Precise Calibration of HART Pressure Transmitters: An Analysis of the Multi-Point Calibration Process

To ensure the measurement accuracy and operational reliability of HART pressure transmitters, this calibration was performed using three tools: the ALKT702 pneumatic calibration bench, the ALKC400A digital pressure calibrator, and the ALK475TREX (Chinese version). Calibration must be performed in an environment with a temperature of (20±2)°C, relative humidity ≤85%RH, and no interference. Using standardized tools, the pressure transmitter is subjected to pressure increase and decrease tests at multiple calibration points ranging from 0 MPa to 6 MPa. The corresponding 4 mA to 20 mA current values are monitored simultaneously. The zero point, pressure value, and current value are calibrated using a HART handheld communicator, and the process is completed by properly relieving the pressure in accordance with specifications. During operation, ensure that tools are used in accordance with regulations and that readings are taken only after data has stabilized. Additionally, perform annual metrological verification and equipment maintenance on the ALKC400A to ensure the transmitter operates with high precision.

I. Tools for Calibrating HART Pressure Transmitters

Name	Air Pressure Calibration Stand	Digital Pressure Calibrator	HART Handheld Communicator	Pressure Transmitters
Model	ALKT702	ALKC400A	ALK475TREX (Chinese Version)	Various Models
Technical Specifications	Pressure Generation Method: Manual Pressure Range: 0–6 MPa Medium: Clean Air Outlets: 2 Connection: M20×1.5 (F) Net Weight: 10 kg Product Dimensions:400 × 320 × 185 mm	Pressure: 0–6 MPa Accuracy: 0.02% FS Temperature Compensation: With temperature compensation Current Measurement:±25.0000 mA Voltage Measurement: ±50.0000 V DC Output: 24 VDC Communication Interface: RS-232/RS-485 Power Supply: Built-in rechargeable battery Connection: M20×1.5(M) Net Weight: 1 kg Dimensions: 156×47×239 mm	Communication: HART protocol Display: Monochrome screen Language: Chinese DC Output: 24 V DC Power Supply: Built-in rechargeable battery Net Weight: 0.5 kg Dimensions:230 × 130 × 45 mm	Pressure: 0–6 MPa Accuracy: ±0.075% FS ±0.1% FS ±0.2% FS ±0.5% FS Output Signal: 4–20 mA + HART Power Supply: 24 VDC Connection: M20×1.5(M)
Product Dimensions

The entire calibration process must strictly comply with current metrological technical specifications. The primary references include:JJG 882-2019 “Pressure Transmitters,” which applies to the verification, periodic calibration, and in-service inspection of pressure transmitters, and is used to standardize the measurement error, hysteresis, and repeatability of transmitters.

II. Detailed Operational Procedures

(1) Preliminary Preparations

1. Environmental Requirements

Maintain a temperature of (20±2)°C and a relative humidity of ≤85% RH, ensuring there is no condensation. Ensure the site is free of vibration, dust, and corrosive gases.

2. Tool Connection

• Connect the pressure transmitter and the ALKC400A digital pressure calibrator to the ALKT702 outlet using a sealed connection.

• Wiring Method 1 (24V Power Supply for Digital Pressure Calibrator):

（1）Red test lead connection: Insert one end into the standard 24V meter jack, and clip the other end to the transmitter's positive terminal.

（2）Connect a 250-ohm resistor to the negative terminal of the transmitter

（3）Black test lead connection: Insert one end into the standard meter's current terminal, and clip the other end to one side of the 250Ω resistor.

（4）Handheld unit communication cable connection: Plug one end into the handheld unit’s communication port, and attach the alligator clips on the other end to the two terminals of the 250Ω resistor.

（5）Complete the wiring to form a closed circuit

Wiring Diagram (1)

• Wiring Method 2 (24 V power supply for the ALK475TREX handheld unit):

（1）Red test lead connection: Insert one end into the standard meter's current terminal, and clip the other end to the transmitter's negative terminal.

（2）Black test lead connection: Insert one end into the common terminal of the standard meter.

（3）Handheld unit communication cable connection: Plug one end into the handheld unit’s communication port; attach the red clip to the transmitter’s positive terminal, and connect the black clip to the clip on the black test lead.

（4）Complete the wiring to form a closed circuit, then turn on the 24V switching power supply of the ALK475TREX handheld unit.

Wiring Diagram (2)

• If the transmitter does not read zero at the current atmospheric pressure, and the standard manometer indicates that the transmitter’s 4 mA current reading is off, use a HART handheld communicator to calibrate the transmitter’s zero point and current.

3、Specific Calibration Steps for the Handheld Calibrator

（1）Calibrate the transmitter's zero point

Turn on the handheld unit; the handheld unit has successfully established communication with the pressure transmitter.

(2) Calibrate the 4 mA or 20 mA current output from the transmitter as measured by the pressure gauge

(2) Calibration Process for HART Pressure Transmitters

1. Boost Calibration Process

• First, open the ALKT702 pressure regulating valve, turn the fine-tuning knob counterclockwise until it stops, and then close the vent valve.

• Use the lever to pressurize the system to over 1 MPa. Then, use the fine-tuning knob to adjust the pressure clockwise to the calibration point of 3 MPa. If the pressure reading from the transmitter deviates from 3 MPa, or if the current reading of 12 mA from the transmitter measured by the standard gauge deviates, use the handheld calibrator to calibrate the pressure and current values.

• Calibrating the transmitter pressure value:

Select either “Low Range Fine-Tuning” or “High Range Fine-Tuning.” If the pressure measured by the transmitter is below 3 MPa, select “Low Range Fine-Tuning”; if the pressure is above 3 MPa, select “High Range Fine-Tuning.” Then press the right button, enter the pressure value 3 MPa, and finally press the right button again to confirm. Both the pressure and current values will be successfully calibrated.

• Follow the same procedure: apply pressure using the lever, then use the fine-tuning knob to adjust clockwise to 6 MPa. The current value should be 20 mA. If there is an error, calibrate using the handheld calibrator (follow the steps above).

2. Pressure-relief calibration process

• Slowly open the vent valve to relieve pressure until it reaches approximately the calibration pressure, then stop. Close the vent valve and use the fine-tuning knob to reduce the pressure counterclockwise to 3 MPa. The standard gauge should read a current value of 12 mA from the pressure transmitter. If there is a deviation, use the handheld calibrator to adjust the pressure and current values.

• Record the relationship between pressure and current at each point to verify that the hysteresis (the difference in current at the same point during boost and buck operations) meets the requirement (≤0.05% FS).

3. Pressure Relief Process

Open the relief valve and the pressure-regulating valve in sequence to gradually release the pressure in the piping system. Once the pressure has been fully released to 0, the measurement is complete.

III. Important Notes

Proper Use

• Follow standard operating procedures. When pressurizing or depressurizing the ALKT702 pneumatic calibration bench, do so slowly to avoid pressure surges.

• Ensure the connection between the ALKC400A digital pressure calibrator and the handheld controller is correct; take care to prevent reversed connections.

• Readings at calibration points must be taken only after the standard gauge reading has stabilized, ensuring real-time accuracy.

• Upon completion of the test, slowly depressurize to zero and ensure there is no residual pressure. Turn off the equipment power before removing the instrument under test.

Maintenance and Calibration Schedule

• Perform metrological verification on the ALKC400A digital pressure calibrator at regular intervals (recommended once a year) to ensure measurement accuracy meets standards.

• Turn off the power when the pressure gauge is not in use; if it will not be used for an extended period, recharge it once every three months.

• If the pressure gauge will not be used for an extended period, depressurize it before storage and cover it with a cloth cover to prevent dust from entering the instrument.

IV. Conclusion

Calibration of HART pressure transmitters is a critical process for ensuring the accuracy and stability of industrial measurements. Using a complete set of calibration equipment, we perform zero-point calibration, span correction, and pressure ramp-up/ramp-down tests to correct deviations in pressure readings and current output, and rigorously verify the instrument’s hysteresis performance. During the calibration process, we strictly control factors such as slow and steady pressure ramp-up, standardized wiring, and stable readings. By strictly adhering to operating procedures and coordinating with periodic verification and equipment maintenance, we consistently maintain calibration quality to ensure the long-term stable operation of the equipment.