The “Heat Ratio Method (SFM)” is a modification of the “Compensation Heat Pulse Method (CHPM)“. SFM represents an improvement on CHPM in that it can also be used to measure very small flow rates and even reverse flow rates. It is possible using this method to determine sap flows in trunks, stalks and roots of various types and sizes of plants under different ecological conditions, right through to drought.

A short heat pulse is the tracer for this thermometric method of measuring sap flows in xylem tissue. It is possible to calculate the quantity and direction of the sap flow from the ratio of the amount of heat transported to two symmetrically arranged temperature sensors.

The sensor consists of three 35 mm long needles connected to a 16 bit microprocessor. The top and bottom probes each have two thermistors. The third probe is positioned centrally; it is heated across its entire length and is a linear heat source, generating a constant heat pulse along the sap wood.

The integrated microprocessor is the heart of the fully automated, plug & play capable sensors. It controls all operating settings and sensor calculations, converts analog signals to calibrated values and reads these out via the serial interface. Program variables such as heat pulse interval, energy input, distances between probes and measurement frequency are stored in the memory.

Special software to optimise data evaluation is available as an optional extra. In conjunction with the Smart Logger SL5,  the SFM sensor provides a complete measurement report with detailed information and all variables required to calculate the velocity and volume of the sap flow.