Allow VOC/NOx/CO2 offset recovery

[wangwillian0]

If I understood correctly, reseting the device means losing the moving average required for the VOC and NOx indexes and losing the moving minimum required for the CO2 sensor. This becomes a problem specially if you set large learning durations.

Saving the history of the sensors is probably prohibitive because of the amount of writes to the flash, but normally the device is connected to a server (either Airgradient or homeassistant), which means we have means to retrieve the historical data and set it manually just like another configuration.

I havent dig very deep on the code, but I found https://github.com/airgradienthq/arduino/blob/master/src/Libraries/Sensirion_Gas_Index_Algorithm/src/VOCGasIndexAlgorithm.h#L53-L63, which hints a partial implementation of such feature.

Does it make sense? I can try to implement something with a little more context.

[samuelbles07]

Hi @wangwillian0 ,

This idea is cool. Yes for TVOC and NOx indexes we can do this and you're right to point the methods to do this. But for CO2, could you elaborate the goals?

[wangwillian0]

I took a look again and only the VOC seems to support such feature (I opened an issue at Sensirion/gas-index-algorithm#25 to ask about the NOx).

I also thought the CO2 sensor calibration was more customizable and allowed hardcoding the calibration offset, which doesn't seem to be true.

[samuelbles07]

As you might know too, the algorithm needs to be fed with TVOC and NOx raw values every seconds to make it "accurate". Hence, even updating the last state might not working that well since we will miss a couple of raw measurements. But this is only a theory though, needs to be validated.