Testing condition for co2 bme680?

[suads]

Hello,

I'm using the bme-680 and bsec lib v2.4. for co2 equivalent measuring, and I have another sensor an photoacoustic NDIR sensor inside a closed box with dimensions 50x50x50 cm with a candle inside.

This project will be used for air quality in house, So my goal is to achieve similiar results for co2 measurments .

When both sensors are inside box, the values are near 400 maybe 100 ppm difference. But when I light up the candle the values go crazy the NDIR sensor is about 1800 ppm and the bsec is 400 ppm and stays there, the co2 acuuraccy changes 2 or 3. If I blow out the candle and open the box it takes NDIR some time to go to normal value 500 ppm , but then bme680 is going up to 2000 ppm and it takes over 4-5 hours to go normal again 500 ppm.

Are my testing conditions good? Is it posible to test the sensor in this way?  And what is the best way?

[BSTRobin]

Hi suads,

The BME688 is a MOX based sensor and it gives an equivalent CO2 output which is calculated based on the IAQ index. So, if user uses a stand-alone CO2 reference device it will be not a one to one comparison and is possible that it will not match in absolute value but the equivalent CO2 from BME688 can depict the relative changes in CO2 value.

[suads]

Hi,  @BSTRobin thanks for the response 

But I m having a huge diferrence about 1000 ppm under same conditions. I also used an arduino with the official bsec  https://github.com/boschsensortec/BSEC-Arduino-library getting similar results.

How can I be sure that my measuremnts are good? 

[BSTRobin · Accepted solution]

Hi suads,

Based on the information from BME680 datasheet,

• CO2 equivalents(ppm): The sensor does not directly measure CO2, but derives this from the average correlation between VOCs and CO2 in human's exhaled breath;
• b-VOC equivalents(ppm): The scaling is derived from lab tests with the b-VOC gas mixture described in below table,
• 
• The sensor will not directly get the CO2 result, and the concentration of the CO2 is relative to VOCs.

  So when customer just light up the candle and blow off the candle, the CO2 concentration was changed, but the VOCs concentration maybe not changed directly.

  At same time, customer can monitor the b-VOC result from the sensor.