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This article in AJ

  1. Vol. 76 No. 1, p. 166-168
    Received: Mar 15, 1983

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Water Condensation on Peltier-Cooled Thermocouple Psychrometers: A Photographic Study1

  1. Herman H. Wiebe2



Water condensation on Peltier-cooled thermocouple psychrometers (used to measure water potential) was observed and photographed under a microscope. Hundreds of minute droplets condensed on the entire junction within a second. As Peltier cooling continued these grew and coalesced, and after 1 min some 50 to 20 droplets uniformly covered the junction. After 5 min these had grown and merged into one (sometimes 2) large drop(s) that adhered to only part of the junction and to a lead wire. This drop grew by coalescence of the droplets that continued to condense on the junction until the drop eventually attained a constant diameter of about 0.3 mm, 6 times that of the junction. Evaporation during the psychrometric water potential measurement stage appeared uniform from the entire junction surface after short condensation times when it was covered with many small droplets. The two-step procedure used to measure very low water potentials requires a large amount of condensed water. Poor psychrometric cooling is a hazard, however, if a drop only partly covers the junction after prolonged condensation times.

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