The output from a current loop pressure transmitter is independent of supply voltage. However, it is important to ensure that the power supply is able to provide sufficient voltage for all the equipment in the current loop. For details on how to calculate the optimum supply and best possible working loop, see “What is a 4-20mA current loop?”.
For example, when using a 4-20mA transmitter zero pressure will produce 4mA output and full-scale pressure will produce 20mA output. This is the ideal curve, but in practice there will also be a fixed null and span offset to consider. For example:
Zero output is 4mA ±0.4mA
Span output is 20mA ±0.2mA (this is an example only).
In addition, you will need to add all the different accuracy and temperature effects, as detailed in the other Engineering Notes in this series.
In order to demonstrate the principle, the errors are exaggerated (see specific data for individual errors and sensors).
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