The PCMM05 uses electro-chemical sensors and can react to ambient changes in carbon monoxide quickly. It's an ideal tool for measuring CO levels in industrial environments, commercial buildings, or residential dwellings. Measures CO levels from 0 to 1000 PPM. Max hold and data hold functions. A built-in alarm sounds when CO concentration hits 200 PPM or higher. Auto power off keeps the included 9V battery charged as long as possible.