Differential Pressure (DP) Flow

Volumetric Flow Rate

Volumetric flow rate is the volume of fluid which passes per unit of time. Common units of measure include litres/sec (l/s) or cubic feet per second (ft³/s).

Differential pressure flow meter uses a variation of Bernoulli’s principle to infer the volumetric flow rate in a pipe. For our application of the principle, it states that the velocity of the flow increases and the pressure decreases as the flow moves through a restriction in the pipe; therefore, the pressure is higher before the restriction than after. Knowing this differential pressure, the flow rate can be determined. The square root of the differential pressure across the restriction in the pipe is proportional to flow.

Mass Flow Rate

Mass flow rate is the amount of mass of a substance which passes per unit of time. Common units of measure include Kg/s or lbs/min.

This type of measurement needs more detailed input than just differential pressure. It also needs temperature, static pressure of the line, the composition of the fluid, and more information of the primary element. 

Differential Pressure Flow Meters consist of a Primary Element, a Secondary Element, and sometimes Tertiary Elements.

Primary Element: The Primary Element generates the differential pressure in the fluid. Primary Elements can include an averaging pitot tube, an integral flow orifice (IFO), or any number of DP producing devices.

Secondary Element: The Secondary Element is the pressure transmitter that is used to measure the DP. The secondary element may or may not include a flow computer with temperature and static pressure input.

Tertiary Element: The Tertiary Elements consist of flow computers, gas chromatographs, static pressure transmitters, or temperature transmitters.