The Blue lab ppm Pen simply tells you that your plants have the right amount of nutrient in your solution and lets you know if adjustments are needed. To feed plants well, we need to know what we're feeding them. Start with a quality nutrient. Measuring the conductivity every day is the best way to avoid over-diluting or over-concentrating your nutrient solution. Completely change the reservoir solution every 7 days. Top up the nutrient level between changes - so your plants always have the right amount of food. The Blue lab ppm Pen also measures the temperature of the solution, as root health is vital to the success of a crop. Temperature affects the growth rate and structure of a plant so a solution temperature of 65 - 72?F is recommended. To make sure the plants can get at the food, you need to measure the pH of the solution. Check out our Handy Blue lab pH Pen so you have every tool required to manage your nutrient solution.