On 08/28/2015 11:05 AM, Matt Ranostay wrote: > On Thu, Aug 27, 2015 at 11:34 PM, Lars-Peter Clausen <lars@xxxxxxxxxx> wrote: >> On 08/27/2015 11:40 PM, Matt Ranostay wrote: >>> On Thu, Aug 27, 2015 at 8:58 AM, Lars-Peter Clausen <lars@xxxxxxxxxx> 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. -- To unsubscribe from this list: send the line "unsubscribe linux-iio" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
