Interpreting the Heat Field Deformation method: Erroneous use of thermal diffusivity and improved correlation between temperature ratio and sap flux density

► The original HFD sap flux density method is thermodynamically incorrect. ► Misinterpretation of thermal diffusivity leads to incorrect sap flux density units. ► The HFD ratio is dependent on sapwood properties and sap flux density itself. ► An improved relationship between the HFD ratio and sap flux density is put forward.