Zero and span define what in instrumentation calibration, and how are they set on a 4-20 mA transmitter?

• A. Zero is the output at the lower end of the range; span is the output at the upper end. For a 4-20 mA transmitter, set zero to 4 mA at the minimum input and span so 20 mA corresponds to the maximum input.
• B. Zero is the maximum output; span is the minimum output; set zero to 20 mA at the maximum input.
• C. Zero and span refer to voltage levels; 4-20 mA is unrelated.
• D. Zero is mid-scale, span is full-scale, and both are independent of input range.

Answer: (A)

In instrumentation calibration, zero and span describe how the input range is translated into the transmitter’s current output. The zero point is the current the transmitter produces when the input is at its minimum, and the span is the amount of current change as the input runs from minimum to maximum. For a 4-20 mA transmitter, the convention is to have 4 mA at the minimum input and 20 mA at the maximum input, so the span is 16 mA (20 mA minus 4 mA). This setup provides a live-zero (the 4 mA baseline) and headroom to indicate faults or loss of signal. Therefore, you set zero to 4 mA at the minimum input and adjust the span so the output reaches 20 mA at the maximum input. Options that suggest the output is 0 or 20 mA only, or that the parameters are voltages, don’t fit how 4-20 mA loops are defined.