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

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

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.