Do you ever wonder why your car is covered with water spots? It's because of all those hard water minerals suspended in your tap water. TDS are the total amount of mobile charged ions, including salts, minerals and metals, in a given volume of water, expressed in units of milligrams per unit volume of water (mg/L). They are also referred to as parts per million (ppm). Our TDS Meter will measure the contaminants in your water and indicate how well your water purification system is working. Now you'll know exactly when to change a water filter or purifier in your home. Easy to read LCD screen for easy use and designed using microprocessor technology. Features an auto-off function to save battery life after 10 minutes of inactivity. Batteries included. A must have to use with our Water Deionizer.