Positive displacement type flow meters divide the fluid in measured portions and then pump it. The flow can be calculated by multiplying the number of strokes with volume of fluid pumped in each stroke. These flow meters are accurate and deliver precise quantities of fluid.
There are different types of flow meter which work on this principle of operation like oval gears, mutating disc, reciprocating pistons etc.
Oval Gear type flow meter
In oval gear type flow meters, two oval shaped synchronized gears are fixed such that their teeth are in perfect contact. As the shaft moves, a specific amount of fluid gets trapped between gears and the casing of the meter and is pumped with every stroke of the gears.
This ensures that only a fixed amount of fluid is pumped in each stroke. The flow can be calculated by multiplying the volume contained in each stroke by number of strokes (which can be counted by monitoring shaft rotations).
The other types implement different mechanisms such as reciprocating pistons, mutating disc etc. but the basic principle of measurement remains the same.