Air Quality Monitor Calibration: Factory vs Field vs Reference Methods

Volume I  ·  May 2026  ·  208 words

Consumer air quality monitors use electrochemical, metal oxide, or optical sensors that drift over time. Calibration restores accuracy. Factory calibration is performed at the manufacturing facility against reference instruments and is valid at shipment — but sensors drift in the months after, and factory calibration cannot be repeated by the user. Field calibration allows the user to place the monitor outdoors in clean air (assumed PM2.5 near zero, CO2 approximately 400-420 ppm) and set the baseline. This corrects for zero-point drift but not for span (sensitivity) drift. Reference calibration involves co-locating the monitor with a research-grade instrument for 24-48 hours and applying a correction factor. The PurpleAir monitor supports applying an EPA-derived correction factor that adjusts the raw readings to match federal reference method instruments. Without periodic calibration, a consumer monitor's readings may drift by 10-30% over 1-2 years — still useful for relative trends (is the air getting better or worse?) but unreliable for absolute health-based comparisons to EPA thresholds.