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

The equilibration between the water potential of air inside the psychrometer and that of calibrating solutions, soils, and plant leaves is examined. The results are compared with those in the literature and with a theory for predicting equilibration times. Techniques aimed at reducing equilibration times are devised and tested. A method which employs a psychrometer to determine the adsorption isotherm of its own wall material is used to assess the suitability of three materials for chamber construction. The main findings include: (1) Times for equilibration of solution and soil samples were as brief as 8 min and 1–3 h, respectively, and times for ‘equilibration’ of tobacco leaf samples obtained with the improved psychrometer are significantly shorter than those reported for previous designs of Spanner-type psychrometers. These improvements enhance the calibration and operational convenience of the method. (2) Current criteria for deciding the attainment of “equilibria” with leaf samples may result in a large error, termed equilibration error. With pine and wheat leaf samples this error may be more than −460 J kg−1, and about −1200 J kg−1, respectively. A criterion for reducing equilibration error is described. (3) Predicted equilibration times for leaf samples are generally far too short. Several factors that are neglected in the theory and which probably account for most of this discrepancy are described. (4) A new technique is outlined which enables up to a 15-fold reduction in the ‘equilibration’ times of pine needle samples. (5) Stainless steel (type 316) appears to be the most suitable chamber material of those tested.