Air tightness vs Water tighness

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In summary: The higher the delta P the less likely that atmospheric pressure will cause a leak. In summary, a vacuum test and a pressure test are not equivilent. Even under the same pressure differentials, there are many factors that effect the sealing between two surfaces. The design of the mating surfaces are paramount.
  • #1
thierry2506
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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 physics 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


:frown:
 
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  • #2
Is it possible that the viscosity of the water is preventing the water from getting into the container?
 
  • #3
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
 
  • #4
jasc15 said:
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.
 
  • #5
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.
 
  • #6
[tex]\int\mbox{What's a surfactant?}dx[/tex]
 
  • #7
actionintegral said:
[tex]\int\mbox{What's a surfactant?}dx[/tex]

A surfactant is something that breaks up surface tension. The most familiar example is soap.

http://en.wikipedia.org/wiki/Surfactant
 
  • #8
Viscosity and size are both the problem. Water does not exist as H2O but as a much larger collection of water molecules. This is why water has a much higer boiling point that a molecule of that weight would suggest. This is caused by hydrogen bonding between water molucules. Because they are stuck together in groups the material is also more viscous than you would expect. This makes the practical size of the molecule much bigger. It may also have to do with polarity and "sticking" to the walls of the sealing material (this is just a guess).

A vacuum test and a pressure test are not equivilent even under the same pressure differentials. There are many factors that effect the sealing between two surfaces, the least of which is the gasket. The design of the mating surfaces are paramount. There are many surfaces that are designed to leak in one direction but not the other. Pump seals are a prime example. Some of them are designed to allow a small amount of liquid (gas) to be sucked into the pump but nothing out. This assures that a leak is controlled in certain manner.

Also remember that the highest delta P you can get from a vacuum test is 1 ATM while a pressure test can go much higher.
 

Related to Air tightness vs Water tighness

What is the difference between air tightness and water tightness?

Air tightness refers to the ability of a material or structure to prevent the passage of air, while water tightness refers to the ability to prevent the passage of water. This means that a material or structure can be air tight without being water tight, and vice versa.

Why is air tightness important?

Air tightness is important for several reasons. It helps to improve the energy efficiency of a building by preventing the loss of heated or cooled air, which can lead to lower energy bills. It also helps to improve indoor air quality by preventing the entry of outdoor pollutants and allergens. Additionally, air tightness can help to prevent moisture damage and improve the durability of a structure.

What are some common methods for testing air tightness and water tightness?

For air tightness, the most common method is the blower door test, which uses a fan to measure the air flow through a building. Water tightness is typically tested by subjecting a structure to simulated rainfall or water pressure and checking for any leaks or water intrusion.

What factors can affect the air tightness and water tightness of a building?

Several factors can impact the air tightness and water tightness of a building, including the quality of construction, the type of materials used, the design and layout of the building, and the presence of any penetrations or openings in the structure. Additionally, weather conditions and natural wear and tear can also affect the performance of a building's air and water barriers.

Can a building be both air tight and water tight?

Yes, it is possible for a building to have both high levels of air tightness and water tightness. This is typically achieved through careful design and construction, the use of high-quality materials and products, and regular maintenance and inspections to identify and address any potential issues or weaknesses in the building's air and water barriers.

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