The surface resistivity of a cooled glass surface at the onset of water vapour condensation

Abstract

The work examines the basis of a dew-point hygrometer which senses the incipience of dew by the change in surface resistance of a cooled insulator. The study is primarily concerned with soda-lime glass surfaces although silica and polymers were briefly examined in the experimental work. The water vapour adsorption process ion glass is reviewed and the ensuing chemical and physical modification of the surface structure is noted. The water vapour to liquid water phase transition was shown to be basically a droplet nucleation process in surface features such as cracks and scratches. A condensation rate equation was developed, and extended to a model which simulates the nucleation process in surface pits and the growth rates of droplets. An experimental apparatus is described which enabled small insulator specimens to be cooled over a carefully controlled temperature range in a gas stream of constant humidity. The surface resistivity and a microscopic examination of the growing dew deposit were simultaneously monitored during the cooling process. The measured resistivity characteristics were in good agreement with earlier work, but have been more accurately compared with the thermodynamic dew-point temperature. Soluble surface materials in the soda-lime glass were shown to have a considerable influence on the temperature at which the dew deposit formed. A computer-based model of the surface film and coalescing droplets has predicted some of the observed resistivity inflection characteristics at the dew-point. The measured droplet growth rates were in good agreement with the predicted rates; and the rate process of condensation has been satisfactorily related to the time dependent surface resistivity characteristics.