Positive Displacement Flowmeters

Oval Gear Flowmeter
(Macnaught)

Positive displacement flowmeters use fluid to mechanically move internal components such as pistons, gears and discs to measure flow.  These devices are both precise and simple to operate. Unlike other liquid flow measurement technologies that rely on the measurement of a physical property of flowing media to produce a volumetric or mass flow measurement, a positive displacement flowmeter provides a direct indication of actual volumetric flow rate. There are a number of different positive displacement flowmeter designs in use throughout industrial and commercial installations:

• Oscillating piston
• Oval Gear
• Nutating disk
• Rotary vane
• Diaphragm 

Oscillating Piston Flowmeter
(Niagara Meter)

Each of the designs, and any others that would be classified as positive displacement, contain a mechanical structure through which the fluid must travel on its path from source to target. The fluid motion drives the mechanical assembly, which contains pathways of known volume. As the fluid motion drives the positive displacement flowmeter assembly, its rotational, oscillating, or other regular movement is counted, often by electronic means using magnetic pickups on moving assembly. The counts can be used to indicate current flow rate, or totalized to measure total flow volume. Additional inputs about fluid properties can be used to calculate mass flow, as well.

Positive displacement flowmeters can be applied to liquid or gaseous media, with the selection of the mechanical internals being a significant factor in the suitability of a design for a particular application. The longstanding use of positive displacement flowmeters across various industries has been a source of stability in terms of design, with the most recent advancements in positive displacement technology focusing on maintaining precision at lower costs.

Nutating Disc Flowmeter
(Niagara Meter)

There are a few known limitations for the use of positive displacement meters. The meters are not the optimal choice for measuring fluids with large particles, and are also non-ideal for measuring fluids with large air pockets. Additionally, systems using positive displacement meters need to account for slight pressure drops in the positive displacement meter.

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