Dynamics of a Heat Balance Stem Flow Gauge during High Flow

Abstract

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 (Helianthus annuus 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.

Determination of the water use and water use response of canola to solar radiation and temperature by using heat balance stem flow gauges

“…1 and 2). Similar observations have been reported by Ham and Heilman (1990), and Rose and Rose (1998). Among the heat flux components, Qv and Qr are obtained by sensor measurements, whereas Qf is estimated as residual heat (Baker and Van Bavel 1987).…”

“…In general, Qv is very small (Fig. 2) and plays a minor role in influencing Qf (Sakuratani 1981;and Ham and Heilman 1990). Qr depends on the thermal conductivity of the gauges and on the radial temperature gradient.…”

“…A number of studies have described the heat balance method in detail (Baker and Van Bavel 1987;Ham and Heilman 1990). In summary, a small flexible foil heater of fixed width that encircles the stem completely is used to emit constant energy (Q) to heat a small portion of the stem.…”

“…Stem gauges (SGA-10, Dynamax Inc, Houston) described by Ham and Heilman (1990) were used. The sap flow gauges were installed for transpiration measurements when stem thickness exceeded 9 mm, which occurred during the early to mid flowering stage.…”

Performance of stem flow gauges in greenhouse and desert environments

“…) Recent developments in the design of stem flow gauges combined with recent advances made in automatic datalogging technologies, show promise for providing accurate, non-invasive, and continuous measurements of transpiration in herbaceous, as well as woody plants (Steinberg et al, 1989;Baker and van Bavel, 1987). Recent studies indicate a 2 10 percent error range when transpiration is considered on a 24-hour basis for experiments in a greenhouse or growth chamber (Senock and Ham, 1991;Ham and Heilman, 1990; Steinberg et al, 1990;Steinberg et al, 1989;Baker and van Bavel, 1987). Reports from field studies have been mixed.…”