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Positive Displacement (PD) Meters


LC positive Displacement Metering Chamber with external pulser. Photo courtesy of Liquid Controls Group. PD meters are classified as direct measurement meters (as opposed to inferential meters). This is because they operate on the principle of volumetric flow measurement by segmenting the flowing stream into distinct portions and providing an output directly proportional to the number of portions which pass through the meter.

In a positive displacement (PD) meter, the product which is being driven through the meter is forced to drive the measuring system directly. This system is usually designed as a series of vanes, rotors or pistons which displace a given amount for each revolution. The drive train is then coupled (usually through a calibrator) to either a mechanical gear head, or more commonly to a pulser(s) which convert the rotation into a series of electrical pulses which are then fed to an electronic register which converts the pulses into a volume indication.

PD meters are available in several common designed, including the following:

PD meters are probably the most widely used meters in the petroleum maesurement fields. They have proven to be reliable and cost effective methods for measuring volumetric flow.

They do however have some weaknesses. One potentially severe limitation of PD meters is that, regardless of design, they rely on the product being measured to provide lubrication. This is adequate in several applications, but has proven to be a limiting factor in others, most notably the measuremnt of very dry products such as LPG. PD meters are also subject to variations in performance due to several other factors including meters size, product pressures, product compatability, product temperature, ambient temperature, product viscosity product contamination and, in some cases, installation orientation.


Last modified: 14 September 2008 00:01:50