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Each model has three stainless steel, 0.25" OD, tube outlets. The longer of the <br /> three tubes is for vacuum/pressure, the next shorter tube is the fluid return tube, <br /> and the third (shortest) tube is for leak testing after fabrication.A stainless steel <br /> cap is provided with each lysimeter to close this third tube. However, some users <br /> prefer to attach 0.25" OD polyethelene tubing to this outlet as well, so that they <br /> can add water directly to the lower porous section of the lysimeter in case the <br /> porous section has dried out. The third tube can also be used to add distilled <br /> water, or tracer solution to the bottom part of the lysimeter. The steel outlet tubes <br /> are connected with stainless steel unions to 1/4 " OD tubing (high density <br /> polyethylene, PTFE or stainless steel)leading from the lysimeter to the surface. <br /> During operation vacuum is applied to the vacuum/pressure tubing, while the <br /> other two tubes are kept closed at ground surface. <br /> When vacuum is applied to the vacuum/pressure tube of a dual chamber <br /> lysimeter, fluid first enters through the porous stainless steel into the lower <br /> chamber of the lysimeter. From there the fluid moves up through a short section <br /> of 1/4" OD tubing with a one-way valve, into the upper chamber. After sufficient <br /> fluid has collected in the upper chamber of the lysimeter, air pressure is applied <br /> to the vacuum/pressure tube, and the fluid return tube is opened and its end <br /> connected to a sample collection bottle. The increased air pressure forces the <br /> fluid up from the lysimeter into the sample collection bottle. The one-way valve <br /> prevents air escaping into the lower chamber. If no more pore water flows into <br /> the sample bottle, all fluid has been transferred to the sample bottle. <br /> Dual chamber models can be used at great depth. Note however, that when the <br /> pore water is forced to the surface by applying pressure to the vacuum /pressure <br /> tube, the fluid return tube fills with pore water. This causes a backpressure to <br /> develop in the fluid return tube, which must be overcome by the applied air <br /> pressure in order for the fluid to reach the sample bottle at ground surface. For <br /> great depths the applied pressure can be quite significant (i.e. a lysimeter placed <br /> at 300 ft (91m) can develop a back pressure of 9.1 bar or 137 psi). In that case the <br /> air pressure needed must be greater than 137 psi in order to bring the pore water <br /> to ground surface. The two chamber design with a one-way valve between the <br /> two chambers makes it possible to use large air pressures to bring the sample to <br /> the surface without blowing the air out of the pores in the lower chamber with <br /> the porous steel walls. Without the one-way valve a pressure greater than 9 psi <br /> (0.6 bar) applied to a lysimeter, would blow the air out of the porous steel and <br /> cause it to leak air. All dual chamber lysimeters have a one-way valve to prevent <br /> this from happening. <br />