Standardised Operating Procedures for the Calibration, Testing and Measurement of Stainless Steel Pressure Gauges

To ensure the accuracy of pressure gauge measurements and the reliability of value traceability, and to standardise the entire process of pressure gauge calibration, testing and measurement, this procedure uses the ALKD91A electric pressure calibration bench and the ALKC400D precision digital pressure gauge as standard equipment to carry out full-range metrological performance calibration on stainless steel pressure gauges. The verification covers both positive pressure (atmospheric pressure) and negative pressure (vacuum) operating conditions. It strictly adheres to standardised procedures regarding ambient temperature and humidity, sealed connections, point-by-point pressure ramp-up verification, pressure reduction re-verification and pressure relief, whilst ensuring that pressure readings are taken at steady pressure and that the indicated values are recorded after gently tapping the case throughout the process. The procedure requires slow pressure application and standardised operations to ensure that the accuracy of the standard gauge exceeds that of the gauge under test. This is complemented by a regular calibration and maintenance mechanism to ensure the compliance of the verification results.

I. Pressure Gauge Calibration Tools

Name	Electric Pressure Calibration Bench	Precision Digital Pressure Gauge	Stainless steel pressure gauge
Model	ALKD91A	ALKC400D	Y-100BF
Technical Specifications	Pressure range: -0.1 to 6 MPa Medium: Clean air Outlets: 4 Connection: M20×1.5 （F） Power supply: AC 220±10%, 50 Hz Total weight: 36 kg Dimensions: 830 × 470 × 340 mm (electric pressure calibration bench) 290 × 210 × 150 mm (external booster pump)	Pressure: -0.1 to 6 MPa Accuracy: Class 0.05 Temperature compensation: With temperature compensation Power supply: Built-in rechargeable battery Connection: M20×1.5 （M） Net weight: 0.6 kg Dimensions: 110×50×185 mm	Pressure: -0.1 to 0 MPa / 0 to 6 MPa Accuracy: Class 1.6 Material: Stainless steel Dial diameter: φ100 mm Connection: M20×1.5 （M） Net weight: 0.35 kg Product dimensions: 100 × 42 × 140 mm
Product Dimensions

The entire verification process must be carried out in strict accordance with current metrological technical specifications. The main references include:  JJG52-2013 ‘General Pressure Gauges, Pressure-Vacuum Gauges and Vacuum Gauges with Elastic Elements’, which applies to the initial verification, periodic verification and in-service inspections of general pressure gauges, pressure-vacuum gauges and vacuum gauges with elastic elements, and covers on-site indicating pressure measuring instruments commonly used in industry.

二、操作流程详解

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

• First, connect the external booster pump to the external pump air supply port and power supply port on the electric pressure calibration bench, and tighten the connections. Connect the calibration bench to a 220 V power supply and switch it on.

• Connect the standard gauge ALKC400D and the pressure gauge under test (Y-100BF) to the outlet of the ALKD91A, ensuring a sealed connection; secure any unused outlets with plugs.

(2) Pressure Gauge Calibration Procedure

1. Pressure Build-up Procedure

• First open the shut-off valve, then turn the fine-adjustment pump anti-clockwise to the appropriate position, and close the relief valve.

• At this point, press the air pressure increase button to build up pressure to the required value; release the button as the pressure approaches the target value, then use the fine-adjustment pump to set the pressure to the calibration value of 1 MPa. Once the pressure has stabilised and the standard gauge reading no longer fluctuates, check the pressure reading on the pressure gauge under test. Then, gently tap the side of the pressure gauge’s casing with your finger (do not tap the glass or tap forcefully), observe the position of the pointer, and record the reading once it has stabilised after tapping. Use the reading after tapping as the actual calibration reading for that pressure point, and calculate the error.

• Continue pressing the air pressure boost button to increase the pressure; stop when the pressure is approximately at the calibration value, then use the fine-adjustment pump to adjust to the calibration value of 2 MPa. Check the pressure reading on the pressure gauge under test, tap the pressure gauge, and record the reading once it has stabilised after tapping.

• Repeat the same procedure for the calibration points 3 MPa, 4 MPa, 5 MPa and 6 MPa. Check the pressure reading on the pressure gauge under test, tap the gauge, and record the reading once it has stabilised. (Note: If the reading is found to drop gradually during pressurisation (indicating a slight leak), close the shut-off valve.)

2. Depressurisation Process

• First, press the depressurisation button to reduce the pressure; stop when the pressure is approximately at the calibration value, then use the fine-adjustment pump to adjust to the calibration value of 5 MPa. Observe the pressure reading on the pressure gauge under test, tap the gauge, and record the reading once it has stabilised after tapping.

• Repeat the same procedure to adjust the pressure to the calibration values of 4 MPa, 3 MPa, 2 MPa and 1 MPa. Check the pressure reading on the pressure gauge under test, tap the gauge, and record the reading once it has stabilised.

3. Depressurisation Process

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

(3) Vacuum Calibration Process for Pressure Gauges

1. Pressurisation Process

• First, remove the pressure gauge under test for positive pressure and fit the one for negative pressure. Turn the fine-adjustment pump clockwise until it can no longer be adjusted, then close the pressure relief valve.

• Press the vacuum boost button to create a vacuum; stop when the pressure reaches approximately the calibration value, then use the fine-adjustment pump to adjust the pressure to the calibration value minus 0.02 MPa. Check the pressure reading on the pressure gauge under test, tap the gauge, and record the reading once it has stabilised.

• Continue to use the vacuum boost button to create a negative pressure, stopping when the pressure is approximately at the calibration value. Then, use the fine-adjustment pump to adjust the pressure to the calibration value minus 0.04 MPa. Observe the reading on the pressure gauge under test, tap the gauge, and record the reading once it has stabilised after tapping.

• Repeat the same procedure to set the pressure to the calibration points of –0.06 MPa, –0.08 MPa and –0.1 MPa. Observe the pressure reading on the pressure gauge under test, tap the gauge, and record the reading once it has stabilised after tapping.

2. Depressurisation process

• First, press the pressure reduction button to reduce the pressure; stop when the pressure is approximately at the calibration value, then use the fine-adjustment pump to set the pressure to the calibration value of –0.08 MPa. Observe the pressure reading on the pressure gauge under test, tap the gauge, and record the reading once it has stabilised after tapping.

• Repeat the same procedure to adjust the pressure to -0.06 MPa, -0.04 MPa and -0.02 MPa below the calibration value. Check the pressure reading on the pressure gauge under test, tap the gauge, and record the reading once it has stabilised.

3. Pressure Relief Procedure

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

III. Precautions

Proper Use

• When measuring positive pressure, press only the ‘Pressurise’ button (do not press the ‘Vacuum’ button); when measuring negative pressure, press only the ‘Vacuum’ button (do not press the ‘Pressurise’ button).

• During calibration, ensure that the permissible error of the standard gauge is less than that of the gauge under test, to ensure that the calibration results are accurate and reliable, and comply with metrological standards.

• Ensure the external pressurisation pump is correctly connected to the ALKD91A electric pressure calibration unit, and that the unit is connected to a 220V power supply; do not connect incorrectly.

• Operations must follow standard procedures; pressurisation and depressurisation of the ALKD91A electric pressure calibration unit must be carried out slowly to avoid pressure surges.

• Pressurise to the calibration point and record the reading once the value on the standard gauge has stabilised; Once the pressure in the pressure gauge under test has stabilised, tap the case gently; use the reading immediately after tapping as the final reading to ensure accuracy.

• Upon completion of the test, first slowly depressurise to zero; only remove the instrument under test once all residual pressure has been released.

Maintenance and Calibration Intervals

• Have the ALKC400D precision digital pressure gauge metrologically verified at regular intervals (recommended once a year) to ensure measurement accuracy meets standards.

• When the pressure gauge is not in use, switch off the power supply; if not used for an extended period, recharge the battery once every three months.

• The lubricating oil in the electric pressure calibration stand should generally be checked and replaced every six months or after 500 hours of operation.

• Without the appropriate technical skills and testing equipment, do not open the cover to carry out maintenance; under no circumstances should components be adjusted or replaced.

• If the electric pressure calibration stand is to be stored 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

Upon completion of the pressure gauge calibration and testing, the measurement data across the full range must be verified item by item to confirm that the indication errors and hysteresis errors at each test point comply with metrological specifications. Standard operating procedures during calibration—such as applying and releasing pressure smoothly, taking readings at a stable pressure, and gently tapping the dial case—are key to ensuring accurate and reliable measurement results. On a daily basis, it is essential to implement regular calibration of standard equipment and maintenance of the electric calibration bench to ensure that testing capabilities remain compliant. Through standardised pressure gauge testing procedures, the metrological performance of instruments can be accurately assessed, providing reliable measurement support for on-site pressure measurement and production operations, and ensuring the accurate traceability of measurement values.