HFD Heat Field Deformation Meter


For the measurement of sap flow or transpiration in plants. Complete plant water status monitoring solutions:

  • Radial Sap Flow Profiles.

  • Mapping Hydraulic Architecture.

  • Hydraulic Redistribution in Roots & Stems.

  • Standalone system with integrated logger and battery.

  • Wireless Connectivity & Data Transfer.

case study

Principle of measurement

The HFD technique is a thermodynamic method based on measuring the ΔT of the sapwood both symmetrically (in the axial direction, above and below) and asymmetrically (in the tangential direction or to the side) around a line heater.

The heater is continuously heated at approx 50 mA and generates an elliptical heat field under zero flow conditions. Sap flow significantly deforms the heat field by elongating the ellipse as shown in the photo of a thermal image of a HFD measurement.

The symmetrical temperature difference (ΔTsym) allows bi-directional (acropetal and basipetal) and very low flow measurements, whereas asymmetrical temperature difference  (ΔTas) is primarily responsible for the magnitude of medium and high sap flow rates.

By using the ratio of measured temperature differences and applying correction for each measurement points local conditions using the adjustable K-values the common features of the medium (such as variable water content, natural temperature gradients and,
wound effects) have negligible impact on sap flow calculations. 

The value for parameter K is equal to the absolute value of ΔTs-a or ΔTas for a zero flow condition. Under flow conditions the parameter K can be extrapolated with accuracy using linear regression.