By Thomas De Wilde

How come your CO2 sensor is measuring below 400 ppm?

How come your CO2 sensor is measuring below 400 ppm?

So your CO2 sensor is measuring below 400 ppm? Don’t panic! There is an easy explanation and solution for this problem. The content of this article is based on measurements from the Crodeon CO2 sensor which uses the SCD30 sensing element by Sensirion AG, but it might be relevant for other self-calibrating NDIR sensors as well.

How much ppm is fresh air?

In 2020 the global average of atmospheric CO2 was 412,5ppm, so if your sensor is measuring below 400ppm it probably needs to recalibrate in the correct conditions. The situation is usually caused by the automatic recalibration process of the sensor.

Crodeon has you covered

We advise our customers that the sensor should see 400 ppm outside air regularly, for example during the evening or night phase. By doing this the sensor will recalibrate and consider the lowest measured values of the past period as “fresh air”, or about 400 ppm.

In the datasheet of the SCD30 sensor provided by Sensirion (which can be found on our product page the self-calibrating functionality, also known as the ASC field-calibration algorithm, is explained. This is what it says:

“To work properly SCD30 has to see fresh air on a regular basis. Optimal working conditions are given when the sensor sees fresh air for one hour every day so that ASC can constantly recalibrate.”

Have a look at the graph below. In the data from this Reporter on November 16th we see that it has measured well above 400 ppm for at least three days.

co2 sensor calibration

As a result the sensor calibration algorithm compensated for this. That is why we see measurements below 400 ppm during the following nights. On the 18th of November we advised our customer that the sensor was not getting enough fresh air at night. On the 22nd of November we see in the data that the calibration algorithm adjusted itself to the new circumstances.

Why does my Crodeon CO2 sensor measure below 400 ppm?

As we see in the chart it can happen that your CO2 sensor measure below 400 ppm. In most of the case this is due to environment surrounding the CO2 sensor in combination with the automatic calibration functionality. The sensor will take an average of the lowest measured CO2 levels during a day and will recalibrate itself with it.

When the CO2 sensor doesn’t get enough “fresh” air during consecutive days, it can start measuring below 400 ppm. Placing it regularly for one hour in 400 ppm outside air will help to avoid this.

Sensor error margin

In addition to the calibration process, take into account the error margin of the sensor. In this case the error margin is ±30 ppm + 3% of the measured value. Also notice that the self calibration can take up to 7 days to work at is best, but result should already been seen after 2-3 days.

Measuring physical quantities remains a complex and delicate job. Our job is to help you get reliable measurements and interpret the data in the right way. If you have any more questions on this topic, don’t hesitate to reach out at

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