12-Pack Blue Extra Heavy Duty 10 ft. Double Chamber Pool Cover Weights for Winter Pool Closing Extra Heavy Duty .42 MM Thick Vinyl constructed 163% 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. Prevents Winter Pool Cover From Falling Into Pool. Double Chamber Design Prevents Rolling. Fill Halfway With Water to Allow For Ice Expansion. If you have trouble closing the valve please use a hair dryer to loosen before closing.