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This article in AJ

  1. Vol. 62 No. 6, p. 705-708
    Accepted: June 26, 1970

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Four-terminal Peltier Type Thermocouple Psychrometer for Measuring Water Potential in Nonisothermal Systems1

  1. A. A. Millar,
  2. A. R. G. Lang and
  3. W. R. Gardner2



The construction, calibration, and tests of a miniature four-terminal thermocouple psychrometer are described. Improved thermal stability under nonisothermal conditions is the main feature of this new type of psychrometer. This is achieved using a separate thermocouple to cool and condense water vapor on the sensing thermocouple.

The sensitivity for this thermocouple psychrometer is about 0.4 #v/bar at 25C, and is operable at temperature fluctuations of about 0.5C during the course of a measurement.

In situ measurements of water potential in onion leaves under a cloudy day differed by 0,5 to 1 bar from measurements on detached leaves. With greater temperature fluctuations, water potentials obtained by the two methods differed by as much as 2 to 3 bars.

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