To convert the pressure measurement in an output signal
Different types of pressure transmitters exist: differential pressure transmitter for measuring a pressure difference ΔP but also a relative or absolute pressure transmitter.
For more specific applications, it will be necessary to use:
- a multivariable differential pressure transmitter to correct variations of pressure and temperature of a gas
- two or more transmitters for the same differential pressure flow element to increase the measuring range
For more details on these cases, please see the Frequently Asked Questions tab on this page.
A differential pressure transmitter is a device that accurately measures a differential pressure (ΔP = upstream pressure - downstream pressure) and converts it into a 4-20mA output signal. The flow rate is calculated with the following formula :
, mass flow in kg/s
, constant
, differential pressure in mbar
, density of the fluid in kg/m3
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The density ρ of an incompressible fluid is constant at a given temperature (a liquid can be considered at first sight as an incompressible fluid) whereas the density ρ of a compressible fluid (gas) varies according to its pressure and of its temperature. Thus, the choice of the transmitter can be provided as follows:
- For a liquid at constant temperature, provide a standard differential pressure transmitter.
- For a gas, at constant temperature and pressure, provide a standard differential pressure transmitter.
- For a gas at variable temperature and pressure, provide a multi-variable differential pressure transmitter. Indeed, it makes it possible to correct the pressure and temperature variations of the gas when it is coupled to a temperature sensor. This correction can also be obtained with a conventional differential pressure transmitter to which is added a temperature sensor, a pressure transmitter and a computer.
The rangeability of a DP flowmeter is the ratio between the maximal flow and the minimum measurable flow with low measurement uncertainty. Thus, the more the flowmeter has a high rangeability, the more it is able to accurately measure flows much lower than the maximum flow.
Deltafluid flowmeters have a rangeability of 1:6 meaning they keep a good accuracy for values of flow from 100 to 16% (Qmax:6) of the maximyum flow.
The coupling of two differential pressure transmitters to a single differential pressure flow measuring element allows to extend the rangeability (1:36 instead of 1: 6 in a conventional way) and to maintain a low measurement uncertainty over a range from 2 to 100% the max.
Two differential pressure transmitters in parrallel for a measure with a rangeability up to 1:36
