The Tri-Rotor type Positive Displacement Flow Meter, commonly called "PD Meter", measures the volume flow rate of a continuous flow stream by momentarily entrapping a discreet segment of the fluid into a chamber of precisely known volume and releasing that fluid back into the flow stream on the discharge side of the meter. This design has tree rotating parts that entrap fluid into the mentioned chamber which is formed between the rotors and an outer wall.
By monitoring the number of entrapments for a known period of time or number of entrapments per unit time, the total volume of flow or the flow rate of the stream can be ascertained.
PD meters are precision instruments whose internal moving components are mass-balanced yet remain hydraulically imbalanced. The result is that the meter can measure very low flows of both liquids and gases without using external power. The PD meter derives the power necessary to work from the energy contained in the flow process.
As illustrated in the pictures below each moving chamber of fluid is separated from the next chamber by a capillary seal, the integrity of which is a function of the precision to which the meter is manufactured. This high precision allows these meters to be almost universally accepted as transfer standards when properly installed and flow-calibrated.
Inside the Tri-Rotor Type PD Meter
Good instrumentation practice requires a filtering mechanism and a capacitance vessel for air removal for liquids or a coalescing filter for gases is an integral part of the metering installation.
Advances in technology allow the meters to be temperature-compensated and interfaced electronically with central control systems as easily as they can be the integral part of a truck-mounted delivery system. The total volume and the flow rate can then be displayed locally or transmitted to a remote monitoring station.
The Advantages of Meters Control PD Meters are the high quality, the high accuracy and the wide range of usage. Furthermore they are very reliable, insensitive to inlet flow profile distortions with long service time period due to lack of points of friction. Also they have low pressure drop across the meter.
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