A small scale dewpoint humidity measurer

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Abstract
A sampling dewpoint measurer is described for small scale humidity determination. Dew is formed on a thermoelectrically cooled plate and is detected by the scattering of a modulated infrared light beam reflected from the plate. A phase sensitive detection system is used, avoiding the need to light-trap the probe. Dewpoint depression temperature is sampled by a copper/constantan thermocouple at the instant dew disappears from the plate. This value is amplified and displayed on a two-digit panel meter, which holds a value until it is updated by the next sample. Knowing the dewpoint depression temperature, the relative humidity can be found from a table given an approximate knowledge (to within 5 degrees C) of the ambient temperature.

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