A transient heat probe sensor for measuring transpiration in the stem of woody plants.

Abstract

A compact portable electronic system has been developed for measurement of sap flow through the xylem in the stem of woody plants. This system utilizes a single heated thermistor probe which is alternately and automatically switched between a constant (heating phase) and a variable (cooling phase) temperature mode of operation. Experiments reported here have been carried out in two distinct porous medium, packed bands and the stem of several apple trees. The system performance was tested to detect the variations in flow rate. In both mediums, results indicated that the single heated thermistor is able to work as flowmeter since its transient thermal response was affected by the local fluid flow. The natural logarithm of the fractional response of the thermistor probe indicated that the system can not be analysed as a first order model. Non-linear regression analysis showed that the relationship between the probe sensor temperature response and the time elapsed from the beginning of cooling phase is adequately fitted by an additive exponential model. A dimensionless heat transfer analysis, applied to the thermistor probe, is outlined. Problems encountered in the transient heat transfer analysis are reported.