Measuring Air Conductivity. The conventional method for measuring the air conductivity of porous media involves continuous-flow permeameters (e.g., Xxxxxxxxxx and Xxxxx, 1989; Springer et al., 1998). Air flow is induced through a sample with a mechanical air pump or a source of compressed air. The pressure drop across the sample is measured with pressure transducers or manometers, and the flow rate is measured with rotameters or soap-film flow meters. More recently, air permeameters have also been developed with pistons and syringes to induce air flow and photosensors to measure flow rates (e.g., Xxxxx et al., 1994). The stand-alone permeameter that was used in the current study is shown in Figure 2.1. This permeameter, which was developed at the University of Washington (Xxxxxxxx and Xxxxxxx, 2001), is constructed with two sections of clear, rigid plastic tubing connected with a plastic fitting or coupling. The upper section of the tubing holds the soil sample, and the lower section is used as a reservoir for pressurized air. Tubes with an inside diameter of 0.5 to 4 inches (1 to 10 cm) and with upper and lower sections each approximately 12 to 20 inches (30 to 50 cm) long work well for fine to medium sands. Longer sections may be required for more permeable materials. A nylon or steel mesh is glued to the bottom of the upper section to hold the sample. The air permeability or conductivity is measured by first placing a sample of dry soil in the upper section of the permeameter. The top of the permeameter is sealed, and the lower section is submerged in a container of water. The seal can be made with a cork or rubber stopper or simply with a thumb. Water will enter the bottom section of the permeameter and will pressurize the air. Because the air pressure in the lower section is greater than atmospheric pressure, air will flow through the sand when the top of the permeameter is opened. As the air flows upward through the permeameter, the air pressure in the lower section decreases, and H, the distance from the water level in the lower section of the permeameter to the water level in the water container, becomes smaller. If the diameter of the water reservoir is large relative to the diameter of the permeameter, the rate of flow through the permeameter, Qair, is directly proportional to the rate of change in H. The air conductivity can be estimated by measuring how quickly the water level in the lower section raises. The following expression gives this relationship (Xxxxxxxx and Xxxx...
