Checking co levels is as easy as pushing just 1 button on the model C010, which measures up to 1,000 ppm. An audible indicator increases in tone starting at 35 ppm. The higher the concentration of co, the faster the alarm will sound. Above 200 ppm, the alarm will sound continuously. A backlight on the display can be used for low light areas. Additional features include: max hold, data hold, and auto-power-off. The meter comes complete with a 9-volt battery, protective holster, and case.