Pressure Transmitter Calibration using Hart Calibrator
Calibrating a pressure transmitter using a HART calibrator involves verifying and adjusting the transmitter’s output to ensure accurate pressure measurements. Below is a step-by-step guide
Requirements*
**HART-enabled pressure transmitter**: The device to be calibrated.
**HART calibrator/communicator**: For configuration and trimming.
**Pressure source**: Hand pump, vacuum pump, or pressure calibrator to apply reference pressures.
**Multimeter or mA meter**: To measure the 4-20 mA output.
**Power supply**: 24 VDC loop power (often provided by the calibrator).
**250-ohm resistor**: Required for HART communication if not already integrated in the calibrator.
**Pressure gauge or reference standard**: To verify applied pressure (4-10 times more accurate than the transmitter).
**Tools and fittings**: Connectors, tubing, and thread seals for secure connections.
**Calibration documentation**: For recording results (e.g., calibration report template).
*Step-by-Step Calibration Procedure*
1. *Preparation*
**Isolate the transmitter**: Close the process isolation valve and open the vent or bleed valve to depressurize the transmitter.
**Clean and inspect**: Ensure the pressure ports and fittings are clean and dry to avoid contamination or leaks.
**Gather documentation**: Refer to the transmitter’s datasheet for the Lower Range Value (LRV), Upper Range Value (URV), and accuracy specifications (e.g., ±0.1% of span).
**Set up equipment**: Connect the pressure source to the transmitter’s pressure port Ensure leak-free connections using appropriate fittings and thread seals.
2. *Wiring and Power Setup*
**Connect the power supply**: Provide 24 VDC to the transmitter.
**Insert a 250-ohm resistor**: Place it in series with the transmitter’s 4-20 mA loop to enable HART communication, unless the calibrator has an integrated resistor.
**Connect the HART communicator**: Attach it across the 250-ohm resistor or directly to the transmitter terminals, depending on the wiring configuration
3. *HART Communicator Setup*
**Power on the communicator**:HART icon from the main menu.
**Verify transmitter parameters**: Use the HART communicator to check the tag number, LRV, URV, and units.
**Select calibration mode**: Navigate to the “Basic Setup” or “Diag/Service” menu and select “Calibration” to prepare for zero and span adjustments.
4. *As-Found Calibration (Check Initial Performance)*
**Apply zero pressure (LRV)**:
For absolute pressure transmitters, create a vacuum (0 PSIA, 0 bara, or 0 torr) using a vacuum pump.
For gauge pressure transmitters, vent to atmospheric pressure (0 psig).
Allow the pressure to stabilize and check the mA output (should be 4 mA) and the digital reading on the HART communicator.
**Apply full-scale pressure (URV)**: Use the pressure source to apply the URV ,Verify the output is 20 mA.
**Test intermediate points**: Apply pressures at 25%, 50%, and 75% of the range . Record mA readings and compare them to expected values (8 mA, 12 mA, 16 mA).
**Check accuracy**: Ensure the output is within the manufacturer’s specified tolerance (e.g., ±0.025% of span). If errors exceed tolerance, proceed to trimming.
5. *Trimming/Adjustment (If Needed)*
**Zero trim (LRV adjustment)**:
Apply the LRV pressure (e.g., 0 bar or vacuum).
Use the HART communicator to navigate to “Zero Trim” or “Lower Range Value” in the calibration menu.
Adjust until the output is exactly 4 mA and the digital reading matches the applied pressure.
**Span trim (URV adjustment)**:
Apply the URV pressure (e.g., 12 bar or 760 torr).
Navigate to “Span Trim” or “Upper Range Value” and adjust until the output is 20 mA.
**Sensor trim (if required)**: For precise input calibration, perform a sensor trim to adjust the analog-to-digital conversion. This compensates for sensor errors and is done using the HART communicator’s “Sensor Trim” option.
**Repeat intermediate points**: Verify the 25%, 50%, and 75% points again to ensure linearity.
6. *As-Left Calibration (Verify Final Performance)*
Repeat the As-Found calibration steps (zero, 25%, 50%, 75%, and full-scale pressures) to confirm the transmitter is within tolerance after adjustments.
Record all readings for the calibration report. If using a documenting calibrator.
7. *Final Steps*
**Disconnect equipment**: Release pressure, close the bleed valve, and disconnect the pressure source, calibrator, and HART communicator.
**Reconnect to process**: Open the isolation valve(s) slowly to avoid damaging the transmitter diaphragm.
**Verify operation**: Ensure the transmitter is functioning correctly in the process loop and check for leaks.
**Document results**: Generate a calibration report with As-Found and As-Left data, including date, technician name, and any deviations.