TDS or Total Dissolved Solids is used to describe the level of nutrients in the solution. A TDS Meter is also know as PPM meter Part Per Million” which is most common in the hydroponic industry. These Conductivity meters help the gardener to test how strong the fertilizer solution …is to determine if it need increased or decreased. Expert gardeners use TDS meters to dial in the amount of nutrients to a specific plant type to achieve the best results possible. PPM meters are also helpful to check how much nutrients is left in the grow medium by testing the run off. By running RO water through the plant medium and then checking the water that runs out the bottom of the container one can determine how much fertilizer is still in the medium. This will help to ensure a well flushed crop leading to the cleanest produce possible.