Bluelab pH PenBLUELAB PH PEN This 2 in 1 Bluelab pH Pen will measure the temperature of your solution, as well as the pH reading. Getting your pH right is important to growing, in order to maximise nutrient uptake and give your plants the best chance. Ideal pH sits between 5. 5 6. 5 for most plants. Temperature can also affect the plant's growth rate, and you should be aiming for between 18 22C. This handheld pen gives you a quick and easy way to check both of