Fix CO2 value interpretation

[Eyenseo]

The zigbee standard specifies that the values of section 4.13 Concentration Measurement are to be represented between 0 and 1 (section 4.13.2.1 Attributes). Further it is explained in section 4.13.2.1.1 MeasuredValue Attribute that "MeasuredValue represents the concentration as a fraction of 1 (one)."

Because the current implementation multiplies the given MeasuredValue by 1000000 (million) the published value is not only wrong, but changes in the inverse direction. The larger the actual measured value becomes (the larger the denominator is), the smaller MeasuredValue becomes.

Device (PPM)	MeasuredValue	z2m (PPM)
800	0.00125	1250
1250	0.00080	800

• I have tried this change as external converter with my own co2 zigbee device.
• Dependent changes
  • modernExtend.ts seems to make similar assumptions too, but I do not know how / when scale and change are used - before or after the conversion took place.
  • some other place?
• Figure out if some existing devices actually implement MeasuredValue in the current way. And provide device specific workarounds.
  • I would need help with that, as I own no other co2 sensor.

[Koenkk]

modernExtend.ts seems to make similar assumptions too, but I do not know how / when scale and change are used - before or after the conversion took place.

What device do you have? Maybe it's best to also refactor that to modernExtend.

[Eyenseo]

DIY project based on nrf52840/zephyr and scd41.

I would like to use modernExtend - I just don't know how to do that / where the calculations take place. But I have also not searched for documentation too much. My guess was that they somehow still make use of the function that I modified, but based on your answer that is not correct, which would mean that they convert via scale?
I wanted to get the PR out as it took me some head scratching why the values where weird and I was hoping to get some help from the community to figure out what to do with this change.

[Koenkk]

Using modern extend should be easy, see

zigbee-herdsman-converters/src/devices/diyruz.ts

Line 325 in cdb1b50

e.co2(),

as an example.

The conversion is done here:

zigbee-herdsman-converters/src/lib/modernExtend.ts

Line 2177 in cdb1b50

value = typeof scale === 'number' ? value / scale : scale(value, 'from');

[Eyenseo]

I'm unsure about reporting: {min: '10_SECONDS', max: '1_HOUR', change: 0.00005}, // 50 ppm change. I guess change is used before scale is applied and is usually used in a way of abs(previous-new) > change?

I haven't reached the point just yet where I can use the reporting feature - but I guess it makes sense that the device will be able to undo the fraction to compare the two values.

[Koenkk]

Thanks!