HM Digital TDS-4 Pocket Size TDS Tester Meter with 0-9990 ppm Measurement Range , 1 ppm Resolution, +/- 2% Readout Accuracy This TDS tester isideal for all water purification applications, wastewater regulation, aquaculture, hydroponics, and colloidal silver ATC (Automatic Temperature Compensation) ; Advanced microprocessor technology The Sleek design of the TDS-4 is perfect for personal or commercial use Alkaline batteries included Hold Function: Freeze the measurement to view it out of the water. Auto-off function: the meter shuts off automatically after 10 minutes of non-use to conserve batteries