Capacitive Sensors

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. 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.

SENSOR LIFE All primary measuring probes are prone to damage by the environment in which they are used and the more sensitive the probe, the more prone it is to damage. Humidity sensors are no exception to the rule and anyone using these devices should accept that the sensor is unlikely to last forever. On the question of "How long will the sensor last ?", we can not say. If the humidity measurement is taking place in a computer suite or a clean room then the sensor should last for many years. On the other hand if the humidity measurement is taking place in a process where the temperature is fluctuating a great deal, the probe experiences regular periods of saturation and the atmosphere contains reducing or oxidising agents, then the sensor life will be shortened. If you have a particular application in mind, please call Lee-Dickens' internal sales office who will offer you the benefit of many years experience.