Mass flow meter operating on the Coriolis principle.

The process connection of the flowmeter is split internally into two identical v-shaped tubes, which are actuated at resonance frequency in opposite direction.

When fluid flows through the tubes, the input legs of the v-shape will vibrate with the same frequency as the output legs, but they are behind in time. This difference in time, called phase shift, is a measure of the mass flow through the tubes.
The magnitude of this phase shift is a measure of the mass flow rate, whilst the frequency of the complete system is a measure of the density of the fluid.

The mass flow rate is the mass of the fluid per unit of time. When the mass flow rate is divided by the measured density, a volumetric flow rate can be obtained.