Stem psychrometers are used to record the plant water potential of living plants without damaging them. Compared to more commonly deployed leaf psychrometers, they offer the advantage of being considerably easier to attach. This minimises disruptions to the energy balance which in turn improves the overall measurement accuracy.

Setup

The stem psychrometer consists of two welded, series-connected chrome contantan thermocouples inside a chrome-plated brass chamber,  forming a large insulating thermal mass. One thermocouple is in direct contact with the test stem, while the second simultaneously measures the air temperature in the chamber and then serves as a wet bulb thermometer.  A third, soldered copper constantan thermocouple used for temperature compensation is situated inside the body of the sample chamber to determine its temperature. All temperature measurements are entered into the calculation of the plant water potential. 

Installation

The stem psychrometer is attached to the stem by clamps using light pressure. These clamps are available in two sizes for stem diameters of up to 2 cm and up to approx. 5.5 cm. The psychrometer can also be attached to thicker stems using special solutions. The measurement can be either psychometric (wet bulb thermometer measurement) or hygrometric (measurement of dew point) and consistently permits the production of precise and reproducible measurement of the plant water potential with the help of automatic correction of the temperature gradients inside the measurement chamber. With good thermal insulation the setting half-time can be reduced to 60 seconds, making this a very fast and reliable instrument.

Data aquisition

Every stem psychrometer contains a Smart Sensor Interface which converts the microvolt measurement signal into calibrated stem water potential. The integrated microprocessor keeps the selected calibration equation and measurement frequency in the memory and provides regulated excitation voltage and the Peltier cooling current so that no complicated programming and wiring of the sensor is necessary. In conjunction with the ICT Smart Logger, use of the stem psychrometer can be flexibly adapted to various operational objectives in an accurate, user-friendly and efficient manner.