Positive Displacement (PD) - PD flowmeters measure the volumetric flow rate of a liquid or gas by separating the flow stream into known volumes and counting them over time. Vanes, gears, pistons, or diaphragms are used to separate the fluid. PD flowmeters provide good to excellent accuracy and are one of only a few technologies that can be used to measure viscous liquids.
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BM Oval Gear positive displacement meter
The Brooks models BM04, BM07, BM10, BM40 and BM50 are positive displacement oval gear flowmeters designed for applications requiring high accuracy measurement of clean liquids with viscosities less than 1000 cP standard or up to 1 million cP with rotors (gears) cut for high viscosity. All meters are available with a pulse output from a reed switch or solid state Hall effect switch for remote registration and/or totalization. Two optional LCD displays offer rate, resetable total, non-resetable total and the option for simple batch.
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The Sliding Vane Meter - Sliding vanes rotate in contact with the meter casing.

The Oval Gear Meter - Oval gears , intermeshed and sliding around in close proximity to a fixed casing. The Piston Displacement Meter - Reciprocating Pistons interconnected with a common crankshaft displacing fluid through a unidirectional valve arrangement.

The Rotary Piston Meter - A rotating piston displacing fluid by moving eccentrically around the inside of the meter casing between inlet and outlet ports, the piston being a sliding fit around the inside of the meter casing.

The Nutating Disc Meter - A nutating disc moving in close contact with a machined casing is caused to displace a fixed volume per revolution by the fluid passage.

The Bi-Rotor Meter - Two Rotors geared to move with a small clearance around the meter casing , containing machined recesses to pass a fixed volume of fluid per revolution.

The Helix Flowmeter - Two radially -pitched helical rotors geared to move together with a small clearance between themselves and the casing . The two rotors when running together thus displace fluid axially from one end of the chamber to the other.