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 the circumference of large trees. Thus multiple probes are recommended in a single tree to make flow calculations accurately. Normally, install two 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.


  • Dual needle, Granier design with Dynamax upgrades
  • Easily inserted and removed for reuse
  • Constant heat, thermal dissipation method
  • Continuous measurement method
  • No waiting periods, no heat pulses
  • Stainless steel needles, Teflon coated
  • Compatible with most dataloggers
  • Differentially wired T-type thermocouples
  • Electronics & connectors sealed - weatherproofed
  • Wires directly to datalogger, 1 differential channel each
  Length Diameter T-Types T/C's Probe Spacing Power Cable Heater Resistance Operating Volts
TDP-10 10 mm (.4") 1.2mm 1ea 40 mm .08 to .12 W 10ft/5 Cond 26 Ohms 2.0 V @~8ºC
TDP-30 30 mm (1.2") 1.2 mm 1ea 40 mm .15 to .2 W 10ft/5 Cond 50 Ohms 3.0 V @~8ºC
TDP-50 50 mm (2") 1.65 mm 1 ea 40 mm .32 W 10ft/5 Cond 78 Ohms 5.0 V @~8ºC
TDP-80 80 mm (3.2") 1.65 mm 2 ea 40 mm .5 W 10ft/6 Cond 110 Ohms 7.5 V @~8ºC
TDP-100 100 mm (3.4") 1.65 mm 3 ea 40 mm .44 W 10ft/7 Cond 145 Ohms 8.0 V @~8ºC