4-20mA Transmitters Calculations

Given a pressure transmitter with a measurement range of 150 to 400 PSI and a signal range of 4 to 20 milliamps, calculate the applied pressure corresponding to a signal of 10.6 milliamps.

Answer : 

Take 10.6 milliamps and subtract the LRV (4 milliamps),

then divide by the span (16 milliamps) to arrive at 41.25% (0.4125 per unit).

Take this number and multiply by the span of the pressure range (400 PSI − 150 PSI, or 250 PSI) and

lastly add the LRV of the pressure range (150 PSI) to arrive at a final answer of 253.125 PSI.

Given a temperature transmitter with a measurement range of −88 degrees to +145 degrees and a signal range of 4 to 20 milliamps, calculate the proper signal output at an applied temperature of +41 degrees.

Answer : 

Take 41 degrees and subtract the LRV (−88 degrees) which is the same as adding 88 to 41,

then divide by the span (145 degrees − (−88) degrees, or 233 degrees) to arrive at 55.36% (0.5536 per unit).

Take this number and multiply by the span of the current signal range (16 milliamps) and

lastly add the LRV of the current signal range (4 milliamps) to arrive at a final answer of 12.86 milliamps.

Given a pH transmitter with a measurement range of 3 pH to 11 pH and a signal range of 4 to 20 milliamps, calculate the proper signal output at 9.32 pH.

Answer : 

Take 9.32 pH and subtract the LRV (3 pH),

then divide by the span (11 pH − 3 pH, or 8 pH) to arrive at 79% (0.79 per unit).

Take this number and multiply by the span of the current signal range (16 milliamps) and

lastly add the LRV of the current signal range (4 milliamps) to arrive at a final answer of 16.64 milliamps.