Hydrologic DM-2 Industrial Dual TDS MonitorMeasure the untreated water coming in vs. the purified water going out of your reverse osmosis system at any time. This is important in determining when the RO membranes need to be changed. Auto Off function conserves battery power. The unit shuts off automatically after 20 seconds. 90 ppm, the resolution is in 10 ppm increments, indicated by a blinking 'x10' image. Factory calibrated: all meters are calibrated with a 342 ppm NaCl solution. Meters can