GE Optica, Operator'S Manual

Page: 16 / 150

January 2006
1-4 Features and Capabilities
Dew Point Sensors
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Model 1111H — Single-stage sensor
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Model 1211H — Two-stage sensor; high pressure and temperature
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Model D-2 — Two-stage sensor
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Model SIM-12H — Two-stage heated sensor
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Model 1311DR — Four-stage, liquid or air cooled sensor
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Model 1311XR — Five-stage, liquid cooled sensor
Temperature Sensor
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Model T-100E
Pressure Sensor
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Model PT-30A or PT-300A
Theory of Operation
Optical condensation hygrometry is a precise technique for
determining the water vapor content in gases by directly measuring
dew point or frost temperatures. Using this technique, a metallic
mirror is cooled until it reaches a temperature at which a thin layer of
condensation begins to form on it. The dew layer is detected optically,
and the mirror is held at that temperature. The mirror temperature,
measured with a platinum resistance thermometer, is an accurate
indicator of the dew or frost point. Because these hygrometers are so
accurate, they are widely used as a standard in many of the world’s
metrology laboratories.
Hygrometer Function
Figure 1-2 on page 1-5 illustrates how GE Infrastructure Sensing
hygrometers detect and measure dew point. The condensate mirror is
illuminated with a high-intensity, solid state, light emitting diode
(LED). A photodetector monitors the LED light reflected from the
mirror. The photodetector is fully illuminated when the mirror is clear
of dew, and it receives less light as dew forms. A separate LED and
photodetector pair are used as a known reference to compensate for
any thermally induced changes in the optical components. The
photodetectors are arranged in an electrical bridge circuit, the output
current of which is proportional to the light reflected from the mirror.
The bridge output controls the electrical current to the thermoelectric
cooler.
A large bridge current develops when the mirror is dry, causing the
mirror to cool toward the dew point. As dew begins to form on the
mirror, less light is reflected, and the bridge output decreases. This, in
turn, causes a decrease in cooling current. A rate feedback loop
within the amplifier ensures critical response, causing the mirror to
stabilize quickly at a temperature that maintains a thin dew or frost
layer on the mirror surface. A precision thermometer element
embedded within the mirror directly monitors this dew point
temperature.