Temperature Sensor; Theory Of Operation; Hygrometer Function - GE Optica Operator's Manual

Chapter 1. Features and Capabilities

1.4.1 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

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Model T-100E

1.4.3 Pressure Sensor

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Model PT-30A or PT-300A

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 metal 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.

Figure 2 on page 5 illustrates how GE Measurement & Control hygrometers detect and measure dew point. The

condensate mirror is illuminated with a solid state, infrared emitter (IR). A photodetector monitors the IR 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.

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Optica™ Operator's Manual