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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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