ZDS-TDS Pen Tester WP (Total Dissolved Solids) Range: 0-1999 ppm (mg/l), Resolution: 1 ppm (mg/l), Accuracy: ¡À2% Full Scale, Battery Life: DC 4x 1.5V, Temp.¡æ: 5--50¡æ (ATC) , Features: Automatic switch-off function: After 8-10 minutes without pushing buttons, the meter will be switched off automatically.