A paper about the comparison of several sap flow sensors, co-authored by Dirk De Pauw (Phyto-IT), was published in Agriculture and Forest Meteorology. A laboratory test and field evaluation were conducted to determine the accuracy of the three commonly used techniques for measuring sap flux density in trees: heat pulse velocity, thermal dissipation and heat field deformation. In the laboratory test a constant flow rate of water was maintained through freshly cut stem segments of diffuse-porous Fagus grandifolia trees. The three sensor types were measured simultaneously and compared against gravimetric measurements. All three techniques substantially underestimated sap flux density. It is concluded that a species-specific calibration is necessary when using any of these techniques to insure that accurate estimates of sap flux density are obtained, at least until a physical basis for an error correction can be proposed.
Full reference details:
Steppe K., De Pauw D.J.W., Doody T.M. and Teskey R.O. (2010). A comparison of sap flux density using thermal dissipation, heat pulse velocity and heat field deformation methods. Agricultural and Forest Meteorology, 150(7-8), 1046-1056.