The Bluelab pH Pen is a hardy, compact, and accurate pH and temperature pen for using in nutrient solution. Use the pH pen to quickly and accurately know if your solution is too acidic or alkaline, or too hot or cold. Designed for everyday use, the Bluelab pH Pen helps you to keep your plants in the  optimal pH and temperature range so that your plants can thrive. 
Getting started is easy. Follow the instructions to hydrate and calibrate your pen, and submerge the probe in your nutrient reservoir to view the measurement on the display.