principle: The principle of measurement is that as the liquid flows through the meter it moves a measuring element which seals off the measuring chamber into a series of measuring compartments each holding a definite volume. As the measuring element moves, these compartments are successively filled and emptied. Thus for each complete of the measuring element a fixed quantity of liquid is permitted to pass from the inlet to the outlet of the meter. The seal between measuring element and the measuring chamber is provided by a film of measured liquid. The number of cycle of the measuring element is indicated by means of a pointer moving over the dial, a digital totalizer or some other form of register, driven from the measuring element through an adjustable gearing.
The most common forms of positive displacement meters are :
1. Reciprocating Piston type.
2. Rotating or Oscillating Piston type.
3. Nutating Disc type.
4. Fluted Spiral Rotor type.
5. Sliding vane type.
6. Rotating vane type.
7. Oval Gear type.