Method 1: On-Site Calibration Method
This method is applicable to high-pressure steam sterilizers with small working volumes (such as portable and vertical types) that are equipped with manual steam release valves. Typically, a metal hose is provided at the end of the steam release valve of a high-pressure steam sterilizer to facilitate the proper discharge of cold air. Therefore, it is only necessary to reliably connect the metal hose to a pressure calibrator for verification and ensure airtightness—after which, on-site calibration of the pressure indicating instrument of the high-pressure steam sterilizer can be carried out in accordance with verification procedures.
Key Operation Points:
- The pressure resistance of the metal hose must be ≥ 400 kPa. If this requirement cannot be met, the hose should be replaced with a pressure connection hose that meets the specifications.
- The pressure airtightness of the calibration system shall not exceed 0.1 kPa/min.
Calibration Equipment:
| Equipment Type | Specifications |
| Standard Instrument | – Digital pressure gauge: Accuracy class not lower than 0.2, upper limit ≤ 500 kPa
– Precision pressure gauge: Accuracy class not lower than 0.4, upper limit determined by the pressure indicator under test |
| Supporting Equipment | Gas pressure calibrator: Stability better than 0.02% FS, working pressure > 600 kPa |
The upper limit of pressure calibration for the on-site calibration method is often restricted by the rated working pressure of the safety valve installed on the high-pressure steam sterilizer. If calibration cannot reach the upper-limit pressure of the pressure indicator under test, the calibration value corresponding to the maximum rated working pressure may be provided.
Method 2: Direct Comparison Method
This method is applicable to high-pressure steam sterilizers equipped with test connectors, usually large-scale, direct steam-type sterilizers.
Key Operation Points:
Connect the pressure standard instrument to the pressure connector in the pipeline of the sterilization chamber. If the pipe thread of the pressure connector is incompatible with that of the pressure standard instrument, an adapter shall be used to ensure reliable connection and meet airtightness requirements.
Calibration Equipment:
- Recommended standard instrument: Digital pressure gauge (accuracy class not lower than 0.2, upper limit ≤ 500 kPa) that can operate at 140℃ without changes to its metrological performance (or with performance that can be corrected).
- Alternative standard instrument: Bourdon-tube precision pressure gauge (accuracy class not lower than 0.4, upper limit determined by the pressure indicator under test). For this type, the allowable error shall take into account the additional error caused by temperature changes.
The indication error ΔPₙ is calculated using the following formula:
ΔPₙ ≤ ±(δ + 0.04Δt)% × Pₘ (1)
Where:
- δ = Accuracy class
- Δt = Difference between the operating temperature and the temperature during calibration
- Pₘ= Measuring range
All calibration items only target the pressure indication error. The direct comparison method uses the steam pressure in the sterilizer as the working medium. Due to the limitation of steam working pressure, calibration points cannot cover the full range of the pressure indicating instrument in accordance with procedures. Instead, calibration points must be determined within the operating pressure or temperature range of the sterilizer, relying on pressure controllers or temperature controllers. Additionally, although sterilizers are required to maintain constant pressure at the selected operating pressure, this is not practically achievable, and pressure fluctuations are inevitable. Therefore, during calibration, readings must be taken quickly and accurately, and the calibration result shall be determined by averaging multiple readings.
Method 3: Real-Time Comparison Method
This method is applicable to all types of high-pressure steam sterilizers, with the standard instrument connected via a wireless measurement method.
Key Operation Points:
Set the working procedure of the standard instrument in accordance with the operating instructions. After completion, place the instrument directly in the sterilization chamber near the pressure indicator under test. Start the sterilizer, and when it reaches normal operating status, calibration can begin. During calibration, attention should also be paid to the impact of pressure fluctuations in the high-pressure steam sterilizer on the calibration result.
Calibration Equipment:
Wireless pressure measurement system: Accuracy class not lower than 0.2, measuring upper limit ≤ 500 kPa, operating environment temperature ≥ 140℃. It should be capable of real-time pressure measurement, display, and recording of measurement results.
The real-time comparison method also uses the steam pressure in the sterilizer as the working medium, so its calibration principle is the same as that of the direct comparison method. The indication values of the standard instrument are generally collected and recorded directly by a computer at set time intervals; during calibration, only the indication values of the pressure indicator under test need to be recorded. Compared with the direct comparison method, it offers flexible installation positions, simple operation, and stronger applicability. Moreover, since a computer is used as the display and recording device during calibration, the process is intuitive, controllable, and more easily accepted by customers.
Calibration result processing follows Formula (2) (see below).
Method 4: Record Comparison Method
This method is also applicable to all types of high-pressure steam sterilizers, with key operation points identical to those of the real-time comparison method. The standard instrument used is a recording-type wireless pressure measurement system, which has the same technical requirements as the system used in the real-time comparison method.
Compared with the standard instrument for the real-time comparison method, the recording-type system has a smaller volume and is suitable for smaller working spaces. However, it lacks real-time measurement data transmission functionality—making it impossible to monitor the calibration process accurately, and only allowing passive acceptance of recorded results. To ensure reliable measurement results, it is often necessary to extend the calibration time; if the calibration result is unsatisfactory, multiple operations may be required to obtain the optimal result.
Calibration result processing follows Formula (2) (see below).
Temperature Calibration Method for High-Pressure Steam Sterilizers
Place the temperature sensor at the central point of the high-pressure sterilizer under test, and run the sterilizer through a complete sterilization cycle. During the heating process, ensure that the air in the sterilizer chamber is fully discharged. Once the sterilization temperature is reached, take a set of data every 2 minutes for 30 minutes (resulting in 15 sets of data).
The temperature fluctuation is calculated as follows:
(Temperature fluctuation) = (Maximum actual temperature – Minimum actual temperature) among the 15 sets of data ÷ 2, expressed with a “±” sign.
GB/T 30690-2014 specifies that during the sterilization cycle:
- The measured value of the sterilization temperature range shall generally not exceed the set value by more than 3℃.
- The pressure range shall correspond to the measured temperature range.
Meanwhile, the measurement uncertainty shall be between 1/5 and 1/3 of the maximum allowable indication error of the equipment under test (i.e., 0.6–1.0℃).
This article focuses on the calibration results of central-point temperature and pressure for high-pressure sterilizers of different volumes using two common calibration methods. The calibration environment conditions are:
- Ambient temperature: 20.0–21.0℃
- Relative humidity: 45%–50%
Unified Calibration Result Processing (Pressure Indication Error)
ΔP = Pₐ – Pₙ (2)
Where:
- ΔP = Pressure indication error of the calibrated pressure indicator (unit: kPa)
- Pₐ= Average value of the indication readings of the calibrated pressure indicator (unit: kPa)
- Pₙ= Average value of the indication readings of the standard instrument (unit: kPa)