Our Nuclear Grade Color Changing Mixed Bed Resin physically changes colors as it becomes exhausted, allowing you to monitor the consumption of the DI resin and know when to change it out. If you use a TDS meter on your RO/RODI system we still recommend color changing resin as it will let you know when to monitor your TDS meter before the total dissolved solids in your filtered water start to increase. Our color changing DI resin is packed in vacuum sealed foil-lined resealable Mylar bags for maximum freshness, efficacy and shelf life. Color Changing DI Resin is necessary to reduce Total Dissolved Solids (TDS) to zero in the final stage of an RO/DI unit. The resin is designed to be used with one of our refillable DI cartridges - refilling DI cartridges is much more economical than throwing away exhausted cartridges and recommended for hobbyists that require large quantities of purified water.