An Improved Design of Sap Flow Gauge without Calibration Using Quad Thin Film Heater

Abstract

Transpiration is an important physical phenomenon for plant survival. Transpiration measurements can provide a variety of information on plant productivity. In particular, the study of transpirational properties has contributed to the development of agricultural technologies and plant hydraulic physiology as before. The stem heat balance (SHB) gauge is a useful tool for measuring transpiration related to sap flow through the plant body, since it can be used for non-destructive measurement of the sap flow rate. However, the stem heat balance method suffers from five theoretical and technical problems. In order to solve these problems, we designed a new gauge called "Quad Gage." We achieved a stable stem heat balance by controlling the temperature of four film heaters using a minute thermistor and microcontroller. Quad Gage zeroed the heat storage and radial heat terms in the stem heat balance equation and enabled the actual measurement of the thermal conductivity of the branch surface. As a result, no assumption is required in the stem heat balance theory. In laboratory and field measurements, Quad Gage exhibited high accuracy similar to that of the calibrated SHB gauge. This improvement of the sap gauge design enabled simplification of the operation for the sap flow gauge, downsizing of the system.