The Bluelab ppm Pen measures conductivity and temperature and ensures plants are receiving the proper amounts of nutrients. It's fully waterproof and also features a backlit LCD display, optional calibration and a successful calibration indicator, hold function, low-battery warning, auto-off function and automatic temperature compensation. It operates on a scale of 500 or 700ppm and has a range of 0 to 5,000 ppm or 0 to 7,000 ppm, with an accuracy of ± 50 ppm or ± 70 ppm. Requires one AAA battery and has a one-year warranty. Battery included.