The ISPRING TDS meter is a digital instrument that measures the TDS (Total Dissolved Solids), in the scale of ppm (parts per million) which indicates the amount of solid dissolved in the water. The lower the TDS, the purer the water. EPA suggests that the TDS of drinking water should be below 500 ppm. Besides the purity, water temperature also impacts TDS value. ISPRING 3-button TDS meter features 2 sensors, 1 for TDS and 1 for temperature. The CPU automatically calibrates the TDS value according to water temperature so the value is always accurate. The Measurement Range is 0 - 9990 ppm at 1 ppm precision, probably the highest in market at this size and cost. It also features a temperature button that turns it into a thermometer with measurement range of 31 - 200°F at 1° precision. The compact design and carrying case make ISPRING TDS a convenient and handy tool for detecting water quality anytime anywhere, either at home, in the restaurant, at work or on the go.