Design, Measurement Techniques and Application of Thermocouple Psychrometers: Historical Review

The determination of soil water potential is useful in the establishment of irrigation guidelines for
agricultural crop management practices, and requires the use of different methods such as
thermocouple psychrometers (TCPs). TCPs have been widely used. However, this complex technique
is difficult to understand and the instrumentation needed is difficult to handle. Therefore, a profound
knowledge of the different aspects involved in the technique–which are not clearly explained in the
different studies published–is required. This review presents the different psychrometers developed
since the first instruments were built back in the 1950s, recalls the different denominations that have
been given to these instruments, and provides an updated classification of TCPs that attempts to
clarify previous classifications that were vague or unclear. Moreover, this review discusses the three
possible modes of operation of TCPs: the psychrometric technique, the hygrometric technique, and
the isopiestic technique. The isopiestic technique has been addressed in few studies and applied
under specific conditions. This review describes the evolution of the different models and instruments
used for laboratory and in situ measurements, and analyzes the problems of each model during use
(temperature gradients and fluctuations, sample size and preservation, equilibration time
measurement, vapour absorption on chamber walls, dirty instrumentation…) and the improvements
suggested to solve these problems. The review provides a detailed analysis of the different
applications of TCPs in the field of agronomy. In agronomy, TCPs were used to conduct
measurements both in the field and in the laboratory using different samples, such as plant material,
soil samples or fruits, among others. The studies have been classified based on the different
applications made and on the comparisons with other measurement methods based on psychrometry
or on other techniques. The most relevant non-psychrometric methods mentioned are the pressure
plate, the dye method or the pressure chamber, among others. Such comparisons enable the
establishment of the discrepancies observed between the different methods and their possible
causes. This review contributes a critical approach to psychrometry and suggests future research
lines from the perspective of practice and research that can be useful in the evaluation of the different
causes of the errors observed in the latest TCPs models, which have not been studied so far.