Engineering Value to 4–20 mA

Method & assumptions

1. EU means your chosen engineering unit: use the same unit for both endpoint values and the engineering input/output. No unit conversion is performed. Current is in mA, not A.
2. Configure the engineering values corresponding to exactly 4 mA and 20 mA. They must differ; decreasing scales are supported. This assumes a linear transmitter output.
3. Forward: fraction = (current − 4)/16; value = valueAt4 + fraction × (valueAt20 − valueAt4). Inverse: fraction = (value − valueAt4)/(valueAt20 − valueAt4); current = 4 + 16 × fraction. Percent span = 100 × fraction.
4. Example: a −50 to 150 °C linear scale gives 50 °C at 12 mA and 75% span at 16 mA. Percent span is measured from the 4 mA endpoint, not from zero engineering value.
5. Results outside 4–20 mA are mathematical extrapolations, not valid process readings or feasible output commands. They are flagged and never clamped. Check device-specific alarms and configured limits; this is not a fault diagnosis or NAMUR compliance test.
6. No square-root extraction, sensor linearization, calibration uncertainty, loop supply/burden check, hardware communication or safety-interlock validation is included.

Sources

• NI: Custom Scales, linear input/output scaling ↗