Different plants need differing strengths of nutrient mixes throughout the growth cycle, especially when fruiting or flowering is to occur. To measure the strength of a solution, a CF (Conductivity Factor) or EC (Electrical Conductivity) meter is needed. CF or EC is the measurement of how much dissolved nutrient is contained within the solution. Therefore, the higher the number, the stronger the nutrient solution.
In general, leafy vegetables like lettuces will need a weaker strength solution than a fruiting or flowering plant, such as peppers, tomatoes or roses.