W151201 Zodiac Duo Clear Cartridge O-RingThe Zodiac/Nature2 DuoClear salt water chlorinator for swimming pools sanitizes pool water automatically while reducing the chlorine level to just 0.5 ppm. Self-cleaning.unit reverses polarity every 5.8 hours to prevent calcium build-up. The DuoClear system gives your pool water a more consistent pH balance and reduces the need to frequently add harsh chemicals and decreases the spikes in the chlorine readings of your pool.In addition to the chlorine generator, Zodiac Duo-Clear adds the Nature2 chlorine-free mineralizer. Nature2™s natural sanitizing cleans the pool water, allowing the chlorine generator to run less, resulting in a lower amount of chlorine in the pool. Trouble-free chlorine produced naturally and automatically By adding a mild salt solution to your pool, the Zodiac LM charges the salt ions and produces chlorine. The salt solution in your pool is at a much lower level that is found in a human tear.Nature2 technology assures you™ll have better water quality than what you would achieve with other types of chlorine.