BME680 adddon board gas response tops out

Has anyone come across problems with the Pimoroni BME680 board gas resistance topping out, I have an old board running that will work up to a sensor gas resistance of just over 100Mohm, but 3 new boards appear to run out of ADC signal at just over 12Mohm gas resistance measurement. It’s a bit of a fundamental problem if Pimoroni have changed to a lower spec ADC on their BME680 boards.

I think there is a technical misunderstanding here: the BME680 has an I2C and a SPI interface (you can choose). So it does not deliver analogue values that an ADC has to convert. So if it tops out, it is an attribute of the sensor.

Yes I realised that after I’d written the post, which set me wondering about the various modes on the BME680 sensor config setup. although I consider that I’m using the same code on each of my 3 main logging systems, on similar pi zero2w, etc, but there is a large data sheet on setting up the heater config and the mode of the BME680 sensor and there is also a trade off in noise reduction , speed and ADC bit size (within the BME680 chip), so maybe I’ve fallen down that rabbit hole,but thanks for the response. I’ll think about it and chat with my ChatGPT4 and Code interpreter friend to see if they are any wiser. Once again thanks for the advice. Just as an aside, I was going to upload a chart, but don’t appear to be able to upload png files, what fotmat image does this site accept. ,

I do wonder why your readings top out. The sensor heats the air and then measures resistance. So low values tell you that there are many VOC in the air. High values are generated by inert gas, which does not react. That is one of the reasons the sensor can’t measure CO2 directly.

I’m not sure what i’m measuring, but it’s real and appears to have some connection with the forrest fires out here. I now have 3 BME680 setup, running at 360 deg C all showing same peaks, I am doing some test to see if sintering at high temp changes the gas sensitivity of the SnO2 . It you look at how the sensor works in ‘clean’ air you can seeon the temp resitance plot how the base resistance of the sensor changes, I’ve looked at it from 100 to 400deg C and there are 3 different regions, but the gas selectivity appears to stay the same. Up this uploads OK, It’s the gas resitsance of the sensor covering just under a month, before the fire the plots are the baseline you can as noise in the bottom. Still work in progress, Thanks for suggestions.

Have a read here: Guide for BME680 Sensor with Arduino (Gas, Temperature, Humidity, Pressure) | Random Nerd Tutorials

They have a nice explanation and a good graphic (just scroll down a little bit) regarding the gas-resistance. Resistance is with with no VOC/pollutants.