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Title: Large Capacity Displacement Meters
Author: J. L. Esola
Source: 1975 International School of Hydrocarbon Measurement
Year Published: 1975
Abstract: A Positive Displacement Meter is one in which a known volume of gas is alternately trapped and released, and the number of cycles recorded. Another term for trapping may be sealing. This principle is applied on both diaphragm type and rotary type meters. Although the operational principle is different, the fact remains that both types measure by means of sealing off a known quantity of gas, and subsequently releasing it. The bulk of the meters in use today are of the positive displacement type. Other measurement principles are applied in the case of the Turbo Meter, Orifice Meter or Swirl Meter. To illustrate the positive displacement principle, imagine a piston and cylinder with an inlet and outlet port. With the inlet port opened, the chamber will fill with gas then close at the end of the piston stroke. At this instant, we know low much gas is in the chamber. The outlet port then opens exhausting the gas. By simply counting the number of times the cylinder fills and empties, re have an effective measurement device. 3y mounting more than one cylinder, we can both increase our efficiency and the smoothness of motion contained within this device. Actually, t h is jrinciple is currently employed in certain liquid neters. For instance, when you drive to your Local service station, the meter contained within :he gasoline pump is actually a four-piston meter. The fact that there are literally thousands of :hese all over the country attests to their accuracy. However, applying this principle to gas neasurement presents some problems. These are mlsation, high operating differentials, and sealing difficulties which could cause leakage.

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