Improved Thermocouple Psychrometer for the Measurement of Plant and Soil Water Potential

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Abstract
This paper is the first of a series which describes: (a) the design and operation of a thermocouple psychrometer which provides improved range of measurement, accuracy, and equilibration rate of water potential determinations of plant and soil samples, and which avoids or minimizes six sources of large error, one or more of which occur with previous psychrometers and (b) studies that provide a better understanding of the thermocouple psychrometric method. The present paper describes the main difficulties encountered with the method, examines those due to absorption phenomena, and describes an improved thermocouple psychrometer which overcomes or reduces some of these difficulties. Improvements include: (i) chamber walls of stainless steel type 316 and samples arranged in shielding geometries to reduce delays in equilibration due to adsorption, (ii) sample holders of geometries that avoid or reduce errors of leaf resistance, adsorption and tissue damage when appropriate cooling periods and tissue segments of adequate size are used, (iii) shielding arrangement of respiring tissue samples to permit thermal equilibration in the region sensed by the ‘active’ thermojunction, a new thermocouple assembly for increased range of measurement of water potential, (v) sample holders that permit plant or soil samples to be accommodated in the one chamber.

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