This type of flow meter trap particular amount of liquid which is directly proportional to flow rate which is calculated. A positive displacement meter can be constructed in several ways.
OVAL – GEAR METERS :
Oval – Gear meter have two rotating oval – shaped gears with synchronized, close fitting teeth. A fixed quantity of liquid passes through the meter for each revolution. Shaft rotation can be monitored to obtain specific flow rate.
NUTATING DISC METERS :
Nutating Disc Meters have movable disks mounted on a concentric sphere in spherical side – walled chambers. The pressure of the liquid passing through the measuring chamber causes the disk to rock in a circulating path without rotating about its own axis. It is the only moving part in the measuring chamber.
RECIPROCATING PISTON METERS :
Piston meters have a piston that oscillates back and forth as fluid flows through the meter. A known volume of water is measured for each rotation, and the motion is transmitted to a register through an arrangement of magnetic drive and gear assembly.