TDS meter (also known as ppm pen, nutrition tester, salt tester) is an inexpensive and convenient digital tool that allows you to easily check the overall water quality. TDS = Fully dissolved solids refers to salt, metal or mineral dissolved in water. Press the TDS-3 button to show the total purity level of water. The screen is displayed in ppm increments. The lower the TDS level, the purity of the water. 0 ppm for pure water. The TDS-3 is perfect for checking your tap water quality or when replacing your filter. (Meters do not test biological contaminants) TDS-3 is the perfect tool for the following applications: Drinking water, water filtration and purifier, colloid silver, hydroponic cultivation (nutrition test), aquariums, RO/DI systems, pool spa, commercial and industrial applications and more. The TDS-3 is a rugged model with a protective carrying case. This is the worlds most famous portable TDS meter suitable for both industry professionals and home use.