56 Field Instrumentation GEN Rev)
3.3 Positive displacement meters Positive displacement meters are measurement devices that trap a known volume of fluid and allow it to pass from meter inlet to outlet. Then the numbers of trapped volumes passing through the meter are counted to obtain the total flow. Displacement means that the fluid flowing through the meter replaces displaces) the volume of fluid that passed through the meter immediately before. Piston meters (reciprocating and oscillating) are normally used as positive displacement meters. They are accurate, precise, and have a wide flow range. These are ideal for measuring low rates of flow.
These are expensive, susceptible to corrosion, and can be damaged by excessive flow.
Metering Pumps Lobbed impeller flow meter Its two lobes (rotors) are mounted on parallel shafts and rotate in opposite directions. Fluid enters the inlet and fills the space between lobes and case. The pressure of the fluid against the lobes causes the lobes to rotate as fluid moves through the meter. A revolution counter on the shaft of one lobe determines total flow.
To determine the flow rate, multiply the revolutions per minute (rpm) of the shaft by the volume per revolution.
57 Field Instrumentation GEN Rev) Lobbed impeller flow meter These meters are normally used to measure gases but can be used for clean liquids.
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