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Evaluation Question

An analog electronic d/p transmitter has an output of 4-20 mA which represents 0 to 100 gpm (0 to 378.5 lpm). On a properly calibrated transmitter, what is the output in gpm (lpm), if the transmitter output is 12 mA?

         a.    25                 (94 lpm)

         b.   33.3             (126 lpm)

         c.   70.7             (267.6 lpm)

         d.   50                 (189.3 lpm)


 This question has to be read carefully because a lot of people including myself , tend to say 50 because 12 mA would normally would represent 50% of your span. However, the question states that you are using a differential pressure transmitter to measure gallons per minute which is a volumetric flow unit. Therefore you have to use the square root because flow is directly proportional to the square root of the differential pressure. This can be done in the transmitter or the DCS, but not both.

The correct formula for the above question is as follows:


 In other words,  take your output signal which was 12 mA and subtract 4, giving you 8. Divide 8 by16 which gives you 0.5. Then you take the square root of 0.5 which is 0.7071 and multiply it by the maximum flow rate of 100.

 The answer is c. 70.7 gpm.


Mark Weisner
Mark Weisner
Mark Weisner is a technical specialist at ISA.

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