TABLE OF CONTENTS Humidity, usually understood to refer to the water content of the air, could also be sensed using silicon based sensor elements. Relative humidity (RH), which is the ratio of absolute humidity to saturation humidity, has a value between 0 and 1 (or 0 per cent and 100 per cent). Several techniques are used to measure the relative humidity using capacitance, resistance, conductivity and temperature based measurements. Thermoset polymer or thermoplastic polymer based materials are used on silicon or ceramic based substrates for RH. A comparison of RH sensors is available in Reference 32.
Capacitive RH sensors dominate both atmospheric and process measurements, and are the only type of full range RH measuring devices capable of operating accurately down to 0 per cent RH. Because of their low temperature effect, they often are used over wide temperature ranges without active temperature compensation.
Thermoset polymer (as opposed to thermoplastic based polymer) capacitive sensors (see Figure 14.43) allow higher operating temperatures and provide better resistivity against chemical liquids and vapours such as isopropyl, benzene, toluene, formaldehyde, oils, common cleaning agents, and ammonia vapour in concentrations common to chicken coops and pig barns. In addition, thermoset polymer RH sensors provide the longest operating life in ethylene oxide (ETO) based sterilisation processes.
An example of a thermoset...
