Short answer: automatic self-calibration can correct drift when a monitor regularly encounters a known fresh-air baseline. If it never does, the same feature can teach the sensor the wrong baseline and make indoor CO₂ readings look lower than they are.
A CO₂ sensor with automatic self-calibration is not continuously checking itself against laboratory gas or an outdoor reference sensor. It uses an assumption about its own measurement history. The feature is only as reliable as that assumption.
Manufacturers use names such as automatic self-calibration (ASC) and automatic baseline correction (ABC). The exact algorithm varies by sensor, but a common approach looks for the lowest concentration observed over a defined period and treats that low point as a known background concentration.
For the Sensirion SCD30, ASC assumes that the lowest concentration regularly observed is 400 ppm. Sensirion's current SCD4x documentation describes a similar default assumption: while measuring, the sensor must encounter outdoor fresh air at 400 ppm for more than three minutes after each accumulated week of operation. The module then uses its measurement history to update its calibration curve.
This can compensate for gradual sensor drift. It cannot independently prove that the room actually reached 400 ppm.
ASC is a sensible option when the installation reliably satisfies the sensor manufacturer's conditions. A continuously powered monitor in a well-ventilated building may see the space become empty overnight, receive enough outdoor air, and return close to the configured background level on a regular schedule.
In that situation, the historical minimum is a reasonable proxy for fresh air. Automatic correction can maintain long-term stability with little user intervention. Sensirion specifically gives a well-ventilated building without people overnight as an SCD30 example.
Indoor use itself is not the problem. The problem is an indoor space that never reaches the assumed baseline. Bedrooms, classrooms, offices, care facilities, basements, greenhouses, and continuously occupied rooms can remain above outdoor background even at their quietest time. Closed windows, demand-controlled ventilation, nearby occupants, combustion sources, and building-wide recirculation can all keep the minimum elevated.
Suppose the lowest true concentration a monitor sees is 750 ppm, but its algorithm assumes that low point represents 400 ppm. The algorithm has no independent reference telling it that 750 ppm is real. It may interpret the difference as sensor drift and adjust readings downward. This example illustrates the direction of the error; the timing and size of a real correction depend on the module's algorithm and safeguards.
| Installation | Lowest recurring CO₂ | Is a 400 ppm assumption credible? |
|---|---|---|
| Empty, well-ventilated building overnight | Regularly approaches outdoor background | Potentially yes, if all manufacturer conditions are met |
| Occupied bedroom with closed windows | May remain elevated all night | No |
| Classroom or office with limited off-hours ventilation | May not return to outdoor background | Do not assume it does; verify first |
| Portable monitor moved between locations | History may not represent one stable environment | Often unreliable as a standing assumption |
The error is especially troublesome because a plausible low number may not look like a fault. It can create confidence precisely when more ventilation is needed.
All VisiblAir monitors that measure CO₂ are configured with automatic self-calibration disabled. VisiblAir monitors can spend months or years inside bedrooms, classrooms, offices, workshops, care environments, and other spaces where a regular return to 400 ppm cannot be guaranteed. We do not want the lowest value in an unknown deployment to silently redefine what “fresh air” means.
This is a product-level choice, not a claim that ASC is defective. Current VisiblAir CO₂ products use Sensirion SCD30 or SCD41 modules depending on the model and hardware revision. The SCD30 feature is left disabled or explicitly turned off, and the SCD41 configuration is explicitly stored with ASC off. The result is the same across the range: calibration changes only through an intentional procedure with a stated reference.
That approach makes long-term data easier to interpret. A correction is a deliberate maintenance event rather than an invisible reaction to occupancy patterns. It also provides one predictable default for fixed, portable, indoor, and optional outdoor installations.
Turning ASC off prevents an invalid historical-minimum assumption from moving the calibration automatically. It does not stop physical drift, undo rough handling, compensate for every pressure or placement effect, or turn an inexpensive monitor into a laboratory reference.
VisiblAir recommends checking CO₂ calibration every 6 to 9 months and after a drop, shock, rough transport, or persistent disagreement with a trusted reference. A short outdoor reading is a practical verification check, not a traceable calibration. Recalibrate only after providing a valid reference under stable conditions.
| Method | Where the reference comes from | Main risk |
|---|---|---|
| Automatic self-calibration | The sensor infers a baseline from measurement history | The environment may never reach the assumed target |
| Forced recalibration | A user or host supplies a known concentration | An incorrect reference deliberately shifts calibration |
| Verification | The monitor is compared with expected outdoor air or a trusted reference | A short or poorly controlled check can be misleading |
| Factory calibration | The manufacturer uses controlled production equipment and gases | Later handling and long-term drift can still affect accuracy |
For broader sensor selection context, see the comparison of NDIR, dual-beam, and photoacoustic CO₂ sensors.