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    BME680: IAQ and eCO2 worse after ventilation

    BME680: IAQ and eCO2 worse after ventilation

    New Poster

    I have recently started using a BME680 and I do operate it in the office. The behavior, respectively the measured values seem a bit strange to me, see screenshot in the attachment.

    Around 11:20 I opened all windows completely for about 5 minutes, so that there is a draft.

    Immediately before venting, IAQ was at 113 and eCO2 was at 660 ppm. During ventilation, both values drop as expected.

    Then, about 30 minutes after opening the windows, the IAQ is 265 and eCO2 2600 ppm. So, within a short period of time, the values have increased dramatically and continue to remain significantly higher than before ventilation.

    I would actually expect IAQ and eCO2 to drop after ventilation and remain well below pre-ventilation levels for a period of time and then gradually rise again.

    So if I were to rely on the data, the air quality would be much better if I refrained from ventilating. That seems pretty implausible to me.

    Apart from the airing, the conditions in the premises have not changed.


    6 REPLIES 6

    Community Moderator
    Community Moderator


    Thanks for your inquiry.

    The IAQ and eCO2 vales come from the BSEC library output for BME680. There is a two-bit status for IAQ accuracy status. "0" means IAQ is not calibrated. "1" and "2" mean IAQ is being calibrated. "3" means IAQ is fully calibrated and the output of IAQ and eCO2 can be trusted. In order to achieve accuracy status "3", you need to use a bad air such as a Sharpie pen to approach BME680 sensor vent hole, then BSEC library will quickly track the base line and figure out the difference between good air and bad air. This means that when you plot IAQ and eCO2 data with windows opened and ventilation on, you also need to monitor the IAQ accuracy status to see if it is "3" or not.


    Here you can see the same section as above including IAQ status:


    Let's take another day:


    It's the same behaviour. Air quality and eCO2 is getting worse after venting. The IAQ status euqals 3 for almost the whole period. It only drops to 2 for 10 minutes around the venting process.

    Community Moderator
    Community Moderator


    Thanks for the plots. I can see IAQ accuracy status of 3 after venting, while temperature and humidity drop suddenly. IAQ and eCO2 output values from BSEC library are related to BME680 temperature and humidity measurements. I wonder if you have a reference meter to monitor the IAQ value and temperature at the same time. Maybe it will show the same behavior as the BME680 board you are using.


    I don't have a reference meter for IAQ, but I do have a SCD30 to measure temperature, humidity an CO2:


    It shows the same behaviour in temperature and humidity - I didn't expect anything else.

    It also shows that eCO2 is completely wrong.