SD Series Dry Cabinets Setup and Operation Manual | A62-3337-00 Rev: 1.0.0 Page 106
Relative Humidity Sensors:
There are two basic types of humidity sensors: mechanical and electrical. Both require precise measurement
of one or more temperature values and, depending on the type, some other electrical property of the sensing
element or physical parameter like barometric pressure. Here are some of the most common methods for rel-
ative humidity measurement:
• Chilled Mirror (AKA: Condensation Hygrometer)
• Wet Bulb, Dry Bulb (AKA: Psychrometer)
• Aluminum Oxide Sensor
• Polymer Capacitor Sensor
• Human Hair Hygrometer (AKA: HHH)
Chilled Mirror (AKA: Condensation Hygrometer) – This system actually measures the dew point temperature
and then calculates Relative Humidity. It does so by bouncing light off the surface of a mirror. It then cools the
mirror until the light is scattered by the formation of dew on the surface, or the dew point temperature. Accu-
rate measurement of the ambient temperature (Ta) and the temperature of the mirror when the dew begins
to form, (Td) or dew point temperature, enables calculation of RH using the following equation:
The constants: 7.5892 and 240.71 are only good for ambient temperatures -20°C to 50°C resulting in an accu-
racy of about +/-0.09%.
Chilled mirror RH sensors are some of the most accurate, since they depend on the measurement of tempera-
ture only, which can be measured very accurately. This measurement is not affected by pressure or other
gases in the sample, but does depend on the ambient air being a stable temperature, as the measurement
takes some time to complete. If the mirror’s surface is contaminated, the measurement can be adversely im-
pacted. Since this system is mechanically complicated, bulky, and costly, its accuracy of better than 1% RH is
best suited for maintaining environments for use as a calibration source for simpler RH sensing systems.
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