The In-Line Dual 3/8" TDS (Total Dissolved Solids) Meter tests the Total Dissolved Salts in the water. It is best for testing the water of pools, hydroponics, aquariums, and water purifiers. You can easily test if the filter cartridge is working properly or not. Put the meter in water before purification and then in the filtered water. It will also help in knowing if the filter cartridge needs changing. Old filters can release bacteria in drinking water. It has a dual range 0-9990 ppm (From 0-999 ppm the resolution is 1 ppm. From 1000 ppm to 9990 ppm, the resolution is 10 ppm, indicated by a blinking 'x 10' image.). The meter has 3/8 inches connections.