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Thermal Dissipation Probe (TDP)
The newest method of transpiration measurement is now available from Dynamax for large trees and plants. The Thermal Dissipation Probe (TDP) transpiration sensor measures sap velocity which is converted to volumetric flow rate. TDP is a simple and affordable device originally proposed by Granier. The basic TDP probe has two thermocouple needles inserted in the sapwood, the upper one containing an electric heater. The probe needles measure the temperature difference (dT) between the heated needle and the sapwood ambient temperature below. The dT variable and the maximum dTm at zero flow provide a direct conversion to sap velocity. For uniform trees in a closed canopy, only one sensor per tree is needed. For irregular canopies or with mixed species, sap flow varies around thecircumference of large trees. Thus multiple probes are recommendedin a single tree to make flow calculations accurately. Normally, installtwo probe sets per tree for trees 3” to 6” (75 to 150 mm) in diameter,and four probe sets per tree for trees 6” to 18” in diameter (>150 mm).Calibration is recommended for new species.
The Dynagage Sap Flow Sensors are the latest technology for measuring the sap flow, and thus the water consumption of plants. These energy balance sensors measure the amount of heat carried by the sap which is converted into real-time sap flow in grams or kilograms per hour. The sensors are non-intrusive and not harmful since the plants are heated up 1ºC to 5ºC typically. The principles of heat balance sensors are scientifically proven and references exist for most major crops and many tree species. Unlike other methods, Dynagages require no calibration since sap flux is directly determined by the energy balance and rates of heat convection by the sap flow. The need for this new technology is great because it is an affordable and practical way to measure the water use by plants of agricultural, economic and ecological importance. Plants in greenhouses, nurseries or in natural environments can be measured with the same ease. Dynamax introduced the first sap flow sensor prototypes in 1988 and todayoffers a full range of sensors from 2 mm up to 150 mm.