New Instrument for Measuring Evaporation; Application to Evaporation of Water through Thin Films

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Abstract
A new instrument quickly and accurately determines evaporation rates of water. A probe passes helium over a small area of the water surface; the vapor picked up by the helium returns through the probe and is continuously measured by a thermal conductivity cell. Accuracy and repeatability are within 1% absolute. The apparatus is sensitive to a change in evaporation rate of less than 1%; response is 5 to 20 sec, depending upon the magnitude of the step change. The instrument, called an evaporometer, is especially useful for rapidly screening water‐evaporation retardants and for determining evaporation rates through monomolecular films spread on the water surface of the film balance. Evaporation rate vs film pressure isotherms, determined simultaneously with pressure vs area isotherms, should yield additional information about the composition and structure of the monolayers. Although the evaporometer was developed for the determination of water evaporation, it is adaptable to nonaqueous liquids.

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