The DM-1 helps you to measure the tap water coming in vs the purified water going out of your water filtration system at any time | The In-Line Dual TDS Meter displays TDS levels of the feed water and product water with in and out electrode probes | This TDS Monitor is highly efficient and accurate due to its advanced microprocessor technology | 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 | Installs quickly and easily