Choosing a CO2 Meter for Indoor Air - What Matters?
A CO2 meter makes indoor air quality visible and shows exactly when a room needs ventilation. This guide explains the NDIR measuring principle, sensible accuracy, the 1000 ppm Pettenkofer limit and correct calibration.
View CO2 metersWhy an NDIR sensor and not a cheap MOX sensor?
A true CO2 meter uses an NDIR sensor (non-dispersive infrared). The gas absorbs infrared light at around 4.26 µm wavelength, and the sensor derives the real CO2 concentration in ppm from that attenuation.
Cheap devices with a MOX or VOC sensor do not measure CO2 directly. They estimate a CO2 equivalent (eCO2) from volatile organic compounds. That estimate is unsuitable for ventilation control because cleaning agents, cooking or hand sanitiser distort it heavily.
- NDIR: measures CO2 directly by infrared absorption, selective and stable long term.
- MOX/eCO2: estimates from VOC only, prone to interference, not for ventilation decisions.
- A measuring range of 0‑5000 ppm is sensible for indoor use.
When does a room actually need ventilation?
The reference value goes back to Max von Pettenkofer: up to 1000 ppm the air is considered hygienically acceptable. Fresh outdoor air sits at about 400 ppm, and indoors the level rises with the breathing of the people present.
CO2 concentration is a reliable indicator of air exchange and, indirectly, of aerosol load. Guidance from environmental agencies rates values below 1000 ppm as harmless, 1000 to 2000 ppm as noticeable and above 2000 ppm as unacceptable.
Which accuracy and features make sense?
For indoor air monitoring an accuracy of ±30 to ±50 ppm plus about 3 percent of the reading is sufficient. A stable calibration and a clearly readable display matter more than the last ppm digit.
NDIR sensors often use automatic baseline calibration (ABC). It assumes the room regularly drops back to the outdoor value of around 400 ppm. In continuously occupied rooms without night-time airing, ABC should be switchable off and manual calibration in fresh air used instead.
- Calibrate against fresh outdoor air (around 400 ppm) as the reference.
- Data logger and USB or app export for documentation and records.
- Combined meters with temperature and relative humidity for the full room climate.
Frequently asked questions
What does the 1000 ppm Pettenkofer limit mean?
It is the classic hygiene reference value for indoor air. Below 1000 ppm the air is considered good; above it you should ventilate because air exchange is declining.
Why is an NDIR sensor better than an eCO2 sensor?
NDIR measures CO2 directly via infrared absorption and is selective and stable over time. eCO2 sensors only estimate from VOC and are easily distorted by cleaning agents or cooking.
How often should I calibrate the device?
Many NDIR meters recalibrate themselves via ABC. Without night-time fresh air, a manual outdoor calibration at about 400 ppm every few months is recommended.
Does a CO2 meter also detect carbon monoxide?
No. CO2 (carbon dioxide) and CO (carbon monoxide) are different gases. For CO as a combustion or exhaust gas you need a separate CO detector.
Looking for the right CO2 meter?
We supply CO2 meters with a true NDIR sensor, traffic-light display and data logger - for healthy air in offices, schools and workshops.
True NDIR measurement
Direct infrared reading instead of an eCO2 estimate.
Calibratable
Reference against fresh air at around 400 ppm.
Clear ventilation light
Green-yellow-red at 1000 and 2000 ppm.
Expert advice
Our specialists help you choose the right meter.