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Is the Tension Beneath a Tension Infiltrometer What We Think It Is?

Dep. of Crop and Soil Sciences, 116 ASI Building, The Pennsylvania State Univ., University Park, PA 16802

View larger version (24K): [in a new window]   Fig. 1. Schematic a two-piece tension infiltrometer. The two-piece tension infiltrometer includes an additional pressure transducer on the infiltration disc. The flexible coupling allows for quick changes between different reservoir diameters, providing for more accurate water height measurements for different flow rates.

View larger version (25K): [in a new window]   Fig. 2. Measured tension at the infiltration disc versus flow rate for the tension infiltrometer on a tension table using the standard water supply tube and fittings (12.7 and 8.7 mm, respectively). The tension control was set to the desired tension of either 0, 10, 20, 30 or 60 mm and remained so while the flow rates were varied. The starting point for each curve was the lowest constant flow rate measured. The flow rate was then increased by elevating the tension on the tension table. The error bars represent the standard deviation of the measured tension during each steady-state measurement. The coefficients of the quadratic equation, which was fitted to the points of each desired tension, are shown in the legend of the graph [(a)x2 + (b)x + (c)]. The dotted vertical line (threshold) represents the approximate flow rate in which the tension starts to increase considerably.

View larger version (16K): [in a new window]   Fig. 3. Replicate measurements of tension in the infiltration disc while on a sand column for six desired tensions, 120, 60, 30, 20, 10, and 0 mm, respectively. Each point on the graph represents 2-min tension averages with the standard deviation shown by error bars. The solid line represents the desired tension setting of the infiltrometer.

View larger version (17K): [in a new window]   Fig. 4. Replicate measurements of tension in the infiltration disc placed on the Hagerstown silt loam soil for six desired tensions, 120, 60, 30, 20, 10, and 0 mm, respectively. Each point on the graph represents 5-min tension averages with the standard deviation shown by error bars. The solid line represents the desired tension setting of the infiltrometer.

View larger version (18K): [in a new window]   Fig. 5. Replicate measurements of tension in the infiltration disc placed on the Morrison sandy loam soil for six desired tensions, 120, 60, 30, 20, 10, and 0 mm, respectively. Each point on the graph represents 5-min tension averages with the standard deviation shown by error bars. The solid line represents the desired tension setting of the infiltrometer.

View larger version (9K): [in a new window]   Fig. 6. Results from the tension calculations. The increase in tension, due to friction, was calculated using the orifice equation and the Darcy–Weisbach equation. The tension increase was calculated for flow rates from 75 to 500 cm3 min–1.

View larger version (18K): [in a new window]   Fig. 7. Measured tension at the infiltration disc versus flow rate for the tension infiltrometer on a tension table using the larger water supply tube and fittings (15.9 and 13.2 mm, respectively). The tension control was set to the desired tension of either 0, 10, 20, 30, or 60 mm and remained so while the flow rates were varied. The starting point for each curve was the lowest constant flow rate measured and then the flow rate was increased by elevating the tension on the tension table. The error bars represent the standard deviation of the measured tension during each steady-state measurement. The coefficients of the quadratic equation, which was fitted to the points of each desired tension, are shown in the legend of the graph [(a)x2 + (b)x + (c)]. The dotted vertical line (threshold) represents the approximate flow rate in which the tension starts to increase considerably.