Oxygen in Air

[shvet]

Good day, forum

21.7.5 OXYGEN-DEFICIENT ATMOSPHERES
... The oxygen content of air at sea level is 21%, but at high altitudes it can fall to 19.5%. ...

Can anyone share a source with real O2 concentration vs altitude? May be there is some map with O2 level?

 

[EmmanuelTop]

This is essentially Boyle's Law: the higher the altitude, the lesser is the pressure i.e. the lesser is the number of molecules and hence the O2 molecules as well.

http://www.altitude.org/air_pressure.php

https://www.wildsafe.org/resources/...itude-safety-101/high-altitude-oxygen-levels/

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1114067/pdf/1063.pdf

Dejan IVANOVIC
Process Engineer, MSChE

 

[shvet]

You are talking about effective O2 content which refers to human breathing. Lee is talking about real O2 content which refers to process explosion prevention (for example see AGA Purging Guide or NFPA 56&69). Effective O2 content has no influence on process equipment purging and inerting.

Sorry. I need another information.

 

[georgeverghese]

This is from Wikipedia on the subject "Atmosphere of the Earth"

"The relative concentration of gasses remains constant until about 10,000 m (33,000 ft)".

They are referring to dry basis composition. On dry basis, this relative concentration is predominantly between N2 and O2.

This contradicts your original statement however - perhaps you are referring to wet basis.

 

[shvet]

Sorry - Wikipedia is not a worth source of information.

As I understood CEN/TR 15281, NFPA 56, NFPA 69 and AGA guide "Purging Principles and Practice" deal with dry air. Please correct me. Moreover to mentioned codes and practices some authoritative source (book "Lee's Loss Prevention ...") says that O2 content depends on altitude and facility location. I tried to understood the real situation. Should I correct purging rates and consumption according to O2 content?

As I understood Google and forums - I should not. Real O2 content does not depend on altitude.

 

[georgeverghese]

You may be right - the reference quoted in Wikipedia for this snippet of information is the Engineering ToolBox, which perhaps isnt reliable??

But elsewhere in this same article, it says that concentration gradients occur only at much higher altitudes due to differential centrifugal forces acting on these molecules in rarefied air where collision frequency between molecules is much lower - this seems reasonable, since you need much lower absolute pressures to reduce intermolecular collision frequencies.

In any case, there is little difference between 19% and 21% - for purging purposes, I'd use the more conservative value.