A Dual-Heater Gauge for Measuring Sap Flow with an Improved Heat-Balance Method
1996
Peressotti, Alessandro | Ham, Jay M.
Measurements of sap flow in intact plant stems help quantify biophysical relationships governing water transport in the soil-crop-atmosphere continuum. Research was conducted to develop and test a dual-heater sap flow gauge based on an improved heat-balance theory. The dualheater gauge contains two resistance heaters that, when operated in variable power mode, allow mathematical manipulation of the heat balances, so no empirical calibration or zero set is necessary (i.e., no gauge factor). Prototype dual-heater gauges were fabricated and tested on containergrown corn (Zea mays L.) with stem diameters between 25 and 30 mm. Traditional single-heater heat-balance gauges were also built and tested to compare the two designs. Gauges were tested simultaneously in a greenhouse over a continuous 5-d drying cycle when flows ranged from 0 to 140 g h⁻¹ and daily water use ranged from 137 to 1064 g d⁻¹. Both gauge types measured hourly sap flow to within ± 10% relative to gravimetric estimates of transpiration. The dual- and single-heater gauges measured cumulative sap ttow over the 5-d test period to within 4.0 and 9.5%, respectively. The largest errors in daily sap flow, 34%, occurred in the single-heater gauge when the plants were subjected to water stress. The dual-heater approach could measure sap flow without predetermined or empirically derived calibration factors or zero sets. Accurate measurements of sap flow were obtained 15 rain after the gauge was attached to the plant stem. Additionally, the dual-heater gauge was easier to build, consumed less power, and caused less stem heating than the single-heater gauge. Contribution no. 95-152-J from the Kansas Agric. Exp. Stn., Manhattan, KS.
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