Differential pressure flowmeters use Bernoulli’s equation to measure the flow of fluid in a pipe. Differential pressure flowmeters introduce a constriction in the pipe that creates a pressure drop across the flowmeter. When the flow increases, more pressure drop is created. Impulse piping routes the upstream and downstream pressures of the flowmeter to the transmitter that measures the differential pressure to determine the fluid flow. This technology accounts for about 21% of the world market for flowmeters.

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 flowmeter 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 flowmeter accuracy can be similarly degraded. Therefore, this non-linear relationship can have a detrimental effect on the accuracy and turndown of differential pressure flowmeters. 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.

Plusses and Minuses

The upside of this technology is low cost, multiple versions can be optimized for different fluids and goals, are approved for custody transfer (though it is being used less and less for this), it is a well understood way to measure flow, and it can be paired up with temperature/pressure sensors to provide mass flow for steam and other gasses. Negatives are that rangeability is not good due to a non-linear differential pressure signal (laminar flow elements excepted), accuracy is not the best and can deteriorate with wear and clogging.