Frequently Asked Questions


It is, of course, your meter so you do with it what you will. However, it is recommended to continuously measure the CO2 level in your classroom. Measuring just a few days to "get an idea" of the CO2 level in your classroom gives a false sense of security. CO2 is in fact strongly dependent on the occupancy rate in the classroom, the activity in the classroom, temperature and humidity. 

No, the sensor calibrates itself by measuring the CO2 level at times when the classroom is empty (especially at night :-)) and taking this as the reference.

In principle, the CO2 concentration in a room is homogeneous. However, due to partition walls, open windows, large rooms, etc., the CO2 value can be different in different places within the same room. It therefore makes sense to take measurements in several places in larger rooms (> 50 m²) or in rooms with open windows or doors.

  • Place the meter in the used part of the room as much as possible. 
  • Place the sensor on a table or cabinet, against the wall or in the middle of the room.
  • Set the sensor not next to a door or window or inlet of the ventilation system.
  • Make sure the meter is in a safe place so that it does not fall over or be spilled on.
  • Do not breathe near the sensor; this may affect the measurement. Do not place the meter directly next to a person, keep a distance of 1.5 m. 

When starting up the CO2 sensor, it may take a while before the sensor displays the correct value. So don't panic.

The minimum CO2 value in a room is 400 ppm. This is equal to pure air. In theory, it cannot be lower than this. Yet it can happen that you measure a lower value. Why is this?


ControlCO2 uses a non-dispersive infrared (NDIR) CO2 sensors. These depend on an infrared light source and detector to measure CO2 molecule counts. Over many years, both the light source and the detector deteriorate, resulting in slightly lower CO2 molecule counts. The industry calls this "drift".

To avoid sensor drift, the sensor must be calibrated using a known gas source. The most accurate way is to use 100% nitrogen (= 0 ppm CO2). However, calibrating sensors with nitrogen is very costly. A sealed calibration housing, a tank with pure nitrogen and calibration software are required to match the original factory test environment. Otherwise, the accuracy of the calibration cannot be guaranteed. 

Most commercial CO2 sensors therefore use pure air for calibration. Pure air (outside air) has a CO2 value of 400 ppm (outside air is actually between 350 ppm and 480 ppm at normal geographical locations).


CO2 sensors used in buildings to measure indoor air quality are difficult to calibrateRemoving the units from the wall for calibration is expensive and requires skilled personnel. Calibration schedules are therefore often ignored. 

To solve the problem of CO2 sensor calibration, most sensors use auto-calibration. The theory behind auto-calibration is that at a certain point every day a room is empty and the CO2 level should return to 400 ppm, the same as outside air. By storing the lowest CO2 readings during the day in the EPROM memory, a deviation of up to 400 ppm could be calculated and then added to or subtracted from the actual CO2 readings.

The advantage of auto-calibration is that the CO2 sensor calibrates itself during the lifetime of the sensor and can gradually make a correction to compensate for the change. 

The disadvantage of auto-calibration is that the indoor air must return to a 400 ppm CO2 level (empty room, clean air) every day. If this does not happen, the sensor will start to give incorrect values.


ControlCO2 is used in a poorly ventilated room. At night, the room is empty but the actual CO2 level only drops to 500 ppm (lowest value for a full day). So 500 ppm is the lowest value measured over 24 hours. ControlCO2 will assume that this is a measurement error and that it should be 400 ppm. It will apply an offset of 100 ppm.

However, if the next day the actual ppm value goes to 400 ppm, ControlCO2 will display a value of 300 ppm. This is because it has applied an offset of 100 ppm based on the previous calibration. ControlCO2 will therefore display a value lower than 400 ppm.


Always ensure that a CO2 level of 400 ppm is reached every 24 hours!

Does your sensor measure an incorrect value for several days? Put it outside (protected from the rain) for at least 24 hours. The sensor is now in clean air and the calibration should be perfect. Several days may be necessary (there is a chance that "dirty air" is trapped in the CO2 chamber).

Fact sheet

The above problem of incorrect calibration is also encountered with other commercial CO2 sensors. However, these sensors are often programmed in such a way that they never display values less than 400 ppm. Under the same circumstances, the erroneous calibration will also occur here. The sensor will therefore also measure a value of 300 ppm after a faulty calibration. However, the sensor will show 400 ppm on its display. So everything seems fine, but in reality it is not.

PS: With some sensors, the displayed value never goes below 413 or 430 ppm. This is because an exact value of 400 ppm could give the idea of manipulation more quickly.

ControlCO2 will always display the measured value from the CO2 sensor. Even if it is lower than 400 ppm. We do not manipulate anything.

Yes, this is very important. The moments at night the sensor uses to calibrate itself. The sensor also needs time to measure the correct value. So if you turn it off, you will have to wait a long time until it measures correctly again.

The Wi-Fi connection of the CO2 sensor will default to Channel 1. If many other Wi-Fi devices are using this channel, this may cause problems. It is then best to change the Wi-Fi channel of the CO2 sensor.

To change the Wi-Fi channel you need to modify (and reprogram) the source code of the sensor. In the file WiFiAP.h, modify the parameter int channel = 1 of function softAP. Choose a channel between 1 and 13.

After adjustment, you have to recompile and reprogram the code.

Gift card

To donate a gift card to your school, place a gift card with a minimum amount of 10 € in your shopping basket. When adding the card, you can enter your email address. We recommend that you enter your own email address so that the gift card is sent to you. Once you have received the gift card, you can send it to the school or teacher of your choice.

The school or teacher can then use the code on this gift card to pay when purchasing CO2 sensors.

Please note: The code on a gift card can only be used once. Once the code has been used, it cannot be reused.

Yes. The minimum amount is 10 € but there is no limit on the maximum amount.

No, gift cards are always sent to an email address. It is best to give your own email address but it can also be the email address of the school. In the latter case, make sure that this address is correct and that the school is informed.


As an employer, you are legally obliged to ensure that the CO2-concentration is lower than 900 ppm or that the ventilation flow is at least 40 m3/h per person present. This can be done naturally (by opening a window or door) or via an air-replacement system. However, you are not obliged to reduce the CO2-It is not possible to measure the content of the water continuously, but it is recommended.

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