Determining the Degree of Ambiguity in Frost Point Temperatures as Measured by an Optical Dew Sensor

摘要

The design and use of optical dew point sensors has raised questions concerning the ambiguities which might occur if measurements are made while the instrument is controlling on a supercooled dew layer at temperatures below freezing. This report outlines the theory of operation of one of these sensors--the ML-592( )/TMQ-11(v) Electronic Dew Point Sensor--and discusses the procedures and results of an extensive evaluation program to resolve these questions. The ML-592 is an optically sensed, cooled-mirror dew point device utilizing a proportionally controlled feedback loop to maintain the mirror at a temperature that permits the liquid (or solid) and vapor phases of water to exist in equilibrium. The tests were performed to determine if ambiguous readings, caused by the presence of supercooled water on the mirror at temperatures below freezing, could occur. If present, over what temperature range might this supercooled layer be expected. Evaluation of the results shows that a persisting supercooled dew layer at temperatures below 32F is highly unlikely during normal field operation. (Author)