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. Therefore, some sections of this article do not apply to these technologies.
As mentioned above, the flow rate through a differential pressure primary flow element is proportional to the square root of the pressure drop across the restriction. The following table illustrates this relationship.
Flow Rate Pressure Drop
100 flow units 100 differential pressure units
50 flow units 25 differential pressure units
31.6 flow units 10 differential pressure units
25 flow units 6.25 differential pressure units
10 flow units 1.00 differential pressure units
This relationship can limit the ability of differential pressure flowmeter technology to measure large flow ranges. In the table above, 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.
Excerpted from The Consumer Guide to Differential Pressure Flow Transmitters