Photo-tube Type Automatic Dew Point Indicator

First Report: Measuring Principle

Abstract

The true dew point should be the temperature at which the thermal equilibrium is established so that the amount of dew on the mirror of dew point indicator neither increases nor decreases, remaining constant. In conventional dew point indicator, however, contamination on the mirror causes water vapor being adsorbed on the surface and low pressure dew is formed before the true dew point is reached rendering the determination essentially uncertain.
A means was devised with which the thermal equilibrium is automatically maintained. By letting first a small amount of dew form on the mirror, subsequent change in the amount of dew form on the mirror, subsequent change in the amount of dew becomes solely depending on the humidity of the surrounding air, the thermal equilibrium being unaffected by the forming of low pressure dew.
Two photo-tubes connected in series receive light from a source, one directly through an adjustable slit and the other by reflection from the indicator mirror. Weakening of the latter intensity due to forming of dew on the mirror is countervailed by narrowing the slit. By means of an electronic circuit and by using proper width of the slit, resultant photo-current automatically controls the mirror temperature to balance the two photo-currents thus quickly establishing and correctly maintaining thermal equilibrium.