Ah true but we could add a few IIO_MOD* modifiers for CO2, tVOC, VOC, etc, etc. I think Jonathan was suggesting that. Thanks, Matt On Fri, Aug 28, 2015 at 2:16 AM, Lars-Peter Clausen <lars@xxxxxxxxxx> wrote: > 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
