A Detailed Explanation of the Operational Procedures for Pressure Measurement and Calibration of Digital Differential Pressure Transmitters

Differential pressure transmitters are core pressure measurement instruments in the field of industrial process measurement and control; their measurement accuracy and output stability influence the accuracy of operating condition parameter monitoring, process control and data metering. To ensure the accuracy and compliance of instrument measurements, calibration and testing must be carried out in accordance with relevant specifications. This calibration focuses on the ALK3051DP differential pressure transmitter, utilising the portable micro-pressure pump ALKB9601, the ALKC400E digital differential pressure calibrator and the ALK475TREX HART handheld communicator to establish a standard testing system. Covering the full range of positive and negative pressures, the test involved zero-point calibration, span point calibration and pressure ramp hysteresis verification, whilst the correspondence between pressure and current was verified through standard comparisons and parameter fine-tuning. Standardised calibration and regular maintenance ensure that the instrument’s metrological performance meets the required standards.

I. Tools for Calibrating Differential Pressure Transmitters

Name	Portable micro-pressure pump	Digital Pressure Calibrator	HART Handheld Terminal	Digital Differential Pressure Transmitter
Model	ALKB9601	ALKC400E (Differential Pressure Type)	ALK475TREX (Chinese version)	ALK3051DP
Technical Specifications	Pressurisation method: Manual Pressure range: ±10,000 Pa Medium: Clean air Outlets: 2 Connection: M20×1.5 female Hose: 400 mm Guide hose: 280 mm Guide connector: M10×1 to φ6 Net weight: 1.35 kg Product dimensions: 200×100×115 mm	Pressure: ±10,000 Pa Accuracy: 0.02% FS Temperature compensation: With temperature compensation Current measurement: ±30.0000 mA Voltage measurement: ±30.0000 V DC output: 24 V DC Communication Interface: RS-232/RS-485 Power Supply: Built-in rechargeable battery Connection: M10×1 Net Weight: 0.85 kg Dimensions: 110×75×190 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: ±10,000 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 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

• Connect the ALK3051DP differential pressure transmitter and the ALKC400E digital pressure calibrator (differential pressure type, hereinafter referred to as the differential pressure gauge) to the outlet of the ALKB9601 portable micro-pressure pump, ensuring a sealed connection.

(1) Connect the high-range port of the differential pressure gauge to one of the output ports of the ALKB9601.

(2) Connect the high-range port of the differential pressure transmitter to the other output port of the ALKB9601.

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

(1) Red test lead: Insert one end into the 24 V port of 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 port 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 port 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 Transmitter, 24V Power Supply):

(1) Red test lead connection: Insert one end into the standard meter’s current socket; 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 differential pressure transmitter does not display zero at the current atmospheric pressure, and the standard meter indicates a deviation in the transmitter’s current reading of 12 mA, use a HART handheld communicator to calibrate the transmitter’s zero point and current.

(2) Positive Pressure Calibration Procedure for Differential Pressure Transmitters

1. Pressure Increase Calibration Procedure

• First, turn the ALKB9601 fine-adjustment handle anti-clockwise to the appropriate position, then turn the micro-adjustment knob anti-clockwise to the appropriate position, and finally close the pressure relief valve.

• Use the fine-adjustment handle to slowly increase the pressure by turning it clockwise until the pressure reaches approximately the calibration value. Then, use the micro-adjustment knob to slowly adjust the pressure to the calibration point of 5000 Pa. If the measured calibration value deviates, and the differential pressure gauge indicates a deviation in the differential pressure transmitter’s current value of 16 mA, use the handheld calibrator to calibrate the pressure and current values.

• Calibrating the transmitter’s pressure value:

Select either the low-range fine adjustment or the high-range fine adjustment. If the pressure value measured by the transmitter is below 5,000 Pa, select the low-range fine adjustment; if the pressure value is above 5,000 Pa, select the high-range fine adjustment. Then press the right button, enter the pressure value of 5,000 Pa, and finally press the right button again to confirm. Both the pressure value and the current value will be successfully calibrated.

• Next, continue to apply pressure gradually by turning the fine-tuning handle clockwise until the pressure reaches approximately the calibration value. Then, use the fine-tuning knob to slowly adjust to the calibration point of 10,000 Pa; the current value should be 20 mA. If there is an error, calibrate using the handheld calibrator (follow the procedure described above).

2. Depressurisation calibration procedure

• Turn the fine-adjustment handle anti-clockwise to reduce the pressure until it is approximately at the calibration value. Then, use the fine-adjustment knob to slowly adjust to the calibration point of 5,000 Pa. The current value measured by the differential pressure gauge should be 16 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 pressure increase and decrease) meets the requirement (≤0.05% FS).

3. Pressure Relief Procedure

Open the relief valve to gradually release the pressure within the pipeline. Once the pressure has been relieved to 0, the measurement is complete.

(3) Negative Pressure Calibration Procedure for Differential Pressure Transmitters

1. Pressure Increase Calibration Procedure

• First, turn the fine-adjustment handle clockwise to the appropriate position, then turn the micro-adjustment knob clockwise to the appropriate position, and close the pressure relief valve.

• Using the fine-tuning handle, slowly turn it anti-clockwise until the reading is approximately at the calibration value, then use the micro-adjustment knob to slowly adjust it to -5000 Pa. If the pressure value measured by the differential pressure transmitter deviates from -5000 Pa, and the current value of 8 mA measured by the differential pressure gauge also deviates, use the handheld calibrator to calibrate the pressure and current values.

• Calibrating the transmitter’s pressure value:

Select either ‘Low Range Fine Adjustment’ or ‘High Range Fine Adjustment’. If the pressure value measured by the transmitter is below –5000 Pa, select ‘Low Range Fine Adjustment’; if the pressure value is above –5000 Pa, select ‘High Range Fine Adjustment’. Then press the right button, enter the pressure value –5000 Pa, and finally press the right button again to confirm. The pressure and current values will then be successfully calibrated simultaneously.

• Next, continue to apply pressure slowly by turning the fine-tuning handle anti-clockwise until the pressure reaches approximately the calibration value. Then, use the micro-adjustment knob to slowly adjust to the calibration point of –10,000 Pa; the current value should be 4 mA. If there is an error, calibrate using the handheld calibrator (follow the procedure described above).

2. Depressurisation calibration procedure

• Rotate the fine-adjustment handle clockwise to reduce the pressure until it is approximately at the calibration value. Then, use the fine-adjustment knob to slowly adjust to the calibration point of –5,000 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 pressure increase and decrease) meets the requirement (≤0.05% FS).

3. Depressurisation Procedure

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

III. Precautions

Proper Use

• Operations must follow standard procedures; pressurisation and depressurisation using the ALKB9601 portable micro-pressure pump must be carried out slowly to avoid pressure surges.

• The wiring of the differential pressure gauge must be clearly identified; take great care to prevent reverse connection.

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

• Upon completion of the test, first slowly depressurise the system 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 ALKC400E 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.

• The portable micro-pressure pump should be depressurised before long-term storage and covered with a cloth cover to prevent dust from entering the instrument.

IV. Conclusion

This differential pressure transmitter calibration operation completed bidirectional testing across the full range of positive and negative pressures, verifying the instrument’s hysteresis characteristics and zero-point stability. Measurement deviations were effectively corrected, ensuring that the correspondence between pressure measurements and current output complies with metrological standards. Standardised testing procedures and operational practices are central to ensuring the accuracy and reliability of industrial pressure measurement. Combined with regular metrological verification and routine operational maintenance, these measures sustain the metrological performance of instruments, providing a solid foundation for the precise monitoring of critical on-site parameters such as differential pressure, liquid level and flow rate, and ensuring the long-term stable operation of production systems.