Positive displacement type flow meters are generally used for accurate measurement of steady flow. These flow meters are working under the following principle.
Positive displacement meters split the flow of liquids into separate known volumes based on the physical dimensions of the meter, and count them or totalize them.
They are mechanical meters in that one or more moving parts, located in the flow stream, physically separate the fluid into increments. Energy to drive these parts is extracted from the flow stream and shows up as pressure loss between the inlet and the outlet of the meter.
The general accuracy of these meters is dependent upon minimizing clearances between the moving and stationary parts and maximizing the length of the flowing path. For this reason, accuracy tends to increase as size increases