Measuring Plant and Soil Water Potentials with Thermocouple Psychrometers: Some Concerns

Thermocouple psychrometers are commonly used to measure soil and plant water status. Users often assume that these instruments can be used routinely with little regard for understanding how they operate, or for the precautions required for accurate measurements of water potential. Poor procedures and inadequate attention to both practical and theoretical requirements can jeopardize the successful performance of these highly sensitive instruments. For example, in addition to understanding the theory of operation of thermocouple psychrometers, calibration and proper cleaning of these instruments are among the most important steps in successfully using them. Important sampling precautions include consistency of technique, prevention of evaporative loss during collection and sealing, and careful sample selection. Major concerns during measurements of water potential include achieving complete vapor pressure equilibrium, interpreting data, and detecting and preventing temperature gradients. Although many precautions for in situ measurements are similar to those for measuring soil and plant samples, each use has specific requirements. Thermocouple psychrometers are useful for accurate and reliable measurements of water potential if proper techniques and precautions are used. Published with the approval of the Director, Agric. Exp. Stn., Univ. of Arkansas, Fayetteville, AR.