Assumed basis of PPM 1

[SomptingGuy]

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

 

[VEBill]

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

 

[gruntguru]

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

 

[tbuelna]

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.

 

[SomptingGuy]

"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

 

[VEBill]

Nice how careful Wiki is on the subject of oxygen in the atmosphere: Oxygen 209,460 ppmv (parts per million by volume)