Re: [RFC] iio: ppm: Add IIO_PPM channel type

[Lars-Peter Clausen <lars@metafoo.de>]

On 08/28/2015 06:27 PM, Matt Ranostay wrote:
> Ah true but we could add a few IIO_MOD* modifiers for CO2, tVOC, VOC,
> etc, etc. I think Jonathan was suggesting that.

What I meant was CO2 measured in what unit per what unit.

Like is it weight per volume or volume per volume or weight per weight or
molecules per volume ...

- Lars

> 
> Thanks,
> 
> Matt
> 
> On Fri, Aug 28, 2015 at 2:16 AM, Lars-Peter Clausen <lars@metafoo.de> wrote:
>> On 08/28/2015 11:05 AM, Matt Ranostay wrote:
>>> On Thu, Aug 27, 2015 at 11:34 PM, Lars-Peter Clausen <lars@metafoo.de> wrote:
>>>> On 08/27/2015 11:40 PM, Matt Ranostay wrote:
>>>>> On Thu, Aug 27, 2015 at 8:58 AM, Lars-Peter Clausen <lars@metafoo.de> wrote:
>>>>>> On 08/27/2015 05:40 PM, Lars-Peter Clausen wrote:
>>>>>>> On 08/27/2015 08:45 AM, Matt Ranostay wrote:
>>>>>>>> There are air quality sensors that report data back in parts per million
>>>>>>>> of VOC (Volatile Organic Compounds) which are usually indexed from CO2
>>>>>>>> or another common pollutant.
>>>>>>>>
>>>>>>>> This patchset adds an IIO_PPM type because no other channels types fit
>>>>>>>> this use case.
>>>>>>>
>>>>>>> Hm, I'm not sure if parts-per-million is a good channel type. It's more of a
>>>>>>> scale. The type would be concentration.[...]
>>>>>>
>>>>>> Reading a bit more[1], concentration doesn't actually seem to be the right
>>>>>> term in this case, the correct term is mole fraction. Maybe we can use that
>>>>>> as the type. That also makes it clear that the unit is molecules per molecule.
>>>>>>
>>>>> Actually we can't use mole fraction for this because we aren't in a
>>>>> chemistry lab, and know the other compounds that make up the local
>>>>> atmosphere. Besides you'd have to include some insane lookup table for
>>>>> molar mass of carbon or whatever VOC being measured :)
>>>>
>>>> I don't think you'd need that. Mole fraction tells you the number of
>>>> molecules of something per total number of molecules. You don't need the
>>>> mass for this.
>>>>
>>>> But what exactly is the sensor measuring? CO2 (or VOC) molecules per total
>>>> number of molecules or number of CO2 molecules in a particular volume?
>>>>
>>> CC'ed my Swiss colleague on this because he knows much on the
>>> interworkings of VOC sensors than I could hope to.
>>> So simply these sensors are finding VOCs (which the sensor in question
>>> does CO2 and tVOC indexes.. probably not too independent of  each
>>> other)
>>>
>>> But molecules are not parts.. think of taking an X volume of air and
>>> figuring what is precent of oxygen is verses nitrogen, argon, etc,
>>> etc. Sure the highest is nitrogen at atomic weight 14.01 with ~78% of
>>> the "air", oxygen is ~20% at the weight of 16.00, and etc.
>>>
>>> Think about cutting cube into a millions of pieces and figuring an X%
>>> is Y substance which you can detect, but can't detect X, Y, and Z
>>> (think any particles in 'air' that aren't bonded with carbon). So mole
>>> fractions are impossible here, and you could only take a parts in a
>>> known volume.
>>>
>>> Most VOCs (if not all, bit of newbie here) use UV LEDs to ionize
>>> particles, and with some maths calculate the parts-per-million.
>>
>> But what per what? PPM is a completely ambiguous unit if you don't specify
>> parts of what per million of what.
>>