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Updated: August 8, 2017
When tubing systems were initially developed, vented ground systems were frequently installed. Today, the use of unvented, aerial systems is considered to be the best alternative. Research shows that unvented, aerial systems will consistantly outperform vented ground systems. This greater efficiency (usually in the range of 40%) is due to the creation of a natural vacuum in the tubing system. This natural vacuum changes the "air pressure" in the tubing system so that the tap hole is actually experiencing "better" sapflow weather even on the poorer days.
Notice how the tubing is following the contours of the slope. Because this system follows the ground (contains numerous sags) and because these systems are frequently vented (so they are not air tight), little or no natural vacuum is created.
Notice how the main line (#1) is suspended with a slope and is tight. The encourages a natural vacuum which can increase sap flow especially on the days with poorer weather.