How do CO2 sensors work?
A CO2 sensor does not count molecules, it measures light: carbon dioxide absorbs infrared at one specific wavelength. What follows from this for accuracy, calibration and selection in building automation – and how to recognise a good data sheet.
How an NDIR sensor measures
The principleAlmost all CO2 sensors in building automation are non-dispersive infrared, NDIR for short. The method is robust, selective for CO2 and can be built small. Our EnoSense® CO2 and EnoPuck® CO2 sensors also work on this principle. Four parts make up the sensor:
Four methods compared
What is inside the housingNot every device that displays “CO2” actually measures CO2. For ventilation decisions according to thresholds such as EN 16798-1, only methods that detect the gas itself are suitable.
| Method | How it measures | Strength | Limit |
|---|---|---|---|
| NDIR, single channel | infrared absorption at 4.26 µm, one detector | Widespread, inexpensive, accurate enough for ventilation | Drifts as the light source ages; needs automatic correction or calibration |
| NDIR, dual channel | additional reference channel at a wavelength without CO2 absorption | Compensates light-source ageing itself, even in rooms that are never ventilated | More expensive, needs more energy |
| Photoacoustic (PAS) | pulsed infrared heats the CO2, a microphone picks up the pressure wave | Very small, economical | Calibration is an issue as with NDIR; more sensitive to vibration and noise |
| Metal oxide (MOX), “eCO2” | measures volatile organic compounds and calculates a CO2 equivalent | Cheap, reacts to odours and emissions | Does not measure CO2 – unsuitable for CO2 thresholds |
Self-calibration: practical, but not everywhere
Automatic correctionThe light source of a single-channel sensor ages, and the reading slowly drifts over the years. Many devices compensate automatically (“Automatic Baseline Correction”, ABC): they remember the lowest value of the past days and equate it with outdoor air, around 420 ppm.
This works as long as the room is regularly ventilated down to outdoor level – an office overnight, for example. It goes wrong where that never happens: in rooms occupied around the clock, in laboratories, greenhouses or poorly ventilated rooms. There the sensor takes an elevated value for outdoor air and then permanently reads too low – exactly where it matters.
The alternative is calibration on demand: the device is deliberately calibrated in fresh air or with test gas. This costs a maintenance visit but delivers a traceable value.
Reading the data sheet
Six key valuesTwo sensors labelled “0 to 5,000 ppm” can behave very differently in operation. These figures make the difference:
| Value | What it means | What to look for |
|---|---|---|
| Accuracy | usually given as “± (x ppm + y % of reading)” | At 1,000 ppm, ± 30 ppm + 3 % add up to ± 60 ppm. Distinguish typical and maximum values. |
| Measuring range | up to which concentration the reading is valid | Around 2,500 ppm is enough to control an office; more for peak values and documentation. |
| Measuring interval | how often a new value arrives | Minutes are enough for ventilation; WELL requires at most ten minutes. |
| Pressure compensation | whether the reading is corrected for air pressure | Without compensation, the sensor reads around ten percent low at 1,000 m altitude. |
| Long-term stability | how much the reading drifts per year | Together with the calibration type, it determines the maintenance interval. |
| Calibration | automatic correction or calibration on demand | Does the type suit how the room is used? See the section on self-calibration. |
Why air pressure matters
Altitude and weatherAn NDIR sensor counts how many CO2 molecules are in the light path. At lower air pressure there are fewer for the same proportion – the sensor reads too low. That is why good data sheets state pressure compensation or an altitude correction.
At a given site, air pressure varies with the weather by only a few percent. Altitude is what matters: a building at 1,000 m is around ten percent below standard pressure. If the sensor is not set for this, all thresholds are off accordingly.
Wireless and energy
Measuring without cablesThe light source is the largest consumer in a CO2 sensor. A device on a power supply can measure every second; a wireless sensor without cables has to budget light and battery and measures less often.
In pure solar operation under typical lighting, the EnoSense® CO2 delivers a reading every five minutes; in longer darkness it extends the interval to 60 minutes, bridged by two AAA cells. The EnoPuck® CO2 traffic light runs on a plug-in power supply and delivers a value every 60 seconds by default.
For demand-controlled ventilation, minutes are entirely sufficient because CO2 changes slowly. A shorter interval brings little there – but an interval longer than ten minutes gives away control quality.
Location and thresholds
Before measuringThe best sensor measures nonsense if it hangs in the wrong place: in the occupied zone, not next to the door, not in the supply air jet and not in the exhaust air. Which values then apply – workplace rules, EN 16798-1 – is covered in separate articles.
Selection checklist
6 points- Ask for the measuring method: NDIR or photoacoustic – no MOX sensor with “eCO2” if ventilation is to follow CO2 thresholds.
- Require accuracy as “± (ppm + % of reading)”, with typical and maximum values separately.
- Choose the calibration type to suit the use: automatic correction only where the room is regularly ventilated to outdoor level; otherwise calibration as a maintenance service.
- Clarify pressure compensation or altitude setting, especially above around 500 m.
- Specify the measuring interval: at most ten minutes for control and verification.
- Specify the power supply: mains adapter, solar with battery or battery – and with it the possible interval.
Frequently asked questions
7 questionsHow does a CO2 sensor work?
Most work on the non-dispersive infrared principle (NDIR): infrared light passes through a measuring chamber, CO2 absorbs the 4.26 µm wavelength, and a detector measures how much light arrives. From this the electronics calculate the concentration in ppm.
What is the difference between CO2 and eCO2?
eCO2 is not a measured value but an equivalent that metal-oxide sensors calculate from volatile organic compounds. It can differ considerably from real CO2 and is not suitable for CO2 thresholds.
How accurate is a CO2 sensor?
Good NDIR sensors typically achieve ± 30 ppm plus 3 percent of reading, i.e. around ± 60 ppm at 1,000 ppm. That is more than accurate enough for ventilation decisions.
Does a CO2 sensor need calibration?
Yes, the light source ages. Either the sensor compensates automatically – which requires the room to be regularly ventilated to outdoor level – or it is calibrated deliberately. On our sensors this is done via the service button.
What is ABC in CO2 sensors?
Automatic Baseline Correction: the sensor equates the lowest value of the past days with outdoor air. In rooms that are never properly ventilated this leads to permanently low readings.
Does altitude affect the reading?
Yes. Without pressure compensation, an NDIR sensor reads around ten percent low at 1,000 m altitude. Good devices compensate for air pressure or can be set to the altitude.
How often should a CO2 sensor measure?
For ventilation, a value every few minutes is enough, at most every ten minutes. CO2 changes slowly; a one-second rate brings no benefit there.
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Subscribe to the newsletterInformation as of 13 November 2026. General measurement technology according to the state of the art; information on our devices from their data sheets and product pages. The data sheets and manuals in their current version are authoritative.
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