Velocity flowmeters give a direct measure of fluid velocity, the characteristic most commonly associated with flow rate.
Positive displacement flowmeters measure flow rate through the continuous filling and emptying of a chamber of known volume.
Parshall flumes are one of the most widely used flumes for permanent flow monitoring.
Mass flowmeters measure flow in one of two ways: inferentially or directly. Inferentially means density and flow rate are measured independently. Coriolis and thermal meters, shown below, are both direct mass flowmeters.
Differential pressure flowmeters are a type of inferential flowmeter where the flow rate is calculated from a non-flow measurement.