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Proper inflation of towables is important for providing the best performance as well as preventing excessive stress on seams.  To get just the right amount of air, pay close attention when inflating.  In general, the tube is full when it is very firm.  Covered tubes should be free of wrinkles.  When an adult stands on the tube, it should only sink a couple of inches.  To achieve this level of inflation, the PVC must stretch.  However, just as under-inflation is a problem, over-inflation will cause undue stress on seams.  Watch carefully when inflating, and stop as soon as the tube is firm and the cover is smooth.