The Heat Field Deformation (HFD) technology is ideal for measuring radial sap flow profiles. This tool can therefore be used to create a hydraulic “map” of the tree architecture. High, low and negative volumes of sap flow can be determined using the HFD sensor technology. HFD measurements are an ideal supplement to SFM-1 measurements.
The HFD technology is a thermodynamic method based on measurement of the temperature gradients in axial and tangential direction inside the sap wood with reference to a linear source of heat.
The heater needle is heated at a constant level of approx. 50 mA, generating an elliptical heat field under zero sap flow conditions. This becomes more markedly deformed the more strongly defined the current sap flow is. The symmetrical temperature difference is used to measure the bidirectional flow in the case of very low flow volumes, while the average and high sap flows are determined using the asymmetrical temperature difference.
The HFD sensor consists of three measurement needles in combination with a heater needle. These are integrally connected to the Smart Interface based on a digital 24 bit microprocessor.
The sensor is exclusively compatible with an SL5 Smart Logger. The dT symmetric (dTsym) and dT asymmetric (dTas) values are stored on a 4GB MicroSD card in °C for every radial measurement by all measurement needles.
Each HFD Sensor includes a precision voltage regulator to control the heater voltage between 0 and 12 V in 0.01 V increments. The voltage can be adjusted independently of other sensors in order to ensure complete freedom when designing experiments. The voltage can be adjusted simply via the sensor setup menu either directly using a wired connection or wirelessly using a wireless modem.
The HFD sensor has a maximum of 8 measurement points per needle, spaced at 10mm intervals (HFD8-100) or 5 mm intervals (HFD8-50). On the sensor setup menu the user can set the number of measurement points to be used on the needle from 1 to 8. The data are recorded in °C, then imported into the HFD software. This automatically detects the correct number of set measurement points per set of needles and calculates the value for the K parameter from this before the sap flow rates are generated from the raw temperature data. The HFD-TOOL software has a modular design and can be individually expanded by adding further modules (e.g. the SFM-1 module). This permits processing of SFM-1 data from the same data logger in order to compare and analyse these with the HFD data at the same time.

• Reliable recording of very small and even negative flow rates
• Autonomously operating plug & play system
• Measurement of radial sap flow profiles