# Air tightness vs Water tighness

by thierry2506
Tags: tighness, tightness, water
 P: 1 Hi, I am currently making test under vacuum and pressure to define sealing performance of a device. Test are made at +/- 0.5 bars. The result I find is that the device tested is water sealed but not air sealed. Is there any physic law showing that a water sealing performance at 0.5 bars will have an air equivalent sealing performance much lower ? Why a sealing performanc vacuum test, weither made under water or air is more severe than a pressure test ? Hope somebody can help ! Regards Thierry
 P: 306 Is it possible that the viscosity of the water is preventing the water from getting into the container?
 P: 156 water molecules are simply larger than air molecules. And what do you mean +/- 0.5 bar? 0.5 bar is approximately 0.5atm, which is by no means a strong vacuum
P: 306
Air tightness vs Water tighness

 Quote by jasc15 water molecules are simply larger than air molecules.
Really? I would have guessed that O2 and H2O would be similar in size. But the H2O would be more polar.
 Sci Advisor HW Helper P: 2,537 Well, I'm pretty sure that water vapor would also get through the gaps just fine. I expect that if you oil or wax the interor of your vacuum vessel, it's going to be much more airtight as well. I think (and this is just wild speculation, really) that the primary reason that the a vessel is watertight, but not air tight (and other gasses) is capilary action: Consider that the gaps are very narrow, which means that the capilary effect is very strong, and, simultaneously, that the force due to pressure difference is very small. Depending on what you're doing, you might be able to test this hypothesis by adding a surfactant to the water, and seeing if the vessel is still watertight.
 P: 306 $$\int\mbox{What's a surfactant?}dx$$
 Quote by actionintegral $$\int\mbox{What's a surfactant?}dx$$