Sometimes high-air entry filters are written into specifications for vibrating wire piezometers. What is the purpose of these filters? Do they alter the performance of the piezometer?
Answer: High air entry filters, are generally not appropriate for standpipe or diaphragm piezometers. The high air entry filter relies on the surface tension of water in its pores to sustain a pressure difference between air and water on the filter surface. This keeps air out of the measuring system and allows measurement of matrix soil suction (negative pore-water pressure) that is present in non-saturated, cohesive soils, such as clays in embankments. The high air entry effect is operative only when the filter is saturated with water. When water drains out of the filter, the high-air entry effect disappears.
Generally speaking, only one type of piezometer, the hydraulic piezometer, is capable of maintaining saturation of the filter in non-saturated soils. Diaphragm piezometers and standpipe piezometers do not normally have this capability, and therefore should not be specified with high air entry filters. In addition, most diaphragm piezometers are not calibrated to read high negative pressures and thus, even if the filter properties were intact, would not be able to read those pressure.