Potential Sources of Errors in Estimating Plant Sap Flow Using Commercial Thermal Dissipation Probes

Thermal dissipation systems are a conventional technique to estimate sap velocity by determining the temperature difference between sap flow probes. The Thermal Dissipation Probe (TDP) method is commercially available and has been used by many researchers and professionals to estimate sap flow in plants. However, some errors and practical issues cause inaccuracy when evaluating plant sap velocity with this technique. Specifically, the sources of these errors are from the effect of the ambient thermal gradient (i.e., error caused by the cold junction location), the underestimation of nighttime sap flow, and the deficient thermal contact between the probe and tree body. This article focuses on errors associated with the traditional TDP, which effect on the thermal difference between the reference and heated probes. These errors vary as a nonlinear function of the transmitter length and the numbers of cold junctions which are affected by changes in the ambient temperature. Keywords: Ambient thermal gradient, Sap velocity, Thermal dissipation probe (TDP), Thermocouple cold junction.