12-Pack Blue Commercial 10 ft. Double Chamber Pool Cover Weights for Winter Pool Closing Commercial .25 MM Thick Vinyl constructed 56% heavier than industry standard. Since 1973 Robelle has been making the best products to close your pool. Use the best pool closing products designed with over 50 years of experience. Double seams to prevent leaking. Easier to use jumbo oversized screw valve. Prevents Winter Pool Cover From Falling Into Pool. Double Chamber Design Prevents Rolling. Fill Halfway With Water to Allow For Ice Expansion.