Appendix D: Chilled Mirror Sensors; Introduction; Depression - GE OptiSonde User Manual

Introduction

Depression

Chilled Mirror Sensors
For use with the OptiSonde hygrometer, GE Sensing offers a choice
of three chilled mirror sensors which differ primarily in their
depression (cooling) capability. Depression capacity determines the
minimum dew point that can be measured. All of the sensors feature
low-noise, infrared optics, a field-replaceable mirror, and can be
located up to 300 ft (91 m) from the electronics.
Many of these features were pioneered by GE Sensing, and are
available only on GE Sensing products.
A chilled mirror sensor is generally selected so that its depression
capability will allow it to measure the lowest dew/frost point
anticipated for the application.
A Peltier device is a solid-state heat pump. It has one surface
thermally bonded to the body (base) of a dew point sensor and the
other surface bonded to the mirror block. When current is supplied to
the Peltier device, heat is "pumped" from the mirror block to the
sensor body where it is dissipated. With full cooling current, the
mirror block will eventually cool to its minimum temperature. The
difference between the temperatures of the mirror block and the
sensor body when the mirror block is at this minimum temperature is
defined as the depression capability of the sensor.
Depression capability is a function of how many "stages" the Peltier
device has stacked in series. Thus, a two-stage sensor typically has
60°C to 65°C (108°F to 117°F) of depression capability, and can
measure lower dew/frost points than a one-stage sensor which has
45°C (81°F) of depression capability. Depression is normally
specified at 25°C (77°F) ambient temperature. As ambient
temperature (and, therefore, sensor body temperature) is decreased,
depression capability also decreases, due to the drop-off in efficiency
of the thermoelectric cooler.
Therefore, there are limitations to using liquid-cooled sensors to
increase low-end measurement range. At nominal dew/frost points,
approximately one third of the additional cooling is lost due to cooler
inefficiency, and does not result in additional measurement range. At
low dew/frost points, as much as one half may be lost. As ambient
temperature is increased, depression capability increases, resulting in
a wider measurement range.
October 2007
D-1