The Commercial In-Line Dual 3/8" TDS (Total Dissolved Solids) Meter measures the TDS of water, both on the raw side and the treated side. The meter can be attached to the untreated side to check the level of TDS before treatment and to the treated side to see the level of TDS after treatment. This meter helps in assessing if the filter needs changing. Old filters can release bacteria in treated water. The meter comes with a dual range, it measures from 0 ppm to 999 ppm, with a resolution of 1 ppm. From 1000-9990 ppm the resolution is in 10 ppm increment that is indicated by a blinking ?x 10? image. It is made from a water resistant material that is strong and fitted with an extra large LCD display.