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    BME680/688 IAQ - results of the index are counterintuitive

    BME680/688 IAQ - results of the index are counterintuitive

    New Poster

    I'm using the BME680 or 688 together with the BSEC library to obtain an IAQ value, everything appears to work as I expect, the index starts low at a steady 25/27 and then after some time starts to report a varying IAQ, apparently with the changing enviroment.

    However, I noticed that when an SCD41 measuring CO2 besides the BME680 starts to detect an increased value of CO2, the BSEC IAQ falls, this does not make any sense to me, surely when in an enviroment with high or increasing CO2 levels, should the IAQ increase and not decrease as it does at present?

    My logic is the IAQ should climb as CO2 concentrations climb...

    The graph on the left is the BME680 CO2 ppm, on the right is BME680 IAQ and at the bottom is the SCD41 CO2 variations in the same environment.

    BME680 CO2 Equivalent.pngBME680 IAQ (BSEC).pngSCD41 CO2.png 


    You can observe that as the SCD41 reports rising CO2, the IQA falls as does the BME680 CO2 equivalent?


    What am I missing here?


    Has the BSEC library been validated and are there any results of the validation?

    8 REPLIES 8

    Community Moderator
    Community Moderator

    Hi G6EJDdavid,

    In the picture you sent for the first time, the IAQ and CO2 change trends outputted by BME680 are synchronized, and there is no obvious problem.


    The BME680 IAQ and SAGP40 IAQ you sent for the second time show big differences in their range of variation. It seems that these results were not obtained from testing in the same environment. Comparing them in different environments has little reference significance.


    Here are the graphs for today that compare the Sensirion SCD41 readings for CO2 with the BME688 IAQ readings and BME688 CO2 Equivalent.

    SCD41-CO2 Level

    SCD41 CO2 Level 11-08-23.png 

    And now the BME688 Results for IAQ:

    BME688 IAQ 11-08-23.png

    And now the BME688 CO2 Equivalent:

    BME688 CO2 Equiv 11-08-23.png

    For the avoidance of any doubt:

    1. All devices are connected via an I2C bus and are being read by an ESP32 processor and use the latest (for the BME688) BSEC library 2.

    2. The SCD41 is using the Sensirion driver.

    3. The devices are seperated by 2.6cm from each other, so for all practical and experimental purposes they are in the same environment.

    4. The graphs are all recorded on the same time base and so are aligned with each other.

    Looking at the results, you can see there is a clear correllation between an increased CO2 reading detected by the SCD41 and a reducing IAQ and CO2 Equivalent result from the BME688, this cannot be correct.

    Further, as the SCD41 records a reducing CO2 level the BME688 IAQ and CO2 Equivalent readings start to increase, this cannot be correct.

    Bosch (through yourself as the only respondent) seems to be in denial that there is anything wrong with the BSEC algorithm and yet there is clear evidence that the Bosch results do-not accord with the SCD41 sensor, which incidently has been validated and costs significantly more than the BME688.

    Do I trust the BME688 results - frankly no.

    I would expect  Bosch to either provide validation results / evidence for the BME688 sensor or to conduct their own trial that runs their sensor in parallel with an SCD41 variant.

    Also, I get similar comparitive results when using the SCD40 (the lower cost Sensirion sensor for CO2) and  previously I included a graph for the SGP40 results too which also provide important validation between the SCD41 and SGP40 for CO2 readings, noting the BME688 was showing the inverse results.

    The scale of the graphs is an irrelevance, it is the qualitative variations that is important, namely as the SCD41 CO2 levels fall the BME688 IAQ and CO2 Equiv values rise!  

    I have conducted some additional tests comparing the SCD41 and BME688 for CO2 values and the results are comparabl (both sensors within 3cm of each other).

    So I conclude that there is an issue with IAQ result in that the IAQ rises as CO2 concentrations fall.


    Has the BME680/688 IAQ been tested in the following conditions:

    1. A temperature considered to be comfortable, say 21°C?

    2. A humidity considered to be comfortable say 40%?

    3. An atmosphere with no CO2?

    4, An Atmosphere with no VOC?

    Then repeat with higher extremes of all 4 parameters to ensure there is both correlation and consistency with the IAQ result?

    Community Moderator
    Community Moderator

    Hi G6EJDdavid,

    We have already mentioned that from your screenshot, the changes in IAQ and CO2 of BME688 are normal. We have not conducted comparative testing with SAGP40, so we are not aware of the comparison between SCD41 and BME688.
    The BME688 IAQ value was measured in the reference VOC mixture, bVOC mixture used for the test are listed in the BME688 datasheet.