Place the temperature sensor at the central point of the high-pressure steam sterilizer to be calibrated, and let the sterilizer undergo a complete sterilization cycle. During the heating process, ensure that the air inside the sterilizer chamber is completely exhausted. After reaching the sterilization temperature, record a set of data every 2 minutes for 30 minutes, resulting in 15 sets of data. The temperature fluctuation is calculated as (maximum actual temperature – minimum actual temperature) among the 15 sets of measured data ÷ 2, and expressed with the symbol “±”.
According to GB/T 30690-2014, during the sterilization cycle, the measured value of the sterilization temperature range should generally not exceed the set value by more than 3°C, and the pressure range should correspond to the measured temperature range. Meanwhile, the measurement uncertainty should be considered to be between 1/5 and 1/3 of the maximum allowable indication error of the equipment under test, i.e., 0.6–1.0°C. This article focuses on discussing the central point temperature and pressure calibration results of high-pressure steam sterilizers with different volumes under two common calibration methods. The calibration environment conditions are: temperature 20.0–21.0°C and relative humidity 45%–50%.
There are two common calibration methods for temperature and pressure calibration:
Method 1: Wireless Temperature and Pressure Data Logger Calibration Method
A wireless temperature and pressure data logger records the temperature and pressure data collected by its sensor internally. Before the test, program the wireless data logger with the corresponding parameters, then place the logger at the central point of the sterilizer chamber to be calibrated for the calibration process. After the test, read the recorded data using a computer with a matching signal converter and supporting software.
Currently, such data loggers have a temperature measurement accuracy of ±0.1°C and a pressure measurement accuracy of ±0.1 kPa. They can record nearly 20,000 data points at a time, powered by an internal lithium battery with a service life of approximately 5 years. This calibration method avoids external connecting wires, ensuring the sterilizer chamber remains well-sealed and the data is true and reliable. The device can directly measure the actual pressure inside the sterilizer chamber. However, the initial cost of this set of equipment is as high as several hundred thousand yuan, and its service life is only about 5 years. Additionally, data reading and analysis require a computer; real-time data monitoring would incur even higher costs. Therefore, this type of device has not been widely promoted among grass-roots metrology calibration institutions and sterilizer users.
Method 2: Thermocouple Calibration Method
A thermocouple is a common temperature-measuring sensor. Before the test, place one end of a thin T-type thermocouple (usually with a wire diameter ≤ 1.0 mm) at the central point of the high-pressure steam sterilizer chamber. Close the sterilizer and ensure it is as well-sealed as possible. After the sterilizer starts operating, a small amount of steam will escape through the thermocouple lead-out port, but the sterilizer can still continue heating. During the heating process, exhaust the air inside the sterilizer chamber. When the sterilizer is in the sterilization state, collect the temperature data using a data logger.
Convert the measured temperature data into theoretical pressure values using the Antoine equation, and compare these values with the data measured by the wireless temperature and pressure data logger. This method uses thermocouples, which are widely available, making the calibration of high-pressure steam sterilizers convenient and simple. However, the introduction of the thermocouple creates a persistent air/steam leakage point throughout the sterilization process, which affects the test results to a certain extent.