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Thermal Dissipation Probe

The Thermal Dissipation Probe (TDP) directly measures sap velocity which can be converted to volumetric flow rate given an accurate measurement of sapwood area for the tree. TDP is an uncomplicated and inexpensive method ideally suited to whole plant transpiration measurements on large diameter trees. The principle of measurement was developed by Dr. Andre Granier of INRA in France and has been widely adopted by the plant science research community since 1996. Granier, A., 1987 Sapflow Measurements in Douglas-Fir Tree Trunks by Means of a New Thermal Method Ann Sci Forest 44, (1): 1-14.

Principle of measurement

The Thermal Dissipation technique measures the difference in temperature (dT) between a heated upper needle and a lower reference needle when placed in the sapwood, or conducting xylem of a woody stem. Using a regulated, known voltage, the stem is constantly heated to approximately 8^oC above ambient. As sap flows past the two needles, the lower reference needle records the ambient sap temperature and the upper heated needle is cooled. If rapid sapflow occurs the difference in temperature between the two needles is low as the heat input from the upper needle is being quickly dissipated. When sap flow is low or close to zero a maximum difference in temperature or (dTmax) is recorded because heat is no longer being dissipated from the heated upper needle. Granier's empirical equation then uses the measured dT and dTmax values to calculate sap velocity.

Sensor design

The standard TDP-30 probe consists of two needles joined together a fixed distance of 40mm apart. A copper and constantan thermocouple is located in each needle at half way distance or 15mm from the base of each needle. The upper needle also contains a fixed line heater that is constantly heated. The needles have a teflon coating designed to aid in removal from stems and assist in reusing the sensors for multiple installations.

Application

The TDP is well suited to whole plant transpiration or water use studies in applications such as forestry, horticulture and viticulture.When measuring sapflow on trees or plants under conditions of equal resource competiton such as a forest plantation, orchard or vineyard, a single sensor per tree or vine may be adequate. For trees growing in mixed species stands or native forests sap flow varies around the circumference of the trees due to varying degrees of competiton for resources. Under these situations it is advisable to measure the sapflow in each of the 4 quadrants of the tree and average these readings to achieve a more accurate total sapflow.