The smallest and lightest TDS meter on the market. * Advanced microprocessor technology for Hydro PPM Water Salt Pool * Hold Function: saves measurements for convenient reading and recording. * Auto-off function: the meter shuts off automatically after 10 minutes of non-use to conserve batteries. * Measurement Range: 0-9990 ppm. From 0-999 ppm, the resolution is in increments of 1 ppm. From 1000 to 9990 ppm, the resolution is in increments of 10 ppm, indicated by a blinking "x10" image.