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What is a Positive Displacement Flow Meter?

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  • Written By: Paul Scott
  • Edited By: R. Halprin
  • Last Modified Date: 01 September 2016
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A positive displacement flow meter is an instrument that measures the flow rate of a fluid by allowing it to move through the meter in balanced, known increments. This action is achieved by placing a rotating, oscillating, or nutating obstruction in the path of the fluid flow which moves or displaces an exact quantity of fluid with each cycle. By reading the number of cycles the fluid flow imparts on the mechanism, exact flow volume values can be calculated. Positive displacement flow meters are available in several guises with reciprocating piston, diaphragm, and oval gear types being common examples. These meters offer several advantages over other types and are widely used as metering devices for consumer points such as domestic residences and businesses.

Flow meters are used to measure real time or totalized volumetric flow rates for liquids and gases. There are several types of flow meter, each of which has its own advantages and specific areas of use. The positive displacement flow meter is a mechanical meter type best suited for clean, non-corrosive mediums. Within those parameters, this meter is able to accurately measure a wide selection of fluids including those with high viscosities.

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The positive displacement flow meter functions by passing the fluid over a mechanism consisting of several cells or divisions of equal volume. The passing fluid turns or motivates this mechanism, thereby causing it to pass the fluid through the meter body in a series of equal portions. The rotating mechanism drives a counter that reads the number of revolutions made and, due to the known nature of each portion passed, calculates the exact volume passed in any given time. The cells or divisions which make up the mechanism of the meter are formed by impeller blades, paddle cups, diaphragm chambers, and lobed or oval gears.

This meter variant has several advantages which include high accuracy and the ability to precisely measure low flow rates. The meters also have a high turndown rate which is the total effective range of flow measurements they are able to deliver. The positive displacement flow meter also requires little pre-metering fluid flow conditioning, thereby allowing it to be placed directly after bends or junctions in the system piping. The only real disadvantage of this type of meter is its sensitivity to dirty or corrosive fluids. Their accuracy and flexibility make the positive displacement flow meter a popular choice for consumer metering in both commercial and domestic applications.

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