A flowmeter is a device that measures the rate of flow of a liquid or a gas. The commonest flowmeter is called an orifice meter.
   An orifice meter is an obstruction placed in the pipe through which the liquid or gas is passing. As the liquid or gas passes the orifice, its speed is temporarily changed; as a result the pressure it exerts on the walls of the pipe is also changed. By measuring this pressure at, and on both sides of, the orifice, the rate of flow may be computed.
   Other flowmeters include the shunt meter, which directs part of the flow past a propeller, and the rotameter.