Product Description: How Pure is Your Water? HM Digital’s DM-1 is a dual inline monitor that measures TDS from 0 to 9990 ppm. It includes batteries, two sensors and two 1/4 quick-connect T-fittings. The DM-1 is an easy and convenient method of determining TDS levels for water filtration/purification systems such as reverse osmosis (RO), deionization (DI) and more. It’s very useful for coolers or under-the-sink systems. For any type of system that removes TDS, you can install the ‚IN‘ sensor on your tap water (before the system) and the ‚OUT‘ sensor on your product water (after your filtration system) to determine the filter/membrane’s effective operating (rejection) rate.