GE Optica, Operator'S Manual

Page: 125 / 150

January 2006
D-2 Chilled Mirror Sensors
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.