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Other Hygrometer
Applications
Theory of Operation and Glossary
Many GE Sensing chilled mirror hygrometers are used in industrial
applications in addition to metrology. The OptiSonde condensation
hygrometer is not readily damaged or contaminated by industrial
process gases that can degrade other secondary measurement schemes
such as saturated salt and polymer-based sensors. If the sensor or
sampling components should become contaminated with oils, salts,
etc., they can be cleaned without harm to the sensor or impairment to
the system accuracy. The performance of the hygrometer can be
checked at any time by heating the mirror above the dew point,
causing the dew deposit to evaporate, then reclosing the servoloop
and checking to see that the system cools and returns to the same dew
point.
The GE Sensing OptiSonde condensation sensors cover a wide range
of applications limited only by the heat pumping capabilities of the
thermoelectrically-cooled mirror.
At high dew points (up to 100°C), the sensor is limited by the thermal
properties of the solid state OptiSonde components as well as the
thermoelectric heat pump capacity.
In a typical application measuring sub-ambient dew points, a two-
stage thermoelectrically-cooled mirror can reach a temperature
approximately 65°C lower than an ambient (heat sink) temperature of
+25°C. The thermoelectric cooler pumps heat from the mirror into the
heat sink. By reducing the temperature of the heat sink, even lower
dew points can be measured. In meteorological applications where
the heat sink temperature is considerably lower, frost points down to
–40°C can be monitored.
For lower levels, GE Sensing's Optica series features four and five
stage chilled mirror sensors.
October 2007
E-3
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