How 4–20 mA Scaling Works

适用场景

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

要使用的工具

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

打开 Four Twenty MA Converter →

步骤

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

示例

  • Tank level 4 mA empty, 20 mA full.
  • Pressure transmitter with elevated zero.

应避免

  • 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.

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常见问题

HART digital values?

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

Fault currents?

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

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