The first psychrometers used to measure relative humidity above a soil sample were of necessity quite small. The two thermometers were made of tiny, fragile thermocouples. A thermocouple is a temperature sensor made from two dissimilar conductors joined at one spot. The thermocouple converts a temperature gradient into electricity, which can be measured to determine temperature changes.
Thermocouple psychrometers were first successfully used to measure water potential by D.C. Spanner before 1951, but it was a difficult measurement to make. To get the results he wanted, Spanner had to make his own wire out of bismuth antimony—according to John Monteith, a fume hood at Rothamsted bore the marks of these experiments for many years.
Others struggled to repeat his measurements. Samples took up to a week to equilibrate, and then the fragile thermocouples would often read just one sample before they were broken. Still, by 1961 Richards clearly saw vapor methods as the future of water potential measurements (Richards and Ogata, 1961).
Decagon (now METER) introduced its first commercial thermocouple psychrometer (the SC-10 Thermocouple Psychrometer Sample Changer, later TruPsi) in 1983. This instrument used a delicate thermocouple but protected it inside a sealed enclosure. Nine samples were equilibrated simultaneously and rotated under the thermocouple to be measured.
Prior to each measurement, the wet bulb thermocouple was dipped in a tiny reservoir of water. The electrical output of the thermocouple was sent to a nanovoltmeter, which had to be monitored to determine when the temperatures stopped changing.