When selecting a humidity sensor for VHP sterilization transfer window, it is important to consider whether it can work stably and accurately in the harsh environment of VHP (vaporized hydrogen peroxide), which is highly oxidative and corrosive, for a long time. Improper selection can lead to rapid sensor failure and reading drift, directly affecting the validation of the sterilization process and product safety. The following are the core considerations, recommended types, and selection suggestions when choosing a humidity sensor:

　　1、 Core challenges and requirements
1. Strong oxidizing environment: High concentrations of gaseous H ₂ O ₂ can corrode the sensing materials or metal components of most sensors.
2. Wide humidity range: It is necessary to measure the high humidity environment from the drying stage (possibly as low as 1% RH or below) to the wetting/sterilization stage.
3. Accuracy and stability: Sterilization process validation requires reliable data, sensors need to be stable for a long time, and drift should be small.
4. High temperature tolerance: Some sterilization cycles or cleaning processes may be accompanied by temperature rise.
5. Quick response: able to track rapid changes in humidity during the sterilization process.

　　2、 Recommended sensor technology types
In VHP environment, capacitive polymer sensors are the least recommended because their moisture sensitive polymer layer is easily oxidized by H ₂ O ₂ and permanently damaged. The mainstream and reliable choices are the following two:
1. Aluminum oxide (Al ₂ O3) humidity sensor
Principle: Based on the capacitance change of porous alumina film, measure the partial pressure of water vapor and output it as dew point/frost point temperature.
Advantages:
1. Strong corrosion resistance: Aluminum oxide itself is an inert material with good resistance to H ₂ O ₂, making it a traditional choice in VHP environments.
2. Measuring extremely low humidity: Very suitable for measuring low dew points during the drying stage (up to -80 ° C or below).
3. Good long-term stability.
Disadvantages:
In high humidity environments (>95% RH), accuracy will decrease and response speed will be relatively slow.
2. The output is usually the dew point value. If relative humidity (RH) is required, it needs to be converted in conjunction with the temperature value.
Applicable scenarios: Very suitable for monitoring and process control during the drying stage of VHP transfer windows.
2. Quartz crystal microbalance (QCM) or surface acoustic wave (SAW) sensor
Principle: Humidity is measured by the frequency change caused by the adsorption of water on the crystal surface, usually directly outputting the partial pressure of water vapor or ppmV.
Advantages:
1. High chemical resistance: The crystal surface usually has a special inert coating (such as silica), which has strong resistance to H ₂ O ₂ corrosion.
2. High precision and fast response: It has high precision throughout the entire range and extremely fast response speed.
3. Direct measurement of water vapor concentration: Output parameters (water vapor partial pressure, ppm) are very suitable for process development and validation.
Disadvantages:
1. The price is usually higher than that of alumina sensors.
2. It is also necessary to calculate relative humidity in conjunction with temperature.
Applicable scenarios: The choice for high-performance and high reliability applications, especially suitable for VHP sterilization processes that require fast response and high precision throughout the entire process.

　　3、 List of Key Selection Parameters
When selecting a specific model, please be sure to verify the following parameters:
Resistant to H ₂ O ₂ concentration: Clearly label the long-term tolerable H ₂ O ₂ gas concentration (e.g. up to 1000 ppm or higher).
Measuring range:
1) Humidity: Covering at least 1% -100% RH, or the corresponding dew point range (such as -80 ° C DP to+60 ° C DP).
2) Temperature: Covering the temperature range that may be involved in the sterilization cycle (such as 0 ° C-60 ° C or wider).
Accuracy: Accuracy is crucial in low humidity areas (<5% RH), typically requiring ± 1% RH or ± 2 ° C DP.
Long term stability: The smaller the annual drift, the better (such as<0.5% RH/year).
Response time: T63 should be as short as possible (e.g.<10 seconds) to track rapid changes.
Output signal: Common 4-20mA, 0-10V or digital interfaces (such as RS485, Modbus), compatible with transfer window PLC or SCADA system.
Probe material: The sensor housing and protective cover should be made of 316L stainless steel or equivalent corrosion-resistant materials.
Protection level: At least IP65 to prevent water ingress during cleaning.
Certification: Whether there are relevant certificates applicable to the pharmaceutical industry (such as CE, RoHS), or compliance with GMP requirements for data integrity.