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CROP ECOLOGY, MANAGEMENT & QUALITY

Correction of Thermocouple Psychrometer Readings for the Interaction of Temperature and Actual Water Potential

Boyce Thompson Institute for Plant Research, Tower Road, Ithaca, NY 14853 USA

jpc8{at}cornell.edu

The standard method of temperature correction for a thermocouple psychrometer only yields satisfactory results for a limited range of water potential and temperature. Contrary to assumptions in standard practices, there is an interaction between the actual water potential under measurement and the temperature correction. Fortunately, the errors associated with ignoring this interaction are often small, and data are presented here that permit an assessment of whether the error is within acceptable limits under specific experimental conditions. More elaborate algorithms are given that can be used more robustly across a wide range of measurement conditions if needed. The temperature responses of two commonly used commercial psychrometers were examined, and it was found that this interaction resulted in model-specific correction algorithms. More specifically, it was found that the frequently made assumption of a correction factor that changes linearly with temperature is a satisfactory approximation across a range of at least 15 to 35°C. However, it was also found that the slope describing how this correction factor changes with temperature itself changes as a function of the actual water potential being measured. The details and magnitude of this effect were model specific.

Abbreviations: RH, relative humidity • µV, microvolt