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Note that with this method the vacuum in the lysimeter drops as the lysimeter is <br /> filling with pore water. <br /> The second method uses a vacuum pump that runs continuously during the <br /> sampling period. The pump makes it possible to maintain a constant vacuum or <br /> negative pressure inside the lysimeter. Generally a negative pressure between <br /> 300 and 400 mbar is optimal.A source of electricity is necessary for this method. <br /> The pump is connected to the vacuum/pressure tubing. Leave the pump on for a <br /> long enough time to collect an adequate volume of pore water. The time it takes <br /> for sampling depends on the moisture content of the soil, and can vary between <br /> 30 minutes and 24 hours. If the soil is too dry (if the tension in the soil around the <br /> lysimeter is greater than 500 cm water pressure (0.5 bar)), most likely no pore <br /> water will be collected. After turning the pump off, connect a pressure pump to <br /> the vacuum/pressure line. This forces the fluid up to the soil surface into the <br /> sample collection bottle. <br /> A good bicycle pump can inflate to at least 100 psi, equivalent to 220 feet of water <br /> pressure, and can be used for this purpose. <br /> Note: In very moist soil the lysimeter could fill up quickly. The pump could <br /> possibly then draw the fluid up in the vacuum/pressure line and into the <br /> vacuum pump. This would ruin the pump. To prevent this from happening <br /> always install an overflow vessel between the collection bottle and the pump. <br /> It is also recommended to run the pump about 15 minutes after each use, without <br /> being connected to the lysimeter system. Pumping ambient air through the pump <br /> will remove any accumulated water from the pump head. This will prolong its <br /> life. <br /> As explained above, the vacuum applied to the lysimeters should be no greater <br /> than 6 psi (0.4 bar). If for some reason a large vacuum was applied to the <br /> lysimeters, the lysimeters could have lost the water held in the porous steel. The <br /> porous steel can also loose its water if the soil around the porous steel is so dry <br /> that it draws all the water out of the porous steel. <br /> To rewet the porous steel add distilled water to the third tube that connects the <br /> shortest outlet in the top of the lysimeter to ground surface. Of course this only <br /> works if the third tube was installed during the lysimeter installation. <br /> Warranty and disclaimer: <br /> Soil Measurements will repair or replace those items that have been returned, <br /> and that it finds defective. This is the Buyer's sole remedy and is in lieu of all <br />