Temperature Gradient Effects on in situ Hygrometer Measurements of Water Potential1

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Abstract
The effects of temperature gradients on water potential measurements with various commercial and laboratory soil hygrometers were tested in a soil column at —18 bars or in a sponge column wetted with a —18 bar sodium chloride solution. Temperature gradients were induced in the medium by warming or cooling either the base or the top of the column for several hours. Hygrometers in which the measuring (wet) junction was located in the middle of the cylindrical sample surface gave small temperature gradient errors, less than 3 bars when the temperature gradient in the surrounding medium was 1 degree cm−1. Locating the measuring thermocouple near the base of the cylindrical sample surface gave errors ranging from 10 to 18 bars per gradient of 1 degree cm−1. Hygrometers with an “end window” sample surface gave errors ranging from 23 to 62 bars. Water potential readings were consistently too low (dry) when the sample surface was cooler than the measuring junction, and too high (near zero) when the sample surface was warmer. Accurate water potentials were obtained with all models when medium temperature was uniform. Double junction, so called “temperature compensated” units, gave the largest temperature gradient errors. The zero offset voltage of single junction models was a systematic function of the temperature gradient in the medium, and could be used as a warning that temperature gradients exist and in calibrating individual sensors.

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