The Effect of Air Laden Soluble Salts on Dew Point Measurement Using Condensation Hygrometers

摘要

Condensation hygrometers are used as humidity transfer standards for calibrating a wide variety of humidity sensors and instruments. Condensation hygrometers measure the partial pressure of water in gases by directly measuring the dew point temperature. Dew point temperature measurement is accomplished by controlling the temperature of a condensation target to equilibrium where the mass of condensate (dew or frost) is constant. The temperature of the condensation surface is, by definition, the dew point. A variety of feedback control methods to maintain mass constancy are utilized including optical, acoustic wave, capacitance, resistance or mass measurements. When used to measure environments laden with soluble salts, deposits that accumulate on the condensation target will affect the localized water vapor pressure resulting in errors. Raoult's. law provides a model to describe the rationale for errors due to salt deposits. Since there are differences betWeen the vapor pressure over water versus ice, the colligative properties of a salt dissolved in solution will also affect the freezing point of water, thereby creating ambiguity in humidity measurements. An experiment was conducted where an optical chilled mirror condensation sensor was connected to a dew point generator that produced constant dew points at various levels. A dilute solution of saltwater was allowed to dry on the surface of an optical chilled mirror sensor to produce a salt residue. The results were compared against a clean chilled mirror. Methods of mitigating the effects of deposited salts such as manual cleaning with prepared solutions and distilled water as well as automated self-cleaning methods were compared to the baseline readings.