The HM Digital SM-1 In-Line TDS Monitor is a Pure Water Meter that tests water quality for Total Dissolved Solids of whatever single water line it is hooked up to. This TDS: *Displays TDS level of a single water line. *Highly efficient and accurate due to its advanced microprocessor technology. *Auto-Off function conserves battery power. The unit shuts off automatically after 10 minutes of non-use. *Wide Range 0-9990 ppm (From 0 - 999 ppm the resolution is 1 ppm. From 1000 to 9990 ppm, the resolution is 10 ppm, indicated by a blinking 'x10' image.) *Factory Calibrated: our meters are calibrated with a 342 ppm NaCl solution. Meters can be recalibrated with a mini-screwdriver. *Comes equipped with John Guest Quick-Connect T-fitting (1/4 inch standard). *Installs quickly and easily.