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 <50 cm h⁻¹. The gauge was calibrated on cut stems of the shrubs Caragana korshinskii and Salix psammophila, with the measured heat-pulse velocity (V_h) compared to gravimetric measurements of sap-flux density (V_s) under controlled conditions. A validation test was also carried out by comparing EHR method with the stem heat-balance (SHB) method through in situ and long-term monitoring of V_s on standing stems of C. korshinskii. V_h in the tested cut stems of both species was linearly correlated with V_s up to approximately 50 cm h⁻¹ (R² up to 0.96, P < 0.001) in a range of stem diameters of 4.1–15.6 mm. An empirical multiplier for converting the measured V_h to V_s, however, varied between the two species, 2.02 and 1.15 for C. korshinskii and S. psammophila, respectively. The EHR technique sensitively captured the diurnal dynamics of V_s in field tests, within a range from zero to nearly 30 cm h⁻¹ on C. korshinskii stems, and hourly V_s was linearly correlated with the reference evapotranspiration (R² = 0.70, P < 0.001) over 26 successive days without drought stress. The tested SHB method, however, poorly detected the sap-flux density, especially at low densities. The gauges for the EHR method were easy to build and capable of accurate estimating bidirectional sap flow, especially at low densities. This technique, with variable EHR gauge configurations, has broader applications than SHB methods for understanding plant-water relations in understories, shrubs and ecosystems dominated by herbage.