How Pure is Your Water? A TDS meter (aka ppm pen, nutrient tester, tds stick) is an inexpensive and convenient digital tool to instantly check your overall water quality. TDS = Total Dissolved Solids, which is any salt, metal or mineral in the water. With the push of a button, the TDS-4 can tell you your overall water purity level, which will be displayed on the screen in ppm (parts per million). The lower the TDS level, the purer the water, with 0 ppm being pure H2O. The TDS-4 is great for checking your tap water quality, or knowing when to change a filter. (The meter does not test for biological contaminants.) The TDS-4 is a great tool for drinking water, water filtration and purification, hydroponics (test your nutrients), colloidal silver, aquariums, RO/DI systems, pools and spas, and more.