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Sap Flow 

Sensors are available in two and three needle configurations


  • Monitoring water use and irrigation scheduling in orchards

  • Estimating water use by trees in agroforestry

  • Monitoring species specific water requirements of trees in watersheds


  • Dimensions: Head is 45mm x 15mm; Needles are 35mm x 1.27mm diameter

  • Temperature Sensors: 10K Precision Thermistor

  • Material: Epoxy and Delrin head, stainless steel needles

  • Cable length: 2m standard (additional cable available.)


The East 30 Sensors Sap Flow Sensor consists of two or three 35mm long stainless-steel needles spaced 6mm apart. One needle contains an Evanohm heater and the others contain three precision thermistor sensors evenly spaced at 5mm, 17.5mm, and 30mm. The needles are inserted into holes drilled in the trunk of a tree with the heater placed below (upstream from) the thermistors in the two needle style or between in the three needle style. A current is applied to the heater for 8 seconds, at which time the temperature of the thermistors is monitored. The flowing sap carries the heat pulse to the sensors. The time taken for the pulse peak to reach the temperature sensors is monitored. This time is directly related to the sap flow velocity. The velocity is used, along with the sapwood area, to compute the transpiration rate of the tree. The three thermistors provide measurements of flow at three depths in the sapwood, from which the velocity profile can be deduced. In the three needle variation, the direction of sap flow can also be easily determined.

What you need

  • Heater control interface: One interface will run up to 5 sensors. 

  • Drilling Guide: for precise spacing of needles in the tree, a drilling guide is recommended for more accuracy in spacing as well as increased ease of installation.

  • Size 55 drill bit for best fitting installation

  • Recommended, but optional mini hex chuck to hold drill bit securely for drilling.

  • Campbell Scientific Datalogger for accurate results                                                  

In the Package

Simple, sturdy and accurate Sap Flow sensor. Free sample programs to help you with datalogger programming as well as Macros to help with data processing can be requested and sent via email. Heater control interfaces, Drilling guides, and the correctly sized drill bits can be ordered for an additional cost.


Sensors are reusable, if they can be safely removed from the tree. Unfortunately this is not always the case- sensors can become cemented in trees!


 Please see Data Logger compatibility to learn more about the the data logger specifications needed to obtain accurate results.

Selected Publications

Bleby, T.M., et al. 2008. Limitations of the HRM: great at low flow rates, but not yet up to speed? In ‘7th International Workshop on Sap Flow: Book of Abstracts’. (International Society of Horticultural Sciences: Seville, Spain)

Cohen, Y., M. Fuchs, and G.C. Green 1981. Improvement of the heat pulse method for determining sap flow in trees. Plant, Cell and Environment 4:391-397

Green, S., et al. 2003. Theory and practical application of heat pulse to measure sap flow. Agronomy Journal 95: 1371-1379.


Green, S., et al. 2008. A re-analysis of heat pulse theory across a wide range of sap flows. ISHS Acta Horticulturae 846: VII International Workshop on Sap Flow. DOI: 10.17660/ActaHortic.2009.846.8

Marshall, D.C. 1958. Measurement of sap flow in conifers by heat transport. Plant Physiology 33: 385-396.

Pearsall, K.R., et al 2014. Evaluating the potential of a novel dual heat-pulse sensor to measure volumetric water use in grapevines under a range of flow conditions. Functional Plant Biology 41: 874-883.

Renner, M., Hassler, S. K., Blume, T., Weiler, M., Hildebrandt, A., Guderle, M., et al. (2016). Dominant controls of transpiration along a hillslope transect inferred from ecohydrological measurements and thermodynamic limits. Hydrology and Earth System Sciences, 20(5), 2063-2083. doi:10.5194/hess-20-2063-2016.