Accuracy
Basic Considerations
Sensors components detecting relative humidity consist
of a sensing element – a polymer in most of the cases –
which absorbs and desorbs water molecules,
depending on the surrounding conditions. The process
of gaseous molecules entering and leaving solid
material is quite sophisticated and bears many special
effects.
However, to make humidity sensors applicable to
commercial products and ease our lives at reasonable
cost, these special effects must be brought to a simple
understanding which can be tested with standard
equipment which is commonly available. It is clear that
such a simple understanding leaves some grey zones,
which are not scientifically correctly covered. Still, a
specification shall give the user a reliable tool to
understand the sensors and plan his own devices.
For a simple but thorough understanding, the accuracy
of relative humidity sensors may be divided into three
different rather independent terms: Calibration
accuracy, hysteresis and long term drift. Beyond this,
short term stability is an issue and there are some
effects at extreme conditions – i.e. very cold, hot, humid
or dry environments.