Evaluation and optimization of a 3D-printed external heat pulse sensor

Sap flow is frequently measured with probes inserted in larger plants to quantify transpiration and irrigation needs, to monitor stress, or to investigate the coupling between plant hydraulic function and growth, but sap flow measurements remain challenging when working with small dimensions as probe insertion causes relatively more damage to the plant tissue. An external sap flow sensor, with external temperature measurements and heat application according to the heat ratio method (HRM), is therefore more suitable for small-diameter plant organs. In this study, the sensor design was optimized to limit measurement errors through an accurate and precise positioning of the electronic components by 3D-printing the sensor casing. Also, a suitable spacing of 3 mm to measure stems ranging from 2 to 4 mm is provided for stem guidance and to ensure a proper stem-sensor contact. A spacing of 5 mm is available for stems ranging from 4 to 6 mm. The 3D-printed HRM-sensor (ExoBeat) is evaluated on 1- to 5-months-old Ficus benjamina L. stems and 1-year-old Populus tremula L. branches. Thermal imaging showed a significant amount of heat loss through the sensor material, which is inherent to the heat-based sensor design. A wide range of heater powers (0.09-0.49 W) resulted in robust heat pulse velocity (v(h)) patterns after temperature gradient correction. Temperature gradient correction is particularly important for ExoBeat measurements as the heat pulse induced temperature rise is relatively small. The conventional temperature gradient correction by subtracting the extrapolated temperature gradient before the heat pulse resulted in slightly less stable sap flow calculations than subtracting the interpolated temperature from a few seconds before to 150 s after the heat pulse. The frequently used 60-100 s time interval for v(h) calculations was confirmed. Furthermore, two calibration methods were evaluated with the calibration on an intact stem under variable greenhouse temperature conditions being more accurate than the one using cut stems. With their broad range of suitable stem diameters, sap flow and ambient temperature stability, ExoBeat sensors show great potential for many applications