@VikeMan
I think it's real that there is a blanket effect, because this is used by people working on solving real problems in real products (be it photography or food preparation) rather than speculating about theory on a homebrewing forum.
But the blanket effect is something that is somehow secondary. If you can expel the air in the fermenter by purging it with CO2 or another gas from a cylinder, then you go back to the initial problem. Homebrewers are interested in better beer not better gas theory understanding.
In a fermenter closed by a
gorgogliatore* yes, the CO2 coming from the beer will "expel" the air, not the way a liquid would do, but certainly will decrease the inner content of O2 because external air does not enter and internal air will be more and more CO2. CO2 hasn't got a preferential pathway toward the exit leaving the previous air inside.
As an example, I have this quotation in my notes:
" As a chemist who works with oxygen reactive and mositure sensitive reagents, learning barrier and air free technique to exclude oxygen is something very basic to most all organic synthetic chemistry. And the food process industry has ALWAYS worked hard to minimize exposure to oxygen during all food process steps excluding baking and tunnel drying. I have always used dry nitrogen gas as a purge and blanket when cider and meadmaking. "
This can be found as a note to this text:
Low Oxygen Brewing; Exploring LODO
You see, this person works with those problems, and he believes there is a blanket effect, and he says "purge and blanket" because he is not interested in the "blanket" question as separate from the "purge" question.
By the same token, in chemical photography people "purge and blanket" chemical solutions which are spoiled by oxygen. That stuff costs, and people do things with a reason.
I answered to "my question" rather than the question asked because I think that the real interest in an homebrewing forum is better beer.
* a
bubbler?