A miniature stem thermocouple hygrometer.

An unprotected chromel-constantan thermocouple was mounted in a cavity (4 x 2 x 1 mm) with rounded corners in a chrome-plated brass block (10 x 6 x 4 mm). When installed against a soybean (Glycine max [L.] Merr.) xylem face, sealed with caulking gum, and insulated with polyurethane foam and aluminum foil, even rapidly changing stem water potentials could be followed accurately. Temperature gradients can be a problem.