Understanding the sap flow in the plant system gives critical information about the hydraulic concentration level between the plant and the soil, and this is possible with the HFD Heat Field Deformation Sap Flux Meter (HFD).

The HFD Heat Field Deformation technique is ideally suited to sap flow research projects that require the measurement of extended radial sap flow profiles to accurately map hydraulic architecture of trees. Similar to the Heat Ratio Method sap flow sensor SFM1x, the Heat Field Deformation Sap Flux Meter can measure high sap flow rates as well as low to zero and reverse sap flow.

HFD1 and HFD2

The HFD Heat Field Deformation Sap Flux Meter is available in two variants, the HFD1 and the HFD2.

• The HFD1 utilises 97 mm thermistor probes with a 117mm heater needle with eight measurement points, spaced at 10 mm (HFD1) intervals
• The HFD2 uses 62 mm thermistor probes with a 82mm heater needle with eight measurement points, spaced at 5mm (HFD2) intervals

To ensure a reliable installation, it is highly recommended that an installation kit is purchased.

The HFD range store data as a stand-alone logging solution, with data stored on an internal MicroSD card. The data is stored in csv format for analysis either within Sap Flow Tool or within other common statistical packages. For advanced users, the data is also stored in a raw form that can be analysed in Sap Flow Tool Software.

Principle of measurement

The HFD technique is a thermodynamic method based on measuring the dT 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 (dTsym) allows bi-directional (acropetal and basipetal) and very low flow measurements, whereas asymmetrical temperature difference (dTas) 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 dTs-a or dTas for a zero flow condition. Under flow conditions the parameter K can be extrapolated with accuracy using linear regression.