When you buy a ZeroWater filter pitcher we include a laboratory-calibrated water tester—called a TDS meter. TDS stands for Total Dissolved Solids, and represents the amount of dissolved substances in parts per million in the water we drink. These substances can negatively affect the way your water tastes. brbr Unlike conventional gravity-fed water filters, ZeroWater’s patented 5-stage ion-exchange filter delivers the only filtered water to measure absolute 000 on the TDS meter. Compare that to the level in your tap water, even after being filtered through the leading filter pitchers.