Appendix D. Chilled Mirror Sensors; Introduction; Depression - GE Optica Operator's Manual

Appendix D. Chilled Mirror Sensors

Appendix D. Chilled Mirror Sensors
D.1 Introduction
GE offers a choice of six, fully interchangeable, 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. Other advanced features, depending on the sensor selected, include heating capability, four- or five-stage
thermoelectric cooling, air and water cooling for additional depression, and modular, field-replaceable optics and
cooling assemblies
Many of these features were pioneered by GE Measurement & Control, and are available only on GE 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.
D.2 Depression
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. For liquid-cooled sensors, it is specified at the coolant 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.
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