Example of 3-15 psi to 4-20mA Conversion

A current-to-pressure transducer is  used  to  convert  a  4-20 mA  electronic  signal  into  a  3-15 PSI pneumatic signal.This particular transducer is configured  for  reverse action instead  of direct, meaning that its pressure output at 4 mA should be 15 PSI  and its pressure  output  at 20 mA should be 3 PSI.

Calculate the necessary current signal value to produce  an output  pressure  of 12.7 PSI.

We are solving this problem using standard 4-20mA conversion Formula. Click here for formula

Reverse-acting  instruments  are  still  linear,  and  therefore  still  follow the slope-intercept  line formula y = mx + b, albeit with a negative slope:

Calculating and substituting the slope (m)  value for this equation, using the full rise-over-run  of the linear function.  Note how the “rise” is actually  a “fall” from 20 milliamps  down to 4 milliamps, yielding a negative value for m:

Therefore,  a current signal of 7.07 mA is necessary to drive the output of this reverse-acting I/P transducer to a pressure  of 12.7 PSI.

Also See: Instrumentation online Tools

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