The Dark Spot Technique - Explained

The optical surface is the key element of the sensor cell and comprises an acid etched, semi-matt surface with a central conical shaped depression. A collimated beam of visible red light is focussed onto the central region of the optical surface to form an annulus ring of light.

Optical detection is made of light dispersed. As the sensor is cooled during measurement cycle, hydrocarbon condensates form on the optical surface and its optical properties are modified - the reflected light intensity of the annulus ring increases and there is a dramatic reduction in the scattered light intensity within the dark spot region. This highly sensitive secondary effect is used to determine the hydrocarbon dew point.

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