Practical Guide to the Calibration of Positive and Negative Pressure Measurements for Differential Pressure Transmitters

The calibration of differential pressure transmitters is central to industrial measurement calibration; to ensure the measurement accuracy of digital differential pressure transmitters, calibration must be carried out regularly. For this calibration, three pieces of equipment were utilised—the ALKT512 precision pressure calibration bench, the ALKC400A digital pressure calibrator and the ALK475TREX (Chinese version) handheld programmer—to perform step-by-step calibration of the differential pressure transmitter under both positive and negative pressure conditions. Prior to operation, environmental conditions were strictly controlled and connection tools standardised. During calibration, pressure was adjusted gradually, calibration errors were monitored in real time and data recorded. Following calibration, pressure was relieved and equipment maintained in accordance with requirements, ensuring compliance throughout the entire process and data accuracy, thereby providing reliable data support for industrial process control.

I. Tools for Calibrating Differential Pressure Transmitters

Name	Micro-pressure calibration bench	Digital Pressure Calibrator	HART handheld communicator	Digital Differential Pressure Transmitter
Model	ALKT512	ALKC400A (differential pressure type)	ALK475TREX (Chinese version)	ALK3051DP
Technical Specifications	Pressurisation method: Manual Pressure: ±1000 Pa Medium: Clean air Outlets: 2 Connection: M20×1.5(F) Net weight: 4.9 kg Product dimensions: 320×190×140 mm	Pressure: ±1000 Pa 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: M10×1 Net weight: 1.25 kg Dimensions: 156×75×239 mm	Accuracy class: ±0.05% FS Communication: HART protocol Display: Black and white 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: ±1000 Pa Accuracy: ±0.2% FS Output signal: 4–20 mA Power supply: 24 V DC Net weight: 3.5 kg
Product Dimensions

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. Tool connections

• Connect the ALK3051DP differential pressure transmitter and the ALKC400A digital pressure calibrator (differential pressure type, hereinafter referred to as the differential pressure gauge) to the output ports of the ALKT512 precision pressure calibration bench in a sealed manner.

(1) Connect the high-pressure end of the differential pressure gauge to one of the output ports of the ALKT512.

(2) Connect the high-pressure end of the differential pressure transmitter to the other output port of the ALKT512.

• Wiring method 1 (24 V power supply for the digital pressure calibrator):

(1) Connecting the red test lead: insert one end into the 24V socket on the standard multimeter, 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) Connecting the black test lead: insert one end into the current socket on the standard multimeter, 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 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)

• The differential pressure transmitter reads 0 at current atmospheric pressure; a standard meter measures the current output of the differential pressure transmitter as 12 mA

(2) The process of positive pressure calibration for differential pressure transmitters

1. Boost calibration process

• First, open the ALKT512 shut-off valve, turn the pressure adjustment knob anti-clockwise until it stops, turn the fine-tuning knob anti-clockwise to the appropriate position, and then close the vent valve.

• Using the pressure adjustment handle, slowly increase the pressure by turning it clockwise until it reaches approximately the calibration value. Use the fine-tuning knob to adjust to the calibration value of 500 Pa. If the pressure reading from the differential pressure transmitter deviates from 500 Pa, and the differential pressure gauge indicates a deviation in the transmitter’s current reading of 16 mA, use the handheld calibrator to calibrate both the pressure and current values.

• Calibrating the transmitter’s pressure reading:

Select either the low-range fine-tuning or high-range fine-tuning. If the pressure reading measured by the transmitter is below 500 Pa, select low-range fine-tuning; if it is above 500 Pa, select high-range fine-tuning. Then press the right button, enter the pressure value of 500 Pa, and finally press the right button again to confirm. Both the pressure and current readings will be successfully calibrated.

• Following the same procedure, use the pressure adjustment knob to slowly increase the pressure clockwise until it reaches approximately the calibration value. Use the fine-tuning function to adjust to the calibration value of 1000 Pa; the current value should be 20 mA. If there is any deviation, calibrate using the handheld calibrator (as described above).

2. Pressure-reduction calibration process

• Use the pressure adjustment knob to slowly reduce the pressure anticlockwise until it is approximately at the calibration value; then use the fine-tuning knob to adjust it to the calibration value of 500 Pa. The current reading on the differential pressure gauge should be 16 mA. If there is any deviation, use the handheld calibrator to calibrate the pressure and current values.

• 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 process

Open the pressure relief valve to gradually release the pressure in the pipework; once the pressure has been reduced to zero, the measurement is complete.

(3) The calibration process for differential pressure transmitters under negative pressure

1. Boost Calibration Procedure

• With the ALKT512 shut-off valve in the open position, turn the pressure adjustment handle clockwise until it stops, adjust the fine-tuning knob clockwise to the appropriate position, and then close the vent valve.

• Use the pressure adjustment handle to slowly create a vacuum by turning it anti-clockwise until the pressure reaches approximately the calibration value. Use the fine-tuning knob to adjust to the calibration value minus 500 Pa. If the pressure value measured by the differential pressure transmitter deviates from –500 Pa, and the differential pressure gauge indicates a deviation in the transmitter’s current reading of 8 mA, use the handheld calibrator to calibrate both the pressure and current values.

• Calibrating the transmitter’s pressure reading:

Select either the low-range fine-tuning or high-range fine-tuning. If the pressure reading measured by the transmitter is below –500 Pa, select low-range fine-tuning; if it is above –500 Pa, select high-range fine-tuning. Then press the right button, enter the pressure value –500 Pa, and finally press the right button again to confirm. Both the pressure and current readings will be successfully calibrated.

• Follow the same procedure: use the pressure adjustment knob to slowly create a vacuum by turning it clockwise until the pressure reaches approximately the calibration value. Use the fine-tuning function to adjust to the calibration value of –1000 Pa; the current value should be 4 mA. If there is an error, calibrate using the handheld calibrator (as described above).

2. Negative Pressure Calibration Procedure

• Use the pressure adjustment knob to slowly reduce the pressure clockwise until it reaches approximately the calibration value. Use the fine-tuning knob to adjust to the calibration value minus 500 Pa. The current value measured by the differential pressure gauge should be 8 mA. If there is a deviation, use the handheld calibrator to calibrate the pressure and current values.

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

3. Depressurisation Procedure

Open the vent valve and gradually release the pressure within the pipeline. Once the pressure has been reduced to 0, the measurement is complete.

III. Points to Note

Proper Use

• Operations must be carried out in accordance with standard procedures. When pressurising or depressurising the ALKT512 precision pressure calibration bench, do so slowly to avoid pressure surges.

• Ensure the connection between the differential pressure gauge and the handheld controller is correct; take great care to avoid reversing the connections.

• Readings at the calibration point must be taken only after the value on the standard gauge has stabilised, and must be recorded accurately and in real time.

• Upon completion of the test, first slowly depressurise the unit to zero, ensuring there is no residual pressure, then switch off the power supply before removing the instrument under test.

Maintenance and Calibration Intervals

• The ALKC400A digital pressure calibrator (differential pressure type) 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 precision pressure calibration bench is not to be used for an extended period, it must be depressurised before storage and covered with a cloth cover to prevent dust from entering the instrument.

IV. Conclusion

The calibration of differential pressure transmitters is a precision technical task, involving step-by-step calibration using positive and negative pressures to precisely control indication error and hysteresis. The process requires standardised wiring, gradual pressure adjustment, real-time calibration of the pressure-to-current relationship, and dynamic correction of errors using a handheld calibrator. At the same time, regular metrological verification and maintenance of the equipment must be carried out to ensure the accuracy of the instruments; the proper implementation of these procedures is key to reliable measurement in industrial process control.