Measurement Range - GE Optica Operator's Manual

Appendix D. Chilled Mirror Sensors
D.3 Measurement Range
The measurement range of a chilled mirror sensor is defined as the temperature range over which a stable dew or frost
layer can be maintained on the mirror. Note that in order to acquire a dew or frost layer on the mirror, the depression
capability of a sensor must extend below its measurement range. The minimum required differential between
depression range and measurement range is 5°C (9°F) at nominal dew/frost points, and increases to 10°C to 12°C
(18°F to 22°F) at very low frost points.
Measurement range is normally specified at 25°C (77°F) ambient temperature in air at atmospheric pressure. For
sensor body temperatures other than 25°C (77°F), measurement range can be estimated by first estimating depression
capability, and then decreasing this range according to the minimum required differential. For most gases other than air,
the effect on measurement range is negligible. However, gases such as hydrogen or helium, which are more thermally
conductive than air, will result in a decrease of several degrees in the measurement range. Measurement range will
decrease as gas pressure is increased, because the increased density (and, therefore, increased thermal conductivity) of
the gas results in an increased heat load. For air or nitrogen, each 50 psi (3 bar) increase above atmospheric pressure
will result in a loss of approximately 2°C (4°F) of depression capability. Conversely, operating under vacuum may
result in a small increase.
Other factors influencing sensor selection include temperature and pressure ratings, and whether anticipated dew points
will be higher than ambient temperature.
98 Optica™ Operator's Manual