Dynamics of a Heat Balance Stem Flow Gauge during High Flow
- J. M. Ham * and
- J. l. Heilman *
The heat balance technique shows promise as a method for determining mass flow of sap in stems of intact plants. Previous work has been limited to plants with stems diameters near 10 mm transpiring at low rates. We evaluated the performance of several heat balance stem flow gauges during periods of high transpiration in sunflower (Heliunthus unnuus L.) with stems approximately 16 mm in diameter. Measurements in a controlled environment were used to compare gauge measurements to gravimetric estimates of transpiration. Gauges having heater widths of 10 mm severely overestimated flow as transpiration approached 200 g/h. Dynamics of the heat balance and temperature profiles suggested that failure of the gauge was due to a lack of thermal equilibrium between the xylem fluid and sensors on the stem surface. Increasing the heat applied to the stem did not improve gauge performance. A gauge with a heater width of 20 mm was constructed and evaluated under similar conditions. Increasing the heater width greatly improved gauge performance and the sensitivity of the measurement during high flow. Additionally, the gauge had the ability to react to large changes in flow over short time intervals. The gauge with the wide heater measured cumulative water loss over a 15-h period to within 5% of that determined gravimetrically. The study indicates that the relationship between stem diameter and heater width is a crucial component of gauge design, and should be rigorously evaluated when applying the technique to stems with previously untested characteristics. Results suggest that heater width should be enlarged as stem diameter increases.
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