The phrase "factory calibrated" appears on almost every air-quality monitor sold today. It means very little.
Calibration is not a one-time event at the end of a production line. It is a relationship between three things: a known reference gas, a sensor's response curve, and a clock.
Why every gas sensor drifts.
- Electrochemical CO sensors lose response as their electrolyte ages. Roughly 5–10% per year, almost always monotonically downward.
- NDIR CO₂ sensors drift at the optical baseline. The infrared LED dims slightly, the detector sees a slightly stronger signal as "no CO₂", and the entire response curve shifts.
- Laser-scattering PM sensors foul. The optics get coated with a thin film of the very thing they are measuring, and signal-to-noise degrades.
- VOC sensors are the worst offender — their substrate poisons over time and they lose specificity entirely.
What calibration actually does.
A bench calibration places the sensor in a chamber with known concentrations (zero air, span gas, mid-range gas) and records the response. The sensor's output curve is then mapped back to ground truth. Without this, the number on the screen is an indication, not a measurement.
What ongoing calibration looks like.
- A signed certificate per serial number at manufacture, with the chamber traceability written down.
- Auto-baseline correction during long quiet periods — CO₂ in particular benefits from this. When a room has been empty 12 hours, the lowest reading should be near 420 ppm, the global background.
- Drift monitoring at the platform level. If a unit reports baseline 510 ppm in a known-empty room, something is wrong before the readings are wrong.
- A schedule. We re-bench-calibrate field units against reference instruments annually.
The tagline of "factory calibrated" sounds like a guarantee. It is the start of a process that runs for the life of the device.