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The rotameter, illustrated in Figure A1, is
an area flow meter so named because a rotating float is the
indicating element.
Figure A1
The rotameter consists of a metal float and a
conical glass tube, constructed such that the diameter
increases with height. When there is no fluid passing through
the rotameter, the float rests at the bottom of the tube. As
fluid enters the tube, the higher density of the float will
cause the float to remain on the bottom. The space between
the float and the tube allows for flow past the float. As
flow increases in the tube, the pressure drop increases. When
the pressure drop is sufficient, the float will rise to
indicate the amount of flow. The higher the flow rate the
greater the pressure drop. The higher the pressure drop the
farther up the tube the float rises.
The float should stay at a constant position
at a constant flow rate. With a smooth float, fluctuations
appear even when flow is constant. By using a float with
slanted slots cut in the head, the float maintains a constant
position with respect to flow rate. This type of flow meter
is usually used to measure low flow rates.
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