Instrumentation
4-20 mA scaling formula and worked examples
Use the linear 4-20 mA scaling equation without losing the live-zero offset, engineering units, or fault-range behavior at the ends of the signal.
- Product
- 4-20 mA instrumentation
- Level
- Field triage
- Read time
- 9 min
- Reviewed
- 2026-07-15
What to establish before troubleshooting
For a linear signal, normalize the measured current inside the configured input range, then apply that fraction to the engineering span. For a 4-20 mA loop the normalized fraction is (mA - 4) / 16.
The live-zero subtraction matters. Dividing the measured current by 20 treats 0 mA as the process minimum and produces an incorrect offset for a 4-20 mA transmitter.
Abbreviated worked example
Scale 12 mA to 0-100 psi
The transmitter is configured for 4 mA at 0 psi and 20 mA at 100 psi.
- 1Normalize: (12 - 4) / (20 - 4) = 8 / 16 = 0.5.
- 2Apply the engineering span: 0 + 0.5 x (100 - 0).
Result: 12 mA represents 50 psi.
Caution: Clamp or alarm behavior outside 4-20 mA must follow the instrument and process fault policy, not an arbitrary display limit.
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