Why Your CO₂ Sensor May Be Reading Too Low: Automatic Baseline Calibration Explained
A CO₂ sensor can be precise yet wrong if its automatic baseline correction assumes the room regularly reaches outdoor-air levels. Here is how to investigate without blindly recalibrating it.
What automatic baseline correction actually does
Many NDIR CO₂ sensors periodically look for the lowest measurement across a defined interval and use it to correct long-term drift. Some designs assume that a typical occupied room becomes close to outdoor CO₂ levels at least occasionally. The baseline period, reference value and whether correction is enabled vary by model: never apply a universal setting to every sensor.
When the assumption fails
A continuously occupied room, an enclosed growing space or an area with inadequate overnight ventilation may never return to near-outdoor CO₂. A sensor applying automatic baseline correction despite that condition could gradually report lower values than the actual concentration. By contrast, sudden isolated spikes are more likely to reflect local exhaled breath, air mixing or placement than baseline drift.
A practical seven-day investigation
Log CO₂ continuously, including times when the space is empty. Note occupancy, door and window position and mechanical ventilation schedule. Compare the lowest readings over successive days rather than trusting a single value. If possible, compare side by side with a recently calibrated reference instrument in the same well-mixed air. Check the manufacturer manual to identify the actual ABC interval, baseline assumption and approved calibration method.
What not to do
Do not force a fresh-air calibration in a room simply because a reading seems high. Do not assume that a displayed 400 ppm proves the sensor is accurate. Do not use unverified readings to certify ventilation compliance or safety.
Decision checklist
Does the space experience meaningful vacancy and outdoor-air exchange? Has the sensor completed its specified warm-up? Is its position away from supply diffusers and exhaled breath? Was it compared against a reference? Does the manufacturer permit disabling ABC for continuous high-CO₂ environments? Keep calibration records before and after any authorized adjustment.
Related AirAlign resources
NDIR CO₂ Sensors · Try the related calculator
References and further reading
Editorial note: This guide is educational and does not replace calibrated measurements, site-specific engineering, code requirements or professional safety advice.
