Analysis of the Multi-Point Calibration Process for Pressure Transmitters (HART Protocol)

In industrial automation systems, pressure transmitters (HART protocol) serve as core sensing devices, and their measurement accuracy directly impacts the stability of production operations. Conducting calibration and testing in accordance with established standards is a critical step in ensuring that instrument performance meets specifications. This procedure is designed for pressure transmitters using the HART protocol and relies on the ALKD101A box-type electric pressure calibration bench, the ALKC400A digital pressure calibrator, and the ALK475TREX (Chinese version) handheld programmer. It covers key steps such as environmental preparation, wiring connections, full-range positive and negative pressure measurements, zero and span calibration, and hysteresis verification. The process strictly adheres to metrological standards and corrects deviations through real-time data comparison. Systematic calibration and measurement ensure accurate and reliable transmitter output, providing credible data support.

I. Tools for Calibrating Pressure Transmitters (HART Protocol)

Name	Box-Type Electric Pressure Calibration Stand	Digital Pressure Calibrator	HART Handheld Programmer	Pressure Transmitters
Model	ALKD101A	ALKC400A	ALK475TREX (Chinese Version)	Various Models
Technical Specifications	Pressurization Method: Manual Pressure: -0.095 to 4 MPa Medium: Clean air Outlets: 2 Connection: M20×1.5(F) Accuracy: ±0.05% F.S. Power Consumption: 100–300 VA Supply Voltage: AC 220 V Overload Capacity: 1.2–1.5 times full scale Weight: 13 kg Dimensions: 260 × 240 × 260 mm	Pressure: -0.1 to 4 MPa Accuracy: ±0.05% 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: Black-and-white screen Language: Chinese DC Output: 24V DC Power Supply: Built-in rechargeable battery Net Weight: 0.5 kg Dimensions: 230 × 130 × 45 mm	Pressure: -0.1 to 4 MPa Accuracy: ±0.1% FS ±0.2% FS ±0.5% FS Output Signal: 4 to 20 mA + HART Power Supply: 24 VDC Connection: M20×1.5(M)
Product Dimensions

The entire verification 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 Explanation of the Operational Process

(1) Preliminary Preparations

1. Environmental Requirements

The temperature must be maintained at (20±2) °C, with relative humidity ≤ 85% RH and no condensation. The site must be free from vibration, dust and corrosive gases.

2. Connecting the Equipment

• Securely connect the pressure transmitter and the ALKC400A digital pressure calibrator to the output port of the ALKD101A box-type electric pressure calibration stand. Connect the calibration stand to a 220V power source and turn on the power switch.

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

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

(2) Connect the negative terminal of the transmitter to a 250Ω resistor.

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

(4) Handheld communicator communication cable connection: Insert one end into the handheld communicator’s communication port, and clip the other end to both ends of the 250Ω resistor

(5) Once the connections are complete, a closed circuit is formed

Wiring Diagram (1)

• Wiring Method 2 (ALK475TREX Handheld Transmitter, 24V Power Supply):

(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 standard meter’s common terminal.

(3) Handheld unit communication cable connection: Insert one end into the handheld unit’s communication port; clip the red clamp to the transmitter’s positive terminal, and clip the black clamp together with the clamp on the black test lead

(4) Once the connections are complete and a closed circuit is formed, turn on the ALK475TREX handheld unit’s 24V switching power supply

Wiring Diagram (II)

• If the transmitter does not display 0 at the current atmospheric pressure, and the standard manometer indicates a deviation in the transmitter’s 4 mA current reading, use a HART handheld communicator to calibrate the transmitter’s zero point and current.

3. Specific Calibration Steps Using the Handheld Communicator

(1) Calibrating the Transmitter’s Zero Point

Turn on the handheld communicator. The handheld communicator has successfully established communication with the pressure transmitter.

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

(2) Positive Pressure Calibration Process for Pressure Transmitters (HART Protocol)

1. Pressure Increase Calibration Process

• First, press the ALKD101A medium switching valve downward (do not switch under pressure), then turn the fine-adjustment valve counterclockwise to the appropriate position, and finally close the pressure relief valve.

• Next, press the pressurization button to increase the pressure. Stop when the pressure reaches approximately the target calibration value, then use the fine-tuning valve to adjust it to the calibration pressure of 1 MPa. If the pressure reading from the transmitter deviates from 1 MPa, or if the current reading of 8 mA measured by the standard gauge also deviates, use the handheld communicator to calibrate the pressure and current values.

• Calibrating the transmitter’s pressure reading:

Select either “Low Range Fine-Tuning” or “High Range Fine-Tuning.” If the transmitter’s measured pressure is below 1 MPa, select “Low Range Fine-Tuning”; if it is above 1 MPa, select “High Range Fine-Tuning.” Then press the right button, enter the pressure value of 1 MPa, and finally press the right button again to confirm. Both the pressure and current values will be successfully calibrated.

• Follow the same procedure: press the pressurization button to increase pressure, stop when the pressure reaches approximately the target calibration value, then use the fine-tuning valve to adjust to the calibration pressures of 2 MPa, 3 MPa, and 4 MPa. The corresponding current values should be 12 mA, 16 mA, and 20 mA, respectively. If there is an error, calibrate using the handheld calibrator (follow the steps above).

2. Depressurization Calibration Process

• Press the depressurization button to reduce pressure until it reaches approximately the target calibration pressure, then stop. Use the fine-tuning valve to adjust the pressure to the calibration values of 3 MPa. If the pressure reading from the transmitter deviates from 3 MPa, or if the current reading of 16 mA measured by the standard gauge deviates, use the handheld calibrator to calibrate both the pressure and current values.

• Repeat the same procedure: press the pressure reduction button to reduce pressure until it reaches approximately the target calibration pressure, then use the fine-tuning valve to adjust to the calibration pressures of 2 MPa and 1 MPa. The corresponding current values should be 12 mA and 8 mA, respectively. If there are any errors, calibrate the pressure and current values using the handheld calibrator (follow the same procedure as above).

• Record the correspondence between pressure and current at each point to verify that the hysteresis (the current difference at the same point during pressure increase and decrease) meets the requirement (≤0.05% FS).

3. Pressure Relief Process

Open the relief valve to gradually release the pressure in the pipeline. Once the pressure is reduced to 0, the measurement is complete.

(3) Negative Pressure Calibration Process for Pressure Transmitters (HART Protocol)

1. Pressure Increase Calibration Process

• Lift the medium switching valve upward (do not switch under pressure), then turn the fine-adjustment valve clockwise to the appropriate position, and finally close the pressure relief valve.

• Next, press the pressurization button to create a vacuum. Stop when the pressure reaches approximately the target calibration value, then use the fine-tuning valve to adjust to the calibration vacuum value of -0.02 MPa. If there is a deviation between the pressure value measured by the transmitter and -0.02 MPa, or if there is a deviation in the current value (16.6 mA) measured by the standard gauge, use the handheld calibrator to calibrate the pressure and current values.

• Calibrating the transmitter’s pressure reading:

Select either the low-range fine-tuning or high-range fine-tuning mode. If the pressure value measured by the transmitter is below -0.02 MPa, select low-range fine-tuning; if it is above -0.02 MPa, select high-range fine-tuning. Then press the right button, enter the pressure value -0.02 MPa, and finally press the right button again to confirm. Both the pressure value and current value will be successfully calibrated.

• Follow the same procedure: press the pressurization button to create a vacuum, stop when the pressure reaches approximately the target calibration value, and then use the fine-tuning valve to adjust to the calibration pressures of -0.04 MPa, -0.06 MPa, -0.08 MPa, and -0.095 MPa. The corresponding current values should be 13.2 mA, 9.8 mA, 6.5 mA, and 4 mA, respectively. If there are any deviations, calibrate using the handheld controller (following the same procedure as above).

2. Pressure Reduction Calibration Process

• Press the pressure reduction button to reduce the pressure. Stop when the pressure reaches approximately the target calibration value, then use the fine-tuning valve to adjust it to the calibration pressure minus 0.08 MPa. If there is a deviation between the pressure reading from the transmitter and -0.08 MPa, or if there is a deviation in the 6.5 mA current value measured by the standard gauge, use the handheld calibrator to calibrate the pressure and current values.

• Repeat the same procedure: press the pressure reduction button to reduce the pressure, stopping when it reaches approximately the target calibration pressure. Then use the fine-tuning valve to adjust the pressure to the calibration values of -0.06 MPa, -0.04 MPa, and -0.02 MPa. The corresponding current values should be 9.8 mA, 13.2 mA, and 16.6 mA, respectively. If there are any deviations, calibrate them using the handheld calibrator (following the same procedure as above).

• Record the correspondence between pressure and current at each point, and verify that the hysteresis (the current difference at the same point during pressure increase and decrease) meets the requirement (≤0.05% FS).

3. Pressure Relief Process

Open the relief valve to gradually release the pressure in the pipeline. When the pressure reaches 0, the measurement is complete.

III. Precautions

Proper Use

• Follow standard operating procedures. Pressurization and depressurization of the ALKD101A Enclosed Electric Pressure Calibration Bench must be performed slowly to avoid pressure surges.

• Do not switch between positive and negative pressure measurements while the system is under pressure.

• Connect the Enclosed Electric Pressure Calibration Bench to a 220V power source; ensure the connection is correct.

• Ensure the communication cable between the ALKC400A Digital Pressure Calibrator and the handheld controller is connected correctly; strictly avoid reversing the connections.

• When taking readings at calibration points, wait until the value on the standard gauge has stabilized to ensure accurate and real-time recording.

• Without the appropriate technical skills and testing equipment, do not open the equipment cover for maintenance, and under no circumstances adjust or replace components; otherwise, the accuracy of the instrument’s measurements cannot be guaranteed.

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

Maintenance and Calibration Schedule

• Perform periodic metrological verification on the ALKC400A Digital Pressure Calibrator (recommended once a year) to ensure measurement accuracy meets standards.

• When the pressure gauge is not in use, turn off the power. If it will not be used for an extended period, recharge it once every three months.

• If the cabinet-type electric pressure calibration stand 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

Standardizing the calibration and testing procedures for pressure transmitters (HART protocol) can effectively control measurement errors and ensure that the pressure-to-current relationship complies with metrological standards across the entire measurement range. Strictly adhering to operating procedures in practice—including thorough wiring verification, step-by-step pressure adjustment, stabilization of readings, and hysteresis verification—can prevent equipment damage and operational risks while ensuring the accuracy and traceability of measurement data. Conducting standardized calibration and measurement activities on a regular basis not only maintains the metrological performance of instruments but also provides a reliable basis for monitoring pressure parameters in industrial settings.