A differential pressure flow measurement system consists of a differential pressure primary flow element and a differential pressure flow transmitter.

When the flow of a fluid in a pipe passes a restriction in the piping system, the pressure in the piping system is reduced. Most differential pressure primary flow elements are designed, constructed and operated in a manner such that the flow rate is proportional to the square root of the pressure drop across the restriction. These differential pressure primary flow elements include orifice plates, Venturi tubes, elbows, flow nozzles, low loss flow tubes, single-port and multiple-port Pitot tubes, segmental wedge and V-Cone flowmeters.

Some differential pressure primary flow elements, such as critical flow elements and laminar flow elements, do not follow this (squared) relationship.

This relationship can limit the ability of differential pressure flowmeter technology to measure large flow ranges. In the table, a “reasonable” flow measurement range of 10-100 flow units (10:1 flow turndown) would require a differential pressure flow transmitter range of 1-100 differential pressure units (100:1 differential pressure turndown). Therefore, the “reasonable” 10:1 flow turndown requires a 100:1 differential pressure flow transmitter turndown.

Because many differential pressure flow transmitters measured accurately with an approximate 10:1 differential pressure turndown, differential pressure flowmeter technology was often considered accurate from approximately 30-100 flow units. Improved performance of differential pressure flow transmitters has increased the differential pressure turndown, so somewhat larger flow turndowns may be possible.

The upstream and downstream pressures associated with a differential pressure primary flow element are available at the taps of the element. Both of these taps are piped to ports on the differential pressure flow transmitter that measures the pressure drop. The differential pressure flow transmitter is a device that converts the differential pressure across its ports into an analog signal. When the differential pressure flow transmitter has an integral square root function, its output signal can be linear with flow rate.

Some (multivariable) differential pressure flow transmitters can make multiple measurements, such as differential pressure, pressure and/or temperature, from which the flow rate can be calculated.