of
flow rate, and so is of no use in automatic control schemes. However, it
is reliable and cheap and used extensively throughout the industry. In
fact, variable area meters account for 20% of all flowmeters sold.
Recently, fiber optics have been incorporated into variable area
flowmeters, where the position of the float is detected by a row of
fibers which sense the reflection of light from the float.
In its simplest form, the instrument consists of a tapered glass tube
containing a float which takes up a stable position where its submerged
weight is balanced by the up thrust due to the differential pressure
across it. The position of the float is a measure of the effective
annular area of the flow passage and hence of the flow rate. The
accuracy of the cheapest instruments is only ±3%, but more expensive
versions offer measurement accuracies as high as ±0.2%. The normal
measurement range is between 10% and 100% of the full-scale reading for
any particular instrument.

More Flow Measurements
|