Total Dissolved Solids, TDS, and Parts per Million, PPM, are two types of measurements use to ascertain the amount of solid materials that are in your hydroponic solution and water systems. These levels are important to measure because they ensure proper water and nutrient delivery. In order to ensure that you are maintaining a proper balance for your plants, be sure to pick up one of our meters or even better, a combination meter, to help ensure proper plant care.