When it comes to low cost CO2 sensors, there are basically three different methods to measure CO2.
- Estimated CO2 (eCO2) through TVOC sensors
- NDIR sensors
- Photo-acoustic sensors
Estimated CO2 (eCO2) through TVOC sensors - not recommended -
The first method via TVOC sensors is extremely unreliable. Some TVOC sensors like the Sensirion SGP30 estimate CO2 via organic components in the air. When you breathe out, you add organic components to the air which these sensors pick up and try to calculate a corresponding CO2 ppm value. The problem is that also other indoor sources like deodorizers add these chemical components to the air and thus the estimated CO2 rises -even though nobody might be in the room. We strongly advise against using TVOC sensors for CO2 estimation and thus will also not analyse it further in the rest of this article.
CO2 gas absorption principle
Both NDIR as well as photo-acoustic sensors are much more accurate than eCO2 from TVOC sensors as these sensors measure the CO2 directly and both are based on the absorption principle of gases. Different gases absorb light at specific ranges of wavelengths. So if the light is tuned at a certain wavelength and a receiver measures the incoming light levels, the rate of absorption can be calculated and with this the concentration of the gas established.
Different gases have different absorption bands:
To measure CO2, the light is tuned to emit a light of close to 4.26 micron (μm) which corresponds to the absorption of CO2.
NDIR CO2 Sensors
NDIR stands for “nondispersive infrared” and is a common technology to measure CO2. Thereby an infrared light emitter, typically an LED, sends light in a chamber filled with ambient air to a receiver.
Some NDIR sensors have dual channels, one for the measurement of the CO2 levels in the ambient air and one with a control gas as a reference. This allows to detect drifts in the measurements and increases accuracy. One of the disadvantages of NDIR sensors is their size. There needs to be a certain minimum distance between the light emitter and receiver to achieve a certain level of accuracy and thus modern NDIR sensors like the SenseAir S8 that we use in our air quality kits are more than 3 cm long. This is not too big but would prevent certain applications, e.g. in a mobile phone.
Photo-Acoustic Sensors
Photo-acoustic sensors use the same working principle of the absorption wavelengths but unlike the NDIR sensors that measure the receiving light from an emitting LED, photo accoustic sensors measure the absorption with a microphone. When the CO2 molecules absorb the IR light, they start to “humm” and this sound can be picked up by a microphone. The biggest advantage of this principle is that the detection does not rely on line-of-sight anymore and thus these sensors can be built much smaller. Some are smaller than 1 cm3.
Comparison between SenseAir S8 (NDIR) and Sensirion SCD40 (Photo-Acoustic)
We have tested two very popular low-cost CO2 sensors, the SenseAir S8 and the Sensirion SCD40. Before looking at the results, here is a comparison of the official specs.
| Criteria | Sensirion SCD 41 | SenseAir S8 |
|---|---|---|
| Working Principle | Photo-Acoustic | NDIR |
| Accuracy | ±(40 ppm + 5 %) | ±(75 ppm +3%) |
| Range | 400 ppm – 5000 ppm | 400 ppm – 5000 ppm |
| Working Temperature | -10 - +60 °C | 0 - +50 °C |
| Connectivity | I2C | UART |
| Size | 10.1 x 10.1 x 6.5 mm | 33.5 x 20.0 x 8.5 mm |
| Picture |  |  |
Except for the size and connectivity, the specs are quite similar. They do also cost around the same (USD 20).
Side-By-Side Comparison
We tested the SenseAir S8 and Sensirion SCD40 / SCD41 for a few weeks and saw them behaving very similarly. Below is a chart over 14 days in a typical indoor home environment (bedroom).
Both sensors deliver very reliable results and the only difference that can be spotted is that the S8 seems to be a bit more sensitive and picks up higher values more. This can be seen on an X Y chart:
We have been using the SenseAir S8 in our air quality monitor and open source kits for a few years and have made very good experience with. The Sensirion SCD4x seems to give a similar accuracy in a smaller form factor and is a good alternative.
