Improved Transient‐Flow Air Permeameter Design: Dampening the Temperature Effects

Abstract Air permeability of soil is an important parameter for soil aeration and contaminant remediation techniques such as soil vacuum extraction. Transient‐flow air permeameters are inexpensive and portable, but have been shown to be sensitive to small changes in temperature in the air tank induced by the changing air pressure. We have developed a simple and inexpensive modification to the design of transient‐flow air permeameters that improves both the accuracy and precision of the method. The apparatus was modified by the addition into the air tank of a heat capacitor, i.e., fiberglass insulation. The fiberglass insulation filling the tank acts as a uniformly distributed heat source‐sink (capacitor) and thereby dampens the temperature changes. Data from the near‐isothermal air tank design were compared with data from the standard air tank design and data from a method that uses the standard air tank design but measures both pressure and temperature simultaneously. An air permeability value determined with a steadystate flow method was used as a reference. The results show that nonisothermal air tank data can produce large inaccuracies relative to the steady‐state permeability. Applying a temperature correction to the nonisothermal air tank data improved the results considerably, but this method requires additional temperature and pressure measurements of extreme precision. The permeabilities produced with the “nearly isothermal” air tank closely matched the steady‐state values without having to apply any correction to the data. It is therefore recommended that the air tank of transient air permeameters be filled with a heat capacitor, such as fiberglass insulation.