BME680 TVOC output does not seem correct

[Pablo]

BME680 TVOC reading (BSEC_OUTPUT_BREATH_VOC_EQUIVALENT) from sensor is super high: > 500ppb (>0.50 ppm), while other sensors returning 12-15 ppb.

Indoor air quality standards are much lower than this. Usually 50ppb to 350ppb (0.05ppm to 0.35ppm) is accepted as acceptable TVOC ranges for IAQ (Indoor Air quality) , however sensor IAQ output (BSEC_OUTPUT_IAQ and BSEC_OUTPUT_STATIC_IAQ) from sensor says it is completely healthy, how are these TVOC and AQI readings correlated? Is there an error in the library?

If the sensor is intended to be used as an IAQ sensor, these readings are completely off, keeping it running longer brings the reading higher as resistance decreases but I hardly ever get any readings lower than 0.50ppm  in a place that has a much lower TVOC concentration.

I have tested 2 different sensors with the same results, can someone explain if this is an error in the library or if there is something I am doing wrong?

Is this sensor not intended to be used to report TVOC values for IAQ?

Thanks.

Sample output from library:

======================================================
Temperature: 23.09 °C
Humidity: 39.01 %
Gas Res.: 193142.78 Ohm
TVOC: 0.51 ppm
Pressure: 102721.70 hPa

At the same time, a different sensor in the same location is returning PPB reading:

TVOC 811: 13 ppb (0.013 ppm)

[handytech]

Your results seem as expected. TVOC-equivalent (bVOC_eq) and CO2-equivalent (CO2_eq) are both calculated based on the sIAQ (static IAQ) output, with scaling derived from lab tests and field studies.

If the sIAQ output better correlates with with your expectations and reference devices, it could also be possible for you to derive your own sIAQ to ppm conversion in your application.

[Pablo]

Thanks for the reply, but then with your answer, we are looking at sensors that can provide reliable TVOC levels that are not possible to use for an indoor air quality monitoring project. 

Is this BME680 not intended to be used as a TVOC sensor that provides accurate results? 

If it is, can you provide a way of obtaining results that I could show to customers about their indoor TVOC concentration in something like the attached picture (Looking mainly at the TVOC part)?

Thanks

[handytech]

The BME680 is a great sensor to track tVOC levels, but is not selective to specific compounds, and as other sensors working on a similar MOX sensing technology, such devices cannot output an absolute tVOC level. One could derive a tVOC-equivalent estimation based on the sensor's outputs history, but as such these 'equivalent estimations' may slightly vary between use-cases and interpretations.

BSEC outputs will provide a reliable overview of the evolution of tVOC levels over time for indoor air quality monitoring projects. It also offers the flexibility to display the results in a way that best fits your project and customer expectations (e.g. sIAQ, bVOC_eq, or even your own algorithm based on compensated gas value or sIAQ).