Abstract Sap-flow techniques had limited application to thin-stemmed woody and herbaceous species and to diverse functional plant organs until the recent development of the external heat-ratio (EHR) method. Existing EHR techniques using miniature gauge configurations, however, are limited to thin stems with diameters of 2–5 mm. This study introduces a new design of an EHR gauge adapted to thin stems with diameters up to 15 mm and sap-flux densities