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Assumed basis of PPM

Assumed basis of PPM

(OP)
I see "PPM" used as a (dimensionless) unit of gas species concentration quite a lot. I kind of assume it's normally an implied mole (volume) fraction, but don't really know why. Or if others do too. Indeed I can't remember ever seeing it clarified in print, just "PPM".

Any/all anecdotal evidence welcomed.

Steve

RE: Assumed basis of PPM

An interesting question. Wiki has a section on this exact question.

RE: Assumed basis of PPM

Yes, an interesting question. For automotive exhaust gas analysis mass fraction would be logical (eg regulations usually specify mass limits for regulated emissions).

je suis charlie

RE: Assumed basis of PPM

It is good practice in science or engineering to never assume anything. As a mechanical design engineer, the most common use of "ppm" I see is with material linear CTE. The imperial units are typically inch/(inch*degF), which for most metals is a value in 10^-6 range. So "ppm" is often used to denote the value.

RE: Assumed basis of PPM

(OP)
"never assume anything"

True. So frustrating when publications don't feel the need to include vital information like this (mass vs volume ratio).

I did a straw poll around some of my colleagues to see if anyone knew whether the 21% oxygen in typical atmospheric air was by volume or by mass. Most looked a bit uneasy and didn't really know the correct answer.

Steve

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