Sensors for efficient measurement of relative humidity in H2O2 environments
ELPRO Lepenik presents a range of sensors from our partner Rotronic for measuring relative humidity. From the range of sensors we highlight HC2A-S-HH sensor for measuring relative humidity, temperature and dew point, with high accuracy.
– The Rotronic HygroMer® HH-1 capacitive humidity sensor does not provide simultaneous results for humidity (inH2O2-containingenvironments) and condensation. In this case, the measurement signals present with condensation depend only on the secondary H2O2 concentration.
– The humidity sensor isH2O2-resistantand provides key measurement results before and after the end of the condensation phase (“dry state”) when in the set state.
– Correctly measured values are displayed again after the end of exposure toH2O2.
– To keep these condensation phases on the sensor element as short as possible, it is advisable to permanently remove the protective filter on the sensor head and place the probe in a position favourable to the flow.
Moisture sensor checks and test results report that hydrogen peroxide gas-water vapour occurs rapidly in the sterilisation chambers during the injection phase. The latter causes condensation on the moisture probes.
As a result, the sensors are exposed to normal ambient temperature (approx. 22 °C) beforeH2O2evaporates. The injection phase is usually very rapid, ranging from 28-30 °C. The relative humidity rises quite quickly above 90% RH due to the injection ofH2O2. The thermal mass of the sensor causes a delayed increase in the temperature of the sensor head. At very high relative humidity, this results in a very small difference between the dew point and the temperature of the sensor head.
If the dew point temperature reaches the temperature of the sensor head, condensation occurs on surfaces that are colder than the dew point temperature. This means that a coating of micro droplets (water withH2O2) appears on these surfaces. This has a negative effect on the humidity sensor and its electrical connections.
The sensor then displays a measurement signal consisting of the actual humidity signal and additional influences. These include: creeping currents in the case of a condensation coating (depending on the density of the coating and the specific conductivity), single droplets on the surface of the sensor, cross-currents due to the formation of galvanic cells in the case of condensation (electrochemical reaction), pre-contamination (evaporation residues from previous charging cycles).
This phenomenon is not constant for the actual humidity signal and depends on the individual physical conditions at the time of measurement. In connection with the measurement signal during the injection and exposure phase, it should also be noted thatH2O2decomposes rapidly at higher temperatures. In the case of condensation, this decomposition reaction generates additional water, which, when evaporated or formed in the gaseous phase, creates additional moisture content. This leads to higher relative humidity values than would be expected. This condensation phase continues until the water layer dries again. In the dry state and in the ventilation phase, the humidity sensor then correctly detects the amount of water vapour in the gaseous phase (standardised to WMO relative humidity).
Pharmaceutical, medical, calibration laboratories, food processing, measurement and processes requiring high accuracy sensors …
Katarina Žunko, source ROTRONIC: ”Measurement of relative air humidity in H2O2 environments”