Variable area meter
Encyclopedia
A variable area meter is a meter that measures fluid flow by allowing the cross sectional area of the device to vary in response to the flow, causing some measurable effect that indicates the rate.

A rotameter
Rotameter
A rotameter is a device that measures the flow rate of liquid or gas in a closed tube.It belongs to a class of meters called variable area meters, which measure flow rate by allowing the cross-sectional area the fluid travels through to vary, causing some measurable effect.-History:The first...

is an example of a variable area meter.
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