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SMS lysimeters (to extract fluid from the unsaturated soil) exceed 9 psi (0.6 <br /> bars). <br /> For successful operation,the negative pressure should be kept between 4.5 and <br /> 6 psi (0.3 and 0.4 bars). <br /> The above-described "pipes with porous membranes" are in fact called suction <br /> lysimeters. Suction lysimeters are available with porous stainless steel, ceramic or <br /> PTFE membranes. <br /> Description and operation: <br /> The lysimeters available from Soil Measurement Systems (SMS) are made of <br /> stainless steel. Stainless steel is the preferred material for sampling groundwater, <br /> and is also an excellent material for vadose zone sampling. The patented design <br /> (US Patent no: 5035149) of Soil Measurement Systems uses high quality porous <br /> stainless steel to allow flow of pore fluid into the lysimeter. <br /> The lysimeters are of welded construction with no glue or plastic. They are <br /> extremely strong and durable, and not easily damaged during installation in <br /> boreholes. The bubbling pressure of the porous steel is 0.6 bars. The dual <br /> chamber models have an outside diameter of 2.0 inches, and have a porous steel <br /> section of 3.7" in length. <br /> The lysimeters are recommended for sampling at depths greater than 12 feet,but <br /> they have been used for sampling the vadose zone down to 300 feet (91 m). The <br /> lysimeters have a threaded collar on top, which allows easy connection of plastic <br /> pipe to the lysimeter. Such a pipe facilitates installation of the lysimeters, and <br /> also protects the two 1/4" tubes leading to the soil surface. <br />