A temperature-controlled plant psychrometer

When measuring plant water potential in conditions of fluctuating temperature, commercial psychrometers exhibit large errors which are caused by a temperature difference between the reference junctions and the chamber air. A novel psychrometer capable of field use is described which incorporates an electronic device for controlling reference junction temperatures. It consists of a chamber with a thermocouple mounted in it for measuring dewpoints and another thermocouple for sensing the temperature gradients. A thermo-electric heat-pump keeps the temperature of the reference junctions the same as that of the chamber air. Good agreement was found between measurements made with this psychrometer and independent measurements made with the destructive pressure chamber method in the glasshouse and in the field under conditions of widely fluctuating temperature. Corresponding measurements with a commercial psychrometer without temperature control failed to show such agreement. Heavy thermal insulation is necessary, despite the temperature control, which can restrict the psychrometers used. Possibilities for further improvements are discussed.