3 lbs Color Changing Nuclear Grade DI Resin. We recommend the color changing resin because it gives you an idea of when to start checking your TDS meter more often. We vacuum package our resin, this removes virtually all of the carbon dioxide within the packaging itself. The resin changes from Blue to light brown as it is depleted. The color indicator in DI resin is just a visual indicator of when the resin is close to exhausting and should not be considered and accurate representation of when the resin is fully depleted, it can often be good much longer. We strongly suggest using inline TDS meters to definitively tell you when it is time to change your resin.