At present, most sap flow sensors are probe-based and therefore inappropriate with small diameter trees, where large comparative amounts of cross-sectional xylem will be disrupted by the probes and heat produced. While external non-invasive sap flow sensors can measure bi-directional flow in trees with diameters less than 10 mm, they are based on the heat ratio method derived from a line heat source. This presents the potential to create localised heating around the heat source, disrupting the natural flow of sap within the tree. Therefore, in this study, we propose a non-invasive sap flow sensor consisting of a ring heat source heating the circumference of the stem along with a modified heat ratio method. Calibration of the measured heat pulse velocity (𝒗𝒉) was conducted by gravimetric measurement of an imposed flow through sections of stem. Two species were examined: white poplar (Populus alba) and paper birch (Betula papyrifera). Preliminary results of the sap flow sensor show linear agreement of both species with their imposed flow, ranging between -20 to 20 g h-1.