Fig. 2. Example of protracted oscillation in power control by the PD algorithm for a representative heat-balance sap flow gauge on a 10-yr-old well-watered Concord vine in a semiarid environment, when flow rates were below those for which the algorithm had been tuned. (A) Power to the gauge heater (Q); (B) temperature difference between upstream and downstream thermopiles (T), with dashed line representing T_target of 1.3°C.; (C) sap flow and global irradiance (R_s). Data were collected DOY 269, 2000.