Bluelab PPM Pen measures conductivity and temperature (selectable units for Celsius and Fahrenheit). This completely waterproof, LCD backlight unit features a hold reading function, auto off function, automatic temperature compensation. The successful calibration indicator will help you achieve the most accurate readings of the following units and ranges: EC (0.0-10.0), 500ppm (0-5000), and 700 ppm (0-7000). With this precision and its loop handle for easy dipping/hanging, the Bluelab PPM pen is a great tool to use in your garden. Accuracy @ 25°C: ±0.1 EC (@ 2.77 EC), ±50ppm (500ppm) (@ 1385ppm), ±70ppm (700ppm) (@ 1940ppm) and ±1 C / ±2 1 year Bluelab guarantee.