The Model 3251, Battery Operated Wireless CO Monitor, is a gas monitor capable of measuring ambient CO gas concentrations. The calibration procedure is a simple two-step process. 100% air is applied first, followed by a known concentration of calibration gas (PBE recommends 25 ppm gas). After the display has settled, press the (+) and (-) buttons until the value matches the rated value of the concentration gas. The Model 3251 is now calibrated and monitoring gas levels.