The combination of water vapor and hydrogen peroxide vapor determines the relative saturation (RS). Relative saturation is dependent on the concentration of water and hydrogen peroxide vapor, as well as the temperature of the air. The higher the temperature, the more water and hydrogen peroxide vapor the air can hold. The lower the humidity level, the more H2O and H2O2 molecules can be added before condensation.
Standard relative humidity sensors are not recommended in vaporized H2O2 conditions because they lack a protective catalytic layer to break down the hydrogen peroxide molecules. When a humidity sensor is exposed to harmful concentrations of hydrogen peroxide it is likely to drift out of accuracy.
The amount of sensor drift will depend on the H2O2 concentration and exposure time. Since humidity sensors are designed for water vapor, hydrogen peroxide vapor will cause a stronger response in the sensor. This means that relative saturation calculated with measurements from normal humidity sensors may cause greater measurement error, especially in higher vH2O2 concentrations.
Alternatively, a humidity sensor with a catalytic layer can be used. The catalytic layer provides additional protection for the sensor by decomposing the harmful vH2O2. As a result, the sensor can only measure a relative humidity level, leaving the relative saturation level unknown. It is also worth mentioning that vH2O2 can cause additional measurement errors during bio-decontamination.
Some of the catalyzed humidity will still be measured in the sensor, creating a small positive error. The magnitude of this error is somewhat uncontrollable due to factors like vapor injection, flow speed, filters, and temperature. The sensor error caused by vH2O2 can be estimated by assuming that all the converted vH2O2 is being measured in the sensor.
For example, the maximum error influence of the catalytic filter is about +1 %RH at 300 ppm vH2O2, or +3 %RH at 900 ppm vH2O2 (at 23°C).
A humidity sensor with a catalytic layer is a natural choice in applications where humidity measurement is not necessarily needed to control the actual bio-decontamination process, but is instead used for controlling humidity conditions between cleaning cycles.
In addition to these humidity sensor features (non-catalytic and catalytic), there is an alternative solution that combines both technologies with added value. Vaisala’s PEROXCAP® technology contains both types of humidity sensors. This combination of humidity sensors, one with and one without a catalytic layer, overcomes the limitations of individual sensors.
Thus, PEROXCAP® sensors contained in Vaisala's HPP270 series probes are able to accurately measure both relative humidity and saturation as well as vH2O2 concentration.
Chemical purge for improved stability
Sensor drift caused by vH2O2 depends on various factors: H2O2 concentration, exposure time, amount of water vapor, and air temperature. For a catalytic sensor without chemical purge or heating, the maximum recommended vH2O2 concentration is 400 ppm in non-condensing environments. Under these conditions, the recommended total exposure time is 100 hours between sensor calibrations. If the concentration exceeds 400 ppm, or the sensor is exposed to condensation, the calibration interval should be adjusted accordingly.
The HPP270 series probes include heating and chemical purge as standard features. These features make the probes an ideal solution for vH2O2 biodecontamination applications due to the high accuracy of the dual-sensor PEROXCAP® technology. Chemical purge is available as an option on several Vaisala humidity products such as Indigo series transmitters, HMM170 humidity module, and HMP smart probes.
A humidity sensor with a catalytic layer benefits from this feature because the periodic heating also improves catalytic layer stability.
If the bio-decontamination process target is to reach a subvisible condensation state, it is recommended that the probe be equipped with a composite sensor for the sensor heating cycle. The HPP270 series probes and humidity probes with chemical purge are suitable for both types of processes: non-condensing and condensing vH2O2 biodecontamination.
