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Explanation: Not to cast aspersions on your source text, but in American English, "gas proofness" sounds like a bad rendering of an idea in English...I would hesitate to use that phrase in learned company...but if that is what they are using in that field, more power to them! The idea is, of course, to what degree is the given material impervious to gas (any other than air) ab(ad)sorption. I would be inclined to use a more meaningful [to me] rendering: "gas tightness"..in other words, a material that is gas-tight is suitable for the purposes of the article described, and therefore would be syntactically in accord with the phrase "air tightness">
Explanation: Dr. R. Bavry has made (in my opinion) a good analysis of the situation.
However I would like to add some more comments.
We know that proofness cannot be found in dictionaries, as opposed to tightness.
We also know that "waterproof" means something like "watertight" with a small difference due to the fact that, originally, it just meant that it could resist to water agression (raincoat) and preserve you from being wet.
Now... with the invasion of the japanese waterproof watches that can even withstand 20 bars of water pressure (200 m), I believe that the perception of "waterproof" for many non-English native people is "watertight" hence the above brand new piece of vocabulary "gas proofness".
A new word was also invented for watches to clarify this new sense of water"proof" : it is water "resistant" : it's different from water"tight"... sorry I mean water"proof": it only can stand being wet under your shower in the case you forget it.
Maketing subtle arguments...
I stick to the idea that the writer made an analogy with waterproof and therefore meant "gastight" which would (should ?) be translated by "étanche au gaz"
reflection trial on a rainy sunday evening in France
Yves Georges France Local time: 22:27 Native speaker of: French PRO pts in pair: 1459
Explanation: May I just say that this illustrates once again the sometimes thorny issues that a translator must confront! But even more, I am somehwat amused that learned discourses in regard to those very same issues were disregarded and, in effect, found null and void by the asker in favor of a terse response (one, by the way, that had no solid backing other than a precious opinion!).
Should I then be unhappy? To some extent, inasmuch as it illustrates that the depth of expertise or research involved so often has no bearing on the validity of the answer accepted...and so it goes!
My fond regards to Yves dripping with knowledge (and rain) in Paris!