HART Transmitter Trim

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Practical Procedure · Calibration

HART Transmitter Trim — Digital Trim vs. Analog (Loop) Trim

Applies to: Smart HART transmitters  |  Equipment needed: HART communicator, pressure/process reference standard, mA reference meter

Two Different Trims — See the Difference

Tap a tab to switch the diagram, then tap "Show Correction" to see the value before and after trim.

Reference Applied Pressure 50.0 units Transmitter Internal Sensor 52.6 mA-eq 50.0 mA-eq reading vs reference HART Comm 4–20 mA Output unchanged by digital trim
Transmitter Sensor OK Output stage HART Comm Loop Output 11.2 mA 12.0 mA trimmed to match PLC / DCS Analog Input Card Reads 12.0 mA target
Digital trim corrects the transmitter's internal sensor reading against a known reference pressure — the fix happens inside the sensor itself.
Loop trim corrects the 4–20 mA output signal to match what the receiving PLC/DCS card sees — the fix happens after the sensor measurement, in the output stage.

Digital (Sensor) Trim

Corrects the transmitter's internal sensor reading against your applied reference pressure. Use this when the sensor's own measurement is off — the raw reading it takes doesn't match the actual applied process value.

  • Apply a known reference pressure (e.g. from a dead-weight tester or precision calibrator).
  • Using the HART communicator, tell the transmitter what the applied reference value actually is.
  • The transmitter adjusts its internal sensor characterization to match — this is a correction inside the sensor, before the signal is even converted to 4–20 mA.

Analog (Loop) Trim

Corrects the transmitter's 4–20 mA output so it matches exactly what the receiving system — PLC or DCS analog input card — actually sees. Use this when the sensor reading itself is correct, but the value arriving at the control system doesn't match.

  • Connect a precision mA reference meter in the loop, or read the value the receiving card is seeing.
  • Command the transmitter to output a fixed reference current (commonly 4 mA and 20 mA).
  • Adjust the loop trim until the value the receiving system sees matches exactly — this correction happens in the output stage, after the sensor measurement is already complete.

Picking the Right One

If the sensor's own reading (viewable via HART) doesn't match your reference pressure — that's a digital trim problem. If the sensor reading is correct but the PLC/DCS is showing a different value than what the transmitter is sending — that's a loop trim problem, and it often means the receiving analog input card has drifted, not the transmitter.

Doing a loop trim to "fix" a sensor problem, or a digital trim to fix a receiving card problem, won't solve anything — it just moves the error somewhere else in the loop.

This is a general field procedure — specifics vary by transmitter brand and plant calibration standards. Adjust to your own SOP where needed.

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