Oxygen Monitor: Ceramatec MAXO2 OM-25ME Oxygen Monitor - Uses two AA batteries - Large, 3 1/2-digit LCD display for readings in the 0-100% range. - Simple operation and calibration using calibrate key functions - Self-diagnostic check of analog and microprocessor circuitry - Low battery indication ** Please reference item #210604-2934-01 when contacting us about this listing. Return Policy Items are sold as-is as described and pictured. Most items come with a 7 day option to return and a 30 day limited parts warranty. Feel free to call before you buy. 888-649-6497