Porous membranes are used in soil columns and tension infiltrometers.
SMS carries two types of membranes:
1. Nylon mesh (bubbling pressure 30 cm H2O).
2. Filter membranes (bubbling pressure 600 to 700 cm H2O).
Nylon mesh membranes are made of threads that are 30 micron in diameter. The resulting mesh opening is 35 micron (which is large enough for microbes to get through), and the open area is 27%. There are 390 mesh counts per inch, and the fabric thickness is 60 micron. The bubbling pressure is 30 cm H2O. The material is hydrophylic.
Nylon mesh is used in soil columns (flow cells) to prevent soil inside the columns from moving through the perforated support plate at the lower end of the soil columns. It is also used for tension infiltrometers. In this latter application, it covers the bottom of the porous disc, which is in contact with the soil below.
The hydrophilic nature of the material, its high permeability, and its ease of use make it most suitable for use in saturated flow studies using soil columns, and for tension infiltrometers.
Infiltrometer membranes can readily be replaced in the field, if necessary.
Note: Using a nylon membrane with a higher bubbling pressure would theoretically extend the range of the tension infiltrometer. However, this comes at the expense of reduced permeability of the nylon material. Furthermore, at higher tensions the infiltration rates are greatly reduced, making it much more difficult (often nearly impossible) to do the field experiments and obtain good data.
Filter membranes are used in soil columns (flow cells) when higher tensions are required. They are mainly used for studies done under unsaturated flow conditions, such as multi-step outflow experiments for determining water retention curves. These membranes are also made of nylon, but are manufactured using an impregnation process, resulting in a nylon filter with uniform pore sizes and consistent flow rates. The pore size is 1.2 micron, resulting in a bubbling pressure between 600 and 700 cm H2O. The pore size is generally too small for many bacteria to get through. The material itself is hydrophilic. The effective permeability of the membrane (typical flow rate is 180 ml/min/sqcm @ 0.7kg/sqcm (10 psi) pressure) is much higher than that of porous ceramic of comparable bubbling pressure, partly due to its much lower thickness (0.1 mm). Multi-step outflow experiments conducted using this material take much less time than when ceramic plates are used. This is not only important for laboratory investigations, but also when teaching the determination of water retention properties in a classroom setting, where time is often limited.
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