A must-have accessory for the BrewRO System! The TDS Monitor hooks up to the incoming and outgoing lines of the BrewRO to measure total dissolved solids. By flipping the switch back and forth you'll be able to see the level of total dissolved solids present in the water entering the system and how pure the water is coming out. Under normal conditions, the Brew RO System will output water that measures between 0 - 2 ppm TDS (99.9998% purity). The best part of the TDS Monitor is that it removes the guesswork of when to change your filters. When the outgoing measurement reads 3 ppm or higher, it's time to order a new set.