I know that air is 20.9% O2, but is that Mm, or L or what?
Thanks.
The units that are used probably depend upon the purpose you have in mind. The amount of oxygen can be stated as partial pressure, gms/cc, moles/km[sup]3[/sup] or whatever suits your purpose.
Alice, percentages are simply a measure of proportion. Neither cubic mm nor liters would make a difference.
Consider: we’ve got a canister filled with 100 cubic mm of air. Since 20.9% of that air is oxygen, there are 20.9 cubic mm of oxygen in our canister.
Now, we pull out another canister filled with 1 liter of air. This canister contais .209 liters of oxygen.
I am almost positive that the 20.2% figure is a measure of volume (ususally expressed in liters). Assuming ideal gas*, then PV=nRT would also give you the same 20.2% figure if you expressed it in terms of particle amount (moles) instead of volume (liters), so either could be considered valid.
However, due to the difference in molecular weights, a percentage derived from mass (expressed in grams) would be different.
*the assumption should be valid here. I can’t imagine that the constants for O[sub]2[/sub] and N[sub]2[/sub] would be different enough to cause the percentage to be off by much.
Percent is a unitless measure.
However, it could be percentage by mass or by volume (or by number of molecules)
Brian
It is by volume (20.9%). e.g see http://www.earth.monash.edu.au/firstyear/esc101102/0232ESC1.htm and lots of others.
By mass it is higher, as O2 weighs 32 and N2 = 28. Approximately 23% by weight
Also note that many sensor readings are on a “wet” or a “dry” basis, and, depending on the situation and the accuracy you need, you may need to take this into account.