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.

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