Precise Measurement, Calibration and Standardised Operating Procedures for Pressure Transmitters (HART Protocol)
Pressure transmitters (HART protocol) are vital instruments in industrial measurement; their measurement accuracy and signal stability determine the level of automation control and production, making regular metrological verification particularly crucial. In this instance, HART pressure transmitters were the subject of calibration, with the calibration work carried out using the ALKT803 hydraulic calibration bench, the ALKC400A digital pressure calibrator and the ALK475TREX handheld programmer. The entire process was carried out in accordance with regulations, covering environmental control, fluid filling and venting of the hydraulic bench, wiring connections, zero-point correction, multi-point pressure ramp-up and ramp-down testing, and hysteresis verification, thereby precisely correcting deviations between pressure readings and the 4–20 mA current signal. This standardised operating procedure provides a practical basis for the calibration of high-pressure transmitters, effectively ensuring the measurement accuracy of instruments and sustaining the stable operation of industrial automation systems.
I. Tools for Calibrating Pressure Transmitters (HART Protocol)
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Name |
Hydraulic calibration bench |
Digital Pressure Calibrator |
HART handheld communicator |
Pressure transmitter |
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Model |
ALKT803 |
ALKC400A |
ALK475TREX (Chinese version) |
Various models |
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Technical Specifications |
Pressurisation method: Manual Pressure: 0–60 MPa Medium: Purified water or calibration oil Outlets: 3 Connection: M20×1.5(F) Net weight: 14 kg Product dimensions: 410×410×180 mm |
Pressure: 0–60 MPa Accuracy: 0.02% FS Temperature compensation: With temperature compensation Current measurement: ±25.0000 mA Voltage measurement: ±50.0000 V DC output: 24 V DC 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
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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 |
Pressure: 0–60 MPa Accuracy: ±0.075% FS ±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 should be maintained at (20±2) °C, with relative humidity not exceeding 85% RH and no condensation; ensure that the site is free from vibration, dust and corrosive gases.
2. Filling, pressurising and testing the hydraulic test bench
• Prepare the pressure transmission medium (purified water or calibration oil) and a syringe.
• Unscrew the pressure relief valve on the fluid reservoir, draw up the medium using the syringe, and inject it into the reservoir. Ensure a sufficient quantity of medium is added. Once the filling is complete, refit the pressure relief valve and tighten it securely.
• Next, bleed any excess air from the hydraulic unit. First, open the pressure-regulating valve, the relief valve and the overpressure shut-off valve; turn the fine-adjustment knob anti-clockwise until it stops; then unscrew the plugs from the three outlet ports and close the relief valve. Next, use the lever to pressurise the system, directing the hydraulic pressure to the three outlet ports.
3. Connecting the Equipment
• Connect the pressure transmitter and the ALKC400A digital pressure calibrator to the two output ports of the ALKT803, ensuring a sealed connection; secure the other port with a plug.
• Wiring Method 1 (24 V power supply for the digital pressure calibrator):
(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 communicator cable connection: Insert one end into the communication socket of the handheld communicator, 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 unit powered by 24 V):
(1) Connecting the red test lead: Insert one end into the standard meter’s current socket and clip the other end to the negative terminal of the transmitter
(2) Connecting the black test lead: 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 read 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 for the handheld programmer
(1) Calibrating the transmitter zero point
Switch on the handheld programmer; the handheld programmer 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) The verification and calibration process for pressure transmitters (HART protocol)
1. Pressure-increasing calibration procedure
• Apply pressure using the ALKT803 lever; stop when the pressure reaches approximately 10 MPa, close the pressure-regulating valve and the overpressure shut-off valve, then use the fine-adjustment knob to turn clockwise to the calibration point of 15 MPa. If there is a deviation between the pressure value measured by the transmitter and 15 MPa, or if the current value of 8 mA measured by the standard gauge for the pressure transmitter also deviates, 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 reading on the transmitter is below 15 MPa, select the low-range fine-tuning mode; if the pressure reading is above 15 MPa, select the high-range fine-tuning mode. Then press the right button, enter the pressure value of 15 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 valve to increase the pressure clockwise to 30 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-adjustment valve to increase the pressure directly to the test points of 45 MPa and 60 MPa; the corresponding current values should be 16 mA and 20 mA respectively. If there is an error, calibrate the pressure and current values using the handheld calibrator (as described above).
2. Decompression Calibration Procedure
• Use the fine-adjustment valve to reduce the pressure anti-clockwise to 45 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 30 MPa and 15 MPa. The corresponding current values should be 12 mA and 8 mA. If there is an error, calibrate using the handheld calibrator (as described above).
• Record the correspondence between pressure and current at each point to 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 pressure relief valve, the pressure-regulating valve and the overpressure shut-off valve in sequence to relieve the pressure to 0; the measurement is then complete.
III. Points to Note
Proper Use
• The pressure-transmitting medium must be clean and free from impurities to prevent blockages in the pipework and damage to the hydraulic test bench and pressure gauge.
• Operations must be carried out in accordance with standard procedures; pressurisation and depressurisation of the ALKT803 hydraulic calibration bench must be performed slowly to avoid pressure surges.
• The connection between the ALKC400A digital pressure calibrator and the handheld controller must be clearly identified; take great care to ensure the cables are not connected in reverse.
• At each calibration point, take readings only after the value on the standard gauge has stabilised, ensuring accurate and real-time recording.
• Upon completion of the test, first slowly depressurise the system 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 ALKC400A digital pressure calibrator must undergo periodic metrological verification (recommended once a year) to ensure measurement accuracy meets standards.
• When the pressure gauge is not in use, switch off the power supply; if it is not used for an extended period, recharge the battery once every three months.
• If the hydraulic test bench is not to be used for an extended period, depressurise it before storage, empty the fluid from the reservoir cup and cover it with a cloth cover to prevent dust from entering the instrument.
IV. Conclusion
To ensure the reliable operation of the pressure transmitter (HART protocol), the ALKT803 hydraulic calibration bench, ALKC400A digital pressure calibrator and ALK475TREX handheld programmer were utilised. In accordance with specifications, the hydraulic bench was filled with fluid and vented, precise wiring was carried out, zero-point correction was performed, multi-point pressure ramp-up and ramp-down testing was conducted, and hysteresis verification was completed to verify the deviation between the system’s calibrated pressure and the 4–20 mA current signal. Operations were conducted in strict accordance with specifications throughout the process to ensure traceability of the calibration, effectively eliminating measurement errors, guaranteeing the stable operation of the pressure transmitter, and contributing to the efficient functioning of industrial automation systems.
