What Is Optical Condensation Hygrometry - GE HumiLab Operator's Manual

Also mounted within the test chamber is a platinum RTD air
temperature sensor, providing air temperature information (dry
bulb) to the chilled mirror hygrometer. A microprocessor calcula-
tion converts the chamber dew point and air temperature data to
percent relative humidity.
A microprocessor-based relative humidity controller receives the
RH information. It also looks at the RH setpoint which has been
selected on the front panel by the operator. If there is a difference
between the two readings, the controller adjusts the ratio of wet to
dry air entering the test chamber, at 5-minute intervals. This closed
loop operation ensures that the resulting relative humidity in the
chamber remains within ±0.2% R.H. of the chilled mirror hygrom-
eter reading.
A multi-point calibration of the test chamber against the built in
chilled mirror has been performed at the factory, at 10% R.H. incre-
ments. At each point, a correction factor, determined by the actual
dew point and temperature measurement, has been stored in
non-volatile memory. Thus, the test chamber has acquired the
accuracy and N.I.S.T. traceability of the fundamental chilled mirror
hygrometer. If desired, this automatic calibration of the test
chamber against the chilled mirror hygrometer may be repeated at
any time in the field.
Optical condensation hygrometry is a precise technique for deter-
What is optical
mining the water vapor content in gases by measuring dew or frost
condensation
temperatures. Because these hygrometers are so accurate, they are
hygrometry?
widely used as a standard in many of the world's metrology labora-
tories.
Optical condensation hygrometry works on the chilled-mirror prin-
ciple and provides a fundamental measurement of dew/frost point
temperature. A metallic mirror surface is cooled until it reaches a
temperature at which condensation begins to form on it. The dew
layer is optically detected and the mirror is held at that tempera-
ture. The mirror temperature, measured with a platinum resistance
thermometer, is an accurate indicator of the dew or frost point.
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Page Chapter 4 — Theory of Operation