Standardised Calibration Procedures for Pressure Transmitters with a Range of 0–100 MPa (HART Protocol)
Pressure transmitters (HART protocol) are core measurement and control instruments in industrial automation systems across sectors such as chemicals, power generation and metallurgy. Their measurement accuracy, zero-point stability and the accuracy of their 4–20 mA current output are critical to production processes and equipment operation. Long-term operation of transmitters is susceptible to environmental and process medium influences, leading to issues such as zero-point drift, span deviation and signal inaccuracy; therefore, regular standardised calibration is of paramount importance. This calibration was carried out using the ALK-T100 hydraulic calibration bench, the ALKC400T precision digital pressure gauge and the ALK475TREX Chinese-language HART handheld communicator, in a standardised, temperature- and humidity-controlled environment free from interference. Following standardised equipment wiring and fluid filling and venting procedures, full-range testing was carried out across the entire pressure range using five standard points from 0 to 100 MPa. The transmitter’s zero point, span and current output were adjusted, hysteresis accuracy was verified, and equipment deviations were corrected to ensure the accuracy and reliability of the transmitter’s measurement data, thereby meeting the requirements of industrial production metrology standards.
I. Tools for Calibrating Pressure Transmitters (HART Protocol)
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Name |
Hydraulic calibration bench |
Precision Digital Pressure Gauge |
HART Handheld Terminal |
Pressure Transmitter |
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Model |
ALK-T100 |
ALKC400T |
ALK475TREX (Chinese version) |
Various models |
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Technical Specifications |
Pressurisation method: Manual Pressure range: 0–100 MPa Medium: Purified water or calibration oil Outlets: 2 Connection: M20×1.5(F) Net weight: 9 kg Dimensions: 330 × 320 × 190 mm
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Pressure: 0–100 MPa Accuracy: ±0.05% FS Current measurement: ±30.0000 mA Voltage measurement: ±30.0000 V DC output: 24 V DC Communication interfaces: RS-232/RS-485 Temperature compensation: With temperature compensation Power supply: Built-in rechargeable battery Net weight: 0.6 kg Connection: M20×1.5 (M) Dimensions: 110×50×185 mm
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Communication: HART protocol Display: Monochrome screen Language: Chinese DC output: 24V DC Power supply: Built-in rechargeable battery Net weight: 0.5kg Dimensions: 230×130×45mm
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Pressure: 0–100 MPa Accuracy: ±0.1% FS ±0.2% FS ±0.5% FS Output signal: 4–20 mA + HART Power supply: 24 V DC Connection: M20×1.5(M)
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Product Dimensions |
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The entire verification process must be carried out in strict accordance with current metrological technical specifications. The main 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 standardise the indication 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. Filling, pressurising and priming the hydraulic test bench
• Prepare the pressure-transmitting medium (purified water or calibration oil).
• First, unscrew the shut-off valve on the ALK-T100 hydraulic calibration bench, remove the cap from the fluid reservoir, and then fill the reservoir with the medium, ensuring it is filled to at least two-thirds capacity. Once filling is complete, tighten the cap securely and screw the shut-off valve back on.
• Next, purge any excess air from the hydraulic test bench. First, loosen the two pressure-regulating valves (one turn is sufficient), then turn the pressure-application rod anti-clockwise to the appropriate position, followed by turning the fine-adjustment valve anti-clockwise until it can be adjusted no further, and finally close the shut-off valve. Unscrew the plugs from the two outlet ports, and use the pressure-application rod to apply pressure in a clockwise direction, forcing the hydraulic fluid through to the two outlet ports to expel any excess air.
3. Connecting the Equipment
• Connect the pressure transmitter and the ALKC400T precision digital pressure gauge to the two output ports of the ALK-T100 hydraulic calibration bench, ensuring a sealed connection.
• Wiring Method 1 (24 V power supply for the precision digital pressure gauge):
(1) Red test lead connection: Insert one end into the 24 V socket on the standard gauge, and clip the other end to the positive terminal of the transmitter
(2) Connect the negative terminal of the transmitter to the 250 Ω resistor.
(3) Black test lead connection: Insert one end into the current socket of the standard gauge, and clip the other end to one side of the 250 Ω resistor.
(4) Handheld unit communication cable connection: Insert one end into the communication socket of the handheld unit, and clip the other end to both ends of the 250 Ω 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 socket and clip the other end to the negative terminal of the transmitter
(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, forming a closed circuit, switch on the ALK475TREX handheld unit’s 24V switching power supply
Wiring Diagram (2)
• If the transmitter does not display 0 at the current atmospheric pressure, and the standard manometer indicates that the transmitter’s current reading of 4 mA is inaccurate, use a HART handheld communicator to calibrate the transmitter’s zero point and current.
4. Specific calibration steps using the handheld communicator
(1) Calibrating the transmitter’s zero point
Switch on the handheld communicator; the handheld communicator 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 Procedure for Pressure Transmitters (HART Protocol)
1. Pressure-rising calibration procedure
• Using the pressure-application rod, slowly increase the pressure in a clockwise direction until it reaches approximately 10–15 MPa, then stop. Next, use the fine-adjustment mechanism to increase the pressure clockwise to the test point of 25 MPa. If there is a deviation between the pressure value measured by the transmitter and 25 MPa, and if the current value of the transmitter measured by the standard gauge (8 mA) also shows a deviation, 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 function. If the pressure reading measured by the transmitter is below 25 MPa, select low-range fine-tuning; if it is above 25 MPa, select high-range fine-tuning. Then press the right button, enter the pressure value 25 MPa, and finally press the right button again to confirm. Both the pressure reading and current value will be successfully calibrated.
• Next, use the fine-tuning function to increase the pressure clockwise directly to the test point of 50 MPa; the current value should be 12 mA. If there is a deviation between the transmitter’s pressure reading and the pressure measured by the standard gauge, or between the transmitter’s pressure reading and the corresponding current value, use the handheld calibrator to calibrate the pressure and current values.
• Repeat the same procedure, using the fine-tuning knob to increase the pressure directly to the test points of 75 MPa and 100 MPa; the corresponding current values should be 16 mA and 20 mA respectively. If there is an error, calibrate using the handheld calibrator (as described above).
2. Depressurisation Calibration Procedure
• Use the fine-adjustment valve to reduce the pressure anti-clockwise to 75 MPa. The current value of the pressure sensor, as measured by the standard gauge, should be 16 mA. If there is a deviation, use the handheld calibrator to calibrate the pressure and current values.
• Following the same procedure, use the fine-adjustment valve to reduce the pressure directly to the test points of 50 MPa and 25 MPa; the corresponding current values should be 12 mA and 8 mA respectively. If there is any deviation, calibrate the pressure and current values using the handheld calibrator (as described above).
• Record the correspondence between pressure and current at each point, and verify that the hysteresis (the difference in current at the same point during pressure increase and decrease) meets the requirement (≤0.05% FS).
3. Pressure Relief Procedure
Open the shut-off valve to gradually release the pressure within the piping system. Once the pressure has been relieved to 0, the measurement is complete.
III. Precautions
Proper Use
• The pressure-transmitting medium must be clean and free of impurities to prevent blockages in the piping and damage to the hydraulic calibration bench and pressure gauges.
• Operations must follow standard procedures; pressurisation and depressurisation of the ALK-T100 hydraulic calibration bench must be carried out slowly to avoid pressure surges.
• Ensure the connection between the ALKC400T precision digital pressure gauge and the handheld controller is correct; take great care to avoid reverse connections.
• Readings at calibration points must be taken only after the standard gauge’s reading has stabilised, ensuring accurate and real-time recording.
• Upon completion of the test, first slowly relieve the pressure to zero, ensuring there is no residual pressure, then switch off the equipment’s power supply before removing the instrument under test.
Maintenance and Calibration Intervals
• The ALKC400T precision digital pressure gauge must undergo periodic metrological verification (recommended once a year) to ensure measurement accuracy meets standards.
• When the pressure gauge is not in use, the power supply should be switched off; if not used for an extended period, it should be recharged once every three months.
• If the hydraulic test bench is to be stored for an extended period, it must be depressurised beforehand; the fluid in the reservoir must be drained, and a cloth cover must be placed over the instrument to prevent dust from entering.
IV. Conclusi on
This calibration of the pressure transmitter (HART protocol) strictly adhered to standardised operating procedures. A five-point pressure ramp-up and ramp-down test covering the full 0–100 MPa range was completed, with targeted adjustments made to the instrument’s zero point, span and 4–20 mA current output parameters, effectively correcting measurement deviations caused by long-term operational conditions. During this calibration, the instrument’s hysteresis performance was rigorously verified, ensuring that the hysteresis error was controlled within the specified standard of ≤0.05% FS, thereby guaranteeing precise correspondence between pressure readings and current signals. Key stages throughout the process—including wiring, venting and pressurisation testing—were strictly controlled in accordance with regulations to minimise external interference and ensure the accuracy and reliability of the calibration data.
