Introduction To Relative Humidity

SENSOR TECHNOLOGY

All of the probes offered by Lee-Dickens use RH sensors of the capacitive type. The sensors are constructed with one plate etched on to a metallised glass substrate which is very thinly coated with active polymer. The second plate consists of a moisture permeable metallic film over the polymer. This method of construction gives the capacitive sensor a very quick response time.

Indeed the sensors we use respond to such minute changes in humidity that when digital meters reading to 0.1mV are used, users may be disconcerted by rapid fluctuations in last digit value.  This is NOT a fault indication. In practice, the use of filter-type sensor guards will reduce the rate of signal fluctuation, as will the use of analogue panel meters.

IMPORTANT NOTE:

Sensors come to equilibrium with the environment regardless of whether probes are switched ON or OFF.  Relative Humidity levels are directly related to the temperature of the area being monitored. As a generalisation it may be stated that the %RH of air with the same water content will change by 0.5% when air temperature changes by only 0.1°C.

It is most important therefore that probes be allowed time to come up (or down) to the same temperature as the ambient atmosphere.

In general, bringing a ‘cold’ probe into a warmer atmosphere will cause initial humidity readings to be too high; conversely initial readings will be ‘low’ when taking a probe into a colder environment.