Differential pressure flowmeters are a type of inferential flowmeter where the flow rate is calculated from a non-flow measurement.
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.
Parshall flumes are one of the most widely used flumes for permanent flow monitoring.
Positive displacement flowmeters measure flow rate through the continuous filling and emptying of a chamber of known volume.
Velocity flowmeters give a direct measure of fluid velocity, the characteristic most commonly associated with flow rate.