How Differential Pressure Flow Meters Work
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Differential Pressure Flow Meters How Differential Pressure Flow Meters Work Differential pressure flow meters use Bernoulli's equation to measure the flow of fluid in a pipe. Differential pressure flow meters introduce a constriction in the pipe that creates a pressure drop across the flow meter. When the flow increases, more pressure drop is created. Impulse piping route the upstream and downstream pressures of the flow meter to the transmitter that measures the differential pressure to determine the fluid flow. Bernoulli’s equation states that the pressure drop across the constriction is proportional to the square of the flow rate. Using this relationship, 10 percent of full scale flow produces only 1 percent of the full scale differential pressure. At 10 percent of full scale flow, the differential pressure flow meter accuracy is dependent upon the transmitter being accurate over a 100:1 range of differential pressure. Differential pressure transmitter accuracy is typically degraded at low differential pressures in its range, so flow meter accuracy can be similarly degraded. Therefore, this non-linear relationship can have a detrimental effect on the accuracy and turndown of differential pressure flow meters. Remember that of interest is the accuracy of the flow measurement system --- not the accuracy of the differential pressure transmitter. Different geometries are used for different measurements, including the orifice plate, flow nozzle, laminar flow element, low-loss flow tube, segmental wedge, V-cone, and Venturi tube. |