E-Zine April 2007
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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. These transmitters are outside the scope of this report.
In general, wetted parts (such as diaphragms) that are in contact with the fluid provide movement or force that is related to the differential pressure across the transmitter pressure ports. Differential pressure flow transmitters have been designed using many technologies including, capacitance, differential transformer, force balance, piezoelectric, potentiometer, strain gage, and vibrating wire.
The quality of the differential pressure flow transmitter signal can be described by its performance specifications. Therefore, given performance specifications, the (internal) sensing technology and details of (internal) operation are not overly important considerations when considering the purchase of differential pressure flow transmitters (although some sensing technologies may have superior reputations).
Due to the nature of the nonlinear relationship between flow and differential pressure, relatively small differential pressure changes can result in relatively large flow changes at low flow rates. To reduce the noise associated with the flow signal at these flows, some differential pressure transmitters force their output signal to zero flow when the signal falls below a certain (preset) differential pressure. Some transmitters with integral square root extraction use a linear relationship between flow and differential pressure below a certain (preset) flow rate. Other algorithms may be available to stabilize the output signal at low flow rates.
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Excerpted from The Consumer Guide to Differential Pressure Flow Transmitters