Measuring nutrient solution strength is a relatively simple process. However, the electronic devices manufactured to achieve this task are quite sophisticated and use the latest microprocessor technology. To understand how these devices work, you have to know that pure water doesn’t conduct electricity. But as salts are dissolved into the pure water, electricity begins to be conducted. An electrical current will begin to flow when live electrodes are placed into the solution. The more salts that are dissolved, the stronger the salt solution and, correspondingly, the more electrical current that will flow. This current flow is connected to special electronic circuitry that allows the grower to determine the resultant strength of the nutrient solution.
The scale used to measure nutrient strength is electrical conductivity (EC) or conductivity factor (CF). The CF scale is most commonly used in hydroponics. It spans from 0 to more than 100 CF units. The part of the scale generally used by home hydroponic gardeners spans 0-100 CF units. The part of the scale generally used by commercial or large-scale hydroponic growers is from 2 to 4 CF. (strength for growing watercress and some fancy lettuce) to as high as approximately 35 CF for fruits, berries, and ornamental trees. Higher CF values are used by experienced commercial growers to obtain special plant responses and for many of the modern hybrid crops, such as tomatoes and some peppers. Most other plant types fall between these two figures and the majority is grown at 13-25 CF.