Some devices use 21 mA for fault; know your device.

How 4–20 mA Scaling Works

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Use this guide when loops read correctly at the transmitter but HMI numbers disagree because span and zero were never written down.

Browser-based preliminary check for Four Twenty MA Converter — fast estimates, not code or stamped design output.

1Confirm whether 4 mA represents live zero or indicates fault depending on device policy.
2Define engineering value at 4 mA and at 20 mA for the measured variable.
3Solve linear relationship between mA and engineering units.
4Cross-check mid-scale points with a calibrator if disputes persist.
5Document alarm thresholds with the same scaling basis.

Using raw mA in SCADA without engineering conversion.
Confusing square-root relationships for orifice DP without configuring properly.
Ignoring square-root extraction done inside the device vs in software.

Digital PV may differ if secondary variables are addressed; verify which variable scales to mA.

Some devices use <3.6 mA or >21 mA for fault; know your device.

Use this guide when loops read correctly at the transmitter but HMI numbers disagree because span and zero were never written down.

Browser-based preliminary check for Four Twenty MA Converter — fast estimates, not code or stamped design output.

1Confirm whether 4 mA represents live zero or indicates fault depending on device policy.
2Define engineering value at 4 mA and at 20 mA for the measured variable.
3Solve linear relationship between mA and engineering units.
4Cross-check mid-scale points with a calibrator if disputes persist.
5Document alarm thresholds with the same scaling basis.

Using raw mA in SCADA without engineering conversion.
Confusing square-root relationships for orifice DP without configuring properly.
Ignoring square-root extraction done inside the device vs in software.

Digital PV may differ if secondary variables are addressed; verify which variable scales to mA.

Some devices use <3.6 mA or >21 mA for fault; know your device.