# Are airtight and watertight the same thing? **URL:** \ **Category:** Factual Questions\ **Created:** [December 21, 2004, 9:13pm UTC](https://boards.straightdope.com/t/are-airtight-and-watertight-the-same-thing/281049 "2004-12-21T21:13:21Z")\ **Posts on this page:** 14\ **Page:** 1
**Author:** ![Chairman\_Pow](https://avatars.discourse-cdn.com/v4/letter/c/838e76/32.png) [@Chairman\_Pow](https://boards.straightdope.com/u/Chairman_Pow)\ **Post date:** [December 21, 2004, 9:13pm UTC](https://boards.straightdope.com/t/are-airtight-and-watertight-the-same-thing/281049/1 "2004-12-21T21:13:21Z")
OK, I know the semantic difference, but can’t think of an example of where something would be watertight but not airtight. ---
**Author:** ![micco](https://avatars.discourse-cdn.com/v4/letter/m/5f8ce5/32.png) [@micco](https://boards.straightdope.com/u/micco)\ **Post date:** [December 21, 2004, 9:29pm UTC](https://boards.straightdope.com/t/are-airtight-and-watertight-the-same-thing/281049/2 "2004-12-21T21:29:29Z")
It’s probably pretty common in laboratory equipment. I knew some combustion researchers who had to take extraordinary measures to seal their gear when they switched fuels from large-molecules gasses like methane or propane to using hydrogen. It is fairly easy to make things airtight, but stopping hydrogen leaks is a whole different matter. I can’t think of a specific example where I’ve had a seal that was watertight but not airtight, but I could probably make one if I tried. ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [December 21, 2004, 9:39pm UTC](https://boards.straightdope.com/t/are-airtight-and-watertight-the-same-thing/281049/3 "2004-12-21T21:39:53Z")
it is possible to have a seal that is watertight but not gas-tight, particularly in a pressurised container - fluids are (generally) so much more dense and viscous than gases that they can get stuck in spaces where gases can be forced through. There are also some plastics that are gas permeable (to some gases), but not water permeable. ---
**Author:** ![Bippy\_the\_Beardless](https://avatars.discourse-cdn.com/v4/letter/b/ac8455/32.png) [@Bippy\_the\_Beardless](https://boards.straightdope.com/u/Bippy_the_Beardless)\ **Post date:** [December 21, 2004, 10:11pm UTC](https://boards.straightdope.com/t/are-airtight-and-watertight-the-same-thing/281049/4 "2004-12-21T22:11:12Z")
Thought of an obvious common example. The wine cork is partially permiable to gas but not to liquid. Barrels used in storeing spirits have a permiability to gasseous ethanol, the so called Angel’s share ---
**Author:** ![Duckster](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/duckster/32/1244_2.png) [@Duckster](https://boards.straightdope.com/u/Duckster)\ **Post date:** [December 21, 2004, 10:33pm UTC](https://boards.straightdope.com/t/are-airtight-and-watertight-the-same-thing/281049/5 "2004-12-21T22:33:04Z")
> [@micco](#): > > I can’t think of a specific example where I’ve had a seal that was watertight but not airtight, but I could probably make one if I tried. The sphincter, but it’s not 100 percent foolproof. ---
**Author:** ![Padeye](https://avatars.discourse-cdn.com/v4/letter/p/a9a28c/32.png) [@Padeye](https://boards.straightdope.com/u/Padeye)\ **Post date:** [December 21, 2004, 10:58pm UTC](https://boards.straightdope.com/t/are-airtight-and-watertight-the-same-thing/281049/6 "2004-12-21T22:58:56Z")
A seal that may keep water in may not keep oil in but IIRC that is because of surface tension rather than molecule size. Plenty of membranes are more permeable to small molecules but less so or not at all to larger molecules. Take three balloons. Fill one with helium, one with air and one with water, each to the same size. Compare sizes over time. ---
**Author:** ![AskNott](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/asknott/32/5790_2.png) [@AskNott](https://boards.straightdope.com/u/AskNott)\ **Post date:** [December 21, 2004, 11:56pm UTC](https://boards.straightdope.com/t/are-airtight-and-watertight-the-same-thing/281049/7 "2004-12-21T23:56:04Z")
In colloquial US English, two examples leap out: An alibi can be airtight, but never watertight. Roughly equal to “Is the Pope Catholic?” is the you-betcha question, “Is a frog’s ass watertight?” Nobody ever asks if it’s airtight. ---
**Author:** ![Chairman\_Pow](https://avatars.discourse-cdn.com/v4/letter/c/838e76/32.png) [@Chairman\_Pow](https://boards.straightdope.com/u/Chairman_Pow)\ **Post date:** [December 22, 2004, 2:26am UTC](https://boards.straightdope.com/t/are-airtight-and-watertight-the-same-thing/281049/8 "2004-12-22T02:26:21Z")
> [@Padeye](#): > > A seal that may keep water in may not keep oil in but IIRC that is because of surface tension rather than molecule size. Now that you mention it I recall a friend trying to describe how Army canteens can keep water in without being unduly hard to unscrew - it was something like the gap between the threads of the nozzle and cap will let some water pass, but it becomes “stuck” and won’t let anything pass. Is that on the right track? The surface tension thing makes sense. ---
**Author:** ![Rick](https://avatars.discourse-cdn.com/v4/letter/r/9f8e36/32.png) [@Rick](https://boards.straightdope.com/u/Rick)\ **Post date:** [December 22, 2004, 2:39am UTC](https://boards.straightdope.com/t/are-airtight-and-watertight-the-same-thing/281049/9 "2004-12-22T02:39:11Z")
One of the problems when car makers changed from R-12 to R-134 back in the early to mid 90s was the problem that flex hoses that did contain R-12 would leak R-134. :smack: Post # 1900. yea me! ---
**Author:** ![micco](https://avatars.discourse-cdn.com/v4/letter/m/5f8ce5/32.png) [@micco](https://boards.straightdope.com/u/micco)\ **Post date:** [December 22, 2004, 2:25pm UTC](https://boards.straightdope.com/t/are-airtight-and-watertight-the-same-thing/281049/10 "2004-12-22T14:25:39Z")
> [@Duckster](#): > > The sphincter, but it’s not 100 percent foolproof. It is airtight when sealed. It just opens under pressure. ---
**Author:** ![Quercus](https://avatars.discourse-cdn.com/v4/letter/q/7ab992/32.png) [@Quercus](https://boards.straightdope.com/u/Quercus)\ **Post date:** [December 22, 2004, 3:00pm UTC](https://boards.straightdope.com/t/are-airtight-and-watertight-the-same-thing/281049/11 "2004-12-22T15:00:23Z")
That’s the whole point of Gore-Tex, and all it’s imitators. Water vapor gets out, liquid water can’t get in. ---
**Author:** ![Lumpy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lumpy/32/446_2.png) [@Lumpy](https://boards.straightdope.com/u/Lumpy)\ **Post date:** [December 22, 2004, 3:12pm UTC](https://boards.straightdope.com/t/are-airtight-and-watertight-the-same-thing/281049/12 "2004-12-22T15:12:02Z")
A substance that dissolved in water could be airtight but obviously wouldn’t remain watertight for very long. A solid block of halite (salt) for example. ---
**Author:** ![1010011010](https://avatars.discourse-cdn.com/v4/letter/1/ea666f/32.png) [@1010011010](https://boards.straightdope.com/u/1010011010)\ **Post date:** [December 22, 2004, 5:54pm UTC](https://boards.straightdope.com/t/are-airtight-and-watertight-the-same-thing/281049/13 "2004-12-22T17:54:56Z")
Well, from an enginneering perspective, you’d have an acceptable leakage rate, and “tight” things leak at less than that rate. Leakage rate is a mixture of the mechnical clearances of the seal, the viscosity of the fluid, pressure of the system, etc. I think it meshes with common experience that gases will pass through a given size opening at a higher volumetric flowrate than liquids. ---
**Author:** ![Colophon](https://avatars.discourse-cdn.com/v4/letter/c/f05b48/32.png) [@Colophon](https://boards.straightdope.com/u/Colophon)\ **Post date:** [December 22, 2004, 6:09pm UTC](https://boards.straightdope.com/t/are-airtight-and-watertight-the-same-thing/281049/14 "2004-12-22T18:09:44Z")
What **Quercus** said. Breathable waterproofs are designed to be watertight but not airtight. Well, gastight.