Positive displacement meter

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A positive displacement meter is a type of flow meter that requires the fluid being measured to mechanically displace components in the meter in order for any fluid flow to occur. Positive displacement (PD) flowmeters make volumetric flow measurements taking finite increments or volumes of the fluid. A basic analogy would be holding a bucket below a tap, filling it to a set level, then quickly replacing it with another bucket and timing the rate at which the buckets are filled (or the total number of buckets for the “totalized” flow). With appropriate pressure and temperature compensation, the mass flow rate can be accurately determined. The meters come in several forms, including:

• Reciprocating piston – each piston is mechanically or magnetically operated to fill a cylinder with the fluid and then discharge the fluid. Each stroke represents a finite measurement of the fluid.
• Oval gear – two rotating oval gears with synchronized teeth “squeeze” a finite amount of fluid through the meter for each revolution.
• Nutating disk – a disk mounted on a sphere is “wobbled” about an access by the fluid flow and each rotation represents a finite amount of fluid transferred.
• Rotary vane – A rotating impeller containing two or more vanes divides the spaces between the vanes into discrete volumes and each rotation (or vane passing) is counted.
• Diaphragm – Fluid is drawn into the inlet side of an oscillating diaphragm and then dispelled to the outlet. The diaphragm oscillating cycles are counted to determine the flow rate.

Positive displacement flowmeters are very accurate and have high turndown. They can be used in very viscous, dirty and corrosive fluids and essentially require no straight runs of pipe for fluid flow stream conditioning. They are widely used in custody transfer of oils and liquid fluids (gasoline) and are applied on residential home natural gas metering. A diaphragm meter, with which most homes are equipped, is an example of a positive displacement meter. This type of meter is appealing in certain custody transfer flow applications where it is critical that the metering be functional in order for any flow to take place.

PD flowmeters are relatively expensive and produce the highest differential pressure (and subsequently greatest pressure drop head loss) of all the flowmeter types.

[edit] See also

• Water meter
• Gas meter
• Flow measurement
• Displacement Volumetric Meter