Extended Field Use of Screen‐Covered Thermocouple Psychrometers1

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Abstract
The use of screen‐covered thermocouple psychrometers for in situ measurements of soil water potential has been criticized because of possible soil and micro‐organism contamination through the screen covering. Double‐junction Peltier psychrometers with a screen covering were installed in the soil under field conditions for periods ranging from 2 to 40 months of continuous exposure, and were then removed and examined for evidence of loss of calibration sensitivity and contamination by soil particles and micro‐organisms. The uncleaned psychrometers were recalibrated in the laboratory and then disassembled and examined under a microscope (250 ✕) for contamination. Although slightly more than half of the units lost some sensitivity after field exposure, the average decrease was less than 5%. The psychrometer cavities were all nearly free of soil particles, the thermocouple junctions were shiny, and no evidence of microbial attack was evident. It appears that long‐term field exposure of screen‐covered psychrometers is no more detrimental to their performance than that of ceramic or other psychrometers.

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