In our blog post "Importance of Steam Quality in Moist Heat Sterilization," we already discussed the need to measure temperature and pressure and their interrelationship for proper sterilization results. In this post, we would like to delve deeper into this topic.
Accurate measurement of temperature, both in a distribution study and in a penetration study, and the precise measurement of chamber pressure are important parameters in the validation of a steam autoclave. The relevant norms differ between the Anglo-American and European areas concerning the parallel measurement of these two parameters. In this blog post, we would like to go into more detail about the importance of pressure measurement.
For further information, we recommend looking at the relevant standards like
- ISO 17665
- EN 285
- HTM2010
- EN 554 (replaced by ISO 17665, yet still widespread in its application)
- PDA Technical Monograph #1 (Validation of Steam Sterilization Cycles)
Why is measuring chamber pressure important in the validation of a steam autoclave?
Measurement of the chamber's pressure is a crucial part of validating a steam autoclave. According to the principles of thermodynamics, as expressed in the Ideal Gas Law (PV=nRT), pressure ("P") plays a pivotal role in regulating temperature ("T") and thus the production of saturated steam, considered as the optimal medium for sterilization. Adequate pressure promotes steam penetration by guiding the steam into hard-to-reach areas of the goods to be sterilized. If the pressure is too low to achieve saturated steam, there is a risk of insufficient sterilization. Pressure measurements are not only essential for real-time monitoring of sterilization cycles but also for ensuring continuous operation within defined pressure limits, as prescribed by standardized operating procedures and guidelines. This helps in verifying the integrity of vacuum systems and identifying leaks that could affect sterilization results.
Reaching and maintaining a state of saturated steam is also vital for sterilization success. Should signs of diminished saturation occur, e.g., irregular temperature or pressure values, it could suggest a deteriorating steam quality that requires further investigation.
Which pressure sensors are used?
There are various types of pressure sensors designed to meet specific demands in industrial applications. Piezoresistive sensors are valued for their changing resistance under mechanical stress. Capacitive sensors use changes in capacitance for pressure measurement, while piezoelectric sensors, producing electric voltages under pressure, are particularly suitable for dynamic pressure measurements. Differential pressure sensors measure pressure differences between two points, and absolute pressure sensors determine the pressure in relation to the perfect vacuum.
In sterilization processes, such as steam sterilization and autoclaves, absolute pressure sensors are widely used. These sensors measure the pressure relative to absolute zero (perfect vacuum), which is advantageous in environments where pressure measurements have to be carried out against a constant reference.
The pressure sensors used in such processes should be capable of precise measurements at high temperatures and under high pressures and should be resistant to materials typically used in sterilization (mostly steam or other gases). Furthermore, they should be easy to clean and sterilizable.
In addition to the mentioned operational, and at times adverse circumstances, that a pressure sensor is exposed to when used in a steam autoclave, optimal calibration plays a crucial role for accurate measurement and subsequent measurement result.
What should be considered during calibration?
Temperature compensation is a crucial factor in the calibration of pressure sensors and other measuring devices. Many sensors react to changes in temperature, which can lead to inaccuracies in the measurement data. A sensor calibrated at room temperature may not yield the same results at higher or lower temperatures.
Temperature compensation is the process of correcting these temperature-related deviations. There are typically two methods of temperature compensation:
1. Physical temperature compensation: This often involves the use of materials or designs that react little or not at all to changes in temperature. But sometimes it can also mean that the response of one component to temperature changes is offset by the opposite response of another component due to clever design.
2. Electronic temperature compensation: This relies on the use of electronic circuits or software algorithms to correct temperature-related deviations. This often involves the use of a separate temperature sensor, measuring the current temperature and adjusting the measurement data of the pressure sensor accordingly.
When calibrating pressure sensors, especially in critical applications like sterilization processes, it's crucial to calibrate the sensor under conditions that come as close as possible to its actual operating environment. If the sensor operates at high temperatures, it should also be calibrated at high temperatures. This ensures all temperature-related deviations are captured and corrected during the process of calibration.
Conclusion
In this blog post, we delve deeper into the importance of accurate temperature and pressure measurements in the validation of steam autoclaves, with a special focus on the pivotal role of chamber pressure. Indeed, proper pressure control is crucial for the production of saturated steam and thus complete steam penetration.
In such sterilization processes, several types of pressure sensors can be used, including piezoresistive, capacitive, piezoelectric, differential, and absolute pressure sensors. Absolute pressure sensors are particularly preferred due to their special features. All sensors used in these processes should be highly resistant to temperature and pressure, sterilizable, and easy to clean.
Proper calibration of the sensors is vital for accurate measurement results, taking into account temperature compensation. Ideally, calibration should be conducted under actual operating conditions. Many sensors react to changes in temperature, which can lead to inaccuracies in the measurements. In particular, a sensor calibrated at room temperature might produce variant measurements at higher or lower operating temperatures.
To combat such temperature-related deviations, calibration is performed under actual operating conditions. This ensures the sensor accurately reflects real conditions and guarantees the accuracy and reliability of the measurements in the given environment.
Therefore, it's essential to scrutinize the calibration certificate closely. Pressure sensors are often calibrated at room temperature, but it's important to insist on calibration at the actual working temperature of the steam sterilization process, typically 121.1°C. Only in this way can an error due to insufficient temperature compensation be detected and minimized.
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