Why Calibration Matters
CO2 sensors can drift over time, leading to readings that are consistently too high or too low. Regular calibration ensures your monitor provides accurate data so you can make informed ventilation decisions. Most quality NDIR sensors need calibration every six to twelve months.
Types of Calibration
Automatic Baseline Calibration (ABC)
Most consumer CO2 monitors feature ABC, which assumes the lowest reading over a seven-day period is close to outdoor air (approximately 420 ppm) and adjusts accordingly. This works well in spaces that are unoccupied for several hours daily — such as offices and classrooms that empty at night.
When ABC fails: If a room is occupied 24/7 (like a hospital ward or continuously occupied bedroom), the sensor never sees true baseline and calibration can drift upward. In such cases, disable ABC if your monitor allows it.
Manual Calibration (Outdoor Reference Method)
This is the gold standard for accuracy:
- Take your monitor outdoors to a location away from roads, exhausts, and crowds.
- Wait 10-15 minutes for the sensor to stabilise.
- Note the reading — it should be close to the current outdoor CO2 level (approximately 420 ppm).
- Trigger manual calibration via the device menu or app, telling the monitor to set its current reading to 420 ppm.
Calibration Gas
Professional-grade calibration uses a certified gas mixture of known CO2 concentration. This is overkill for most home users but essential for laboratory and industrial applications.
Calibration Tips
- Calibrate in mild weather — extreme temperatures can affect sensor readings during the process.
- Let the monitor run for 20 minutes before calibrating to ensure the sensor is fully warmed up.
- Do not breathe near the sensor during outdoor calibration — your exhaled CO2 (40,000 ppm) will skew the reading.
- Document the date — set a reminder to recalibrate every 6-12 months.
How to Tell If Your Monitor Needs Calibration
If your monitor consistently reads below 400 ppm or shows readings that seem implausible, it is time to recalibrate. Our guide on verifying monitor accuracy covers additional diagnostic steps.
For more on choosing a reliable monitor that holds calibration well, see our best home monitors guide or try the CO2 calculator to understand your monitoring needs.