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

How 4–20 mA Scaling Works

When this applies

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.