Piston flow meters are ideal for low viscosity fluids due to the tight tolerances & fits they use. However, due to these tight fits, clean fluid must be used (or filters put in place) as a particle getting trapped can actually block flow. They operate by a piston moving within a chamber of known volume. When fluid passes into this chamber, the piston displaces an exact volume of liquid. Piston meters are great for low volume flow measurements.

Animation Credits : hytekflowmeters.com