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Two Algorithms for Variable Power Control of Heat-Balance Sap Flow Gauges under High Flow Rates

USDA-ARS, Horticultural Crops Research Unit, 24106 N. Bunn Rd., Prosser, WA 99350

View larger version (19K): [in a new window]   Fig. 1. Typical daytime course of (A) power (Q); (B) temperature difference between upstream and downstream thermopiles (T), with dashed line representing Ttarget of 1.3°C.; and (C) sap flow and global irradiance (Rs) under clear skies for a representative heat-balance sap flow gauge on a 10-yr-old well-watered Concord vine in a semiarid environment. The gauge was operated under variable power control using a PD algorithm. Data were collected DOY 202, 2000. Overnight, Q was held at 0.2 W to avoid overheating the stem.

View larger version (24K): [in a new window]   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 Ttarget of 1.3°C.; (C) sap flow and global irradiance (Rs). Data were collected DOY 269, 2000.

View larger version (18K): [in a new window]   Fig. 3. Performance of a representative heat-balance sap flow gauge operated in constant power mode on a 10-yr-old well-watered Concord vine in a semiarid environment. (A) Power to the gauge heater (Q); (B) temperature difference between upstream and downstream thermopiles (T), with dashed line representing T of 1.3°C; (C) sap flow and global irradiance (Rs). The substantial error that can occur due to small T is evident around 10.00 LST. Data were collected on DOY 272, 2000.

View larger version (18K): [in a new window]   Fig. 4. Exemplary diurnal curve of power (Q) applied to the gauge heater by the open-loop algorithm on a day with clear skies. The divergence of Q from irradiance (Rs) at either end of daylight hours reflects the start time of the controller at 60 min before the computed value for sunrise and the stop time of the controller at 60 min after the computed value for sunset. Nighttime Q was held constant at 0.2 W (horizontal reference line).

View larger version (24K): [in a new window]   Fig. 5. Performance of an exemplary heat-balance sap flow gauge on deficit-irrigated Cabernet Sauvignon grapevines during 5 d encompassing the end of a weekly dry-down and rewatering. Irrigation was applied by drip (0.75 L h–1 vine–1) for 17 h overnight DOY 233 to 234 and 16 h overnight DOY 234 to 235. (A) Power to the gauge heater (Q); (B) temperature difference between upstream and downstream thermopiles (T); (C) sap flow and global irradiance (Rs). The gauge was operated under variable power control using an open-loop algorithm. Maximum allowable Q deliberately adjusted on DOY 233. Arrow in (A) denotes time at which program implemented the high-temperature "fail-safe" mechanism, reducing Q to 0.25Q for the remainder of DOY 234. Horizontal reference line in (B) is Ttarget. Data were collected during 2003.

View larger version (16K): [in a new window]   Fig. 6. Performance of a representative heat-balance sap flow gauge operated under variable power control using an open-loop algorithm. The gauge was on an 11-yr-old deficit-irrigated Cabernet Sauvignon vine in a semiarid environment. (A) Power to the gauge heater (Q); (B) temperature difference between upstream and downstream thermopiles (T); (C) sap flow and global irradiance (Rs). Data were recorded on DOY 237, 2003. Horizontal reference line in (B) is Ttarget (1.5°C).