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 needles, whilst the HFD2 uses 62 mm thermistor probes with a 82mm heater needle.

Using up to eight measurement points, spaced at 5mm (HFD2) or 10 mm (HFD1) intervals, 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.

The purchase of the HFD Installation Kit is essential to perform the installation of the instrument on a tree.

The measurement needles of the HFD Sap Flow Meter are made of hollow 1.6 mm stainless steel tubing. They can easily be bent, damaged or broken off if basic care is not taken in handling the instrument. The significant length of the needles relative to this very thin diameter demand additional attention to detail when handling or transporting the needles. Once installed in sleeves within the tree the needles very robust.

NOTE: the needles gain much of their structural strength when combined with the sleeves.

ICT provides sleeves with a closed end to prevent sap and resin from filling the sleeve and gluing the needle in place. This also enables the sleeves to be left in place without the HFD needles installed and to be used again at a later time without the sleeves having become blocked by sap & resin.

NOTE : if it is intended to use the sleeves for more than one growing season it may be advisable to allow a small excess of sleeve protruding from the stem surface to allow for tree growth and increment of the stem, otherwise the sleeves will become engulfed and obscured by the tree making reuse difficult. If this technique is employed it is very important to make allowance for this to ensure the specific depths of insertion for the measurement points are known and adjusted using a insulated spacer

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.