An Improved Thermocouple Hygrometer for In‐situ Measurements of Soil Water Potential

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Abstract
A new dewpoint hygrometer for in‐situ measurements of soil water potential was designed and its performance evaluated. A porous silver membrane serves as the hydraulic connection between the soil and the measuring cavity which encloses a four‐terminal thermojunction made by cross welding chromel and constantan wires 0.001 in. (0.00254 cm) in diameter. The membranes were screwed to the copper body of the hygrometer and such configuration minimized the temperature difference between the vapor source and the reference junctions. When a thermal gradient was imposed on a hygrometer placed horizontally in a soil column, the maximum voltage offset observed was 0.05µV, but the soil water potential values as indicated by the hygrometer readings remained constant at −0.56MPa.This new device has a fast response for water flow with a time constant of approximately 5 min, and the time constant for salt diffusion in and out of the membranes is about 8 min. The results of soil water potential measurements under corn (Zea mays) plant hybrid United 108 grown in a growth chamber environment have shown that this new device is capable of following the changes in soil water potential very well. The difference in water potential observed between leaf and soil after the plant had been left several hours in the dark appeared to be related to uneven moisture distribution in the pot, and the plant was responding to the average soil water potential.
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