A “smart” DP transmitter with built-in square root characterization is used to measure flow through a pipe.

The orifice plate range is 0 to 125 inches WC at 0 to 277 gallons per minute:

A technician removes the transmitter from service and tests it by applying several air pressures to the “H” port while leaving the “L” port vented. Here are the As-Found results:

Calculate the largest error, in percent of (output signal) span.

Also, determine whether this looks like a zero or span error, and whether that error exists in the input (ADC) of the smart transmitter or in the output (DAC).

Answer :

This is a zero error in the input (ADC) of the smart transmitter, the greatest error being at the 0% point (5.431 mA instead of 4.00 mA = +8.94%).

As you can see, the square-root function inside the transmitter exaggerates the sensor calibration error at the low end of the scale.

This same zero-shift error of +1 inch WC is hardly noticeable at the high end (20.006 mA instead of 20.000 mA = +0.4% error).

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