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Testing Empty Housing

Discussion in 'Tips and Techniques' started by divinh, Sep 7, 2018.

  1. giffenk

    giffenk Great White

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    Your explanations do not make sense. I.e. what do you mean by decompress? Maybe try different words?

    Do we all agree the housing is rigid?
     
  2. caruso

    caruso Banned

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    I agree that the housing is rigid.

    I've used all my safe words. Maybe instead of decompress I should have said "A camera housing is not meant to break or implode during normal use".
     
  3. giffenk

    giffenk Great White

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    Agreed - imploding is not good.

    A rigid housing does not compress or shrink in size. This means the contents are not subjected to any change in pressure as depth increases. The contents stay at what ever pressure they where at when the housing door was closed. The contents do not "support" the housing, the housing protects the contents. It is rigid. So it does not matter what is inside the housing.
     
  4. caruso

    caruso Banned

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    The material that a rigid housing is constructed of does not compress or shrink in size, for all practical purposes, that's true. I may have used the word "decompress" but I was speaking about the entire housing including the air space inside of it. If a housing is subject to conditions it's not designed for, such as taking it to depth without a camera inside it, it may very well act like a soda bottle that is brought to depth and we know what happens to the soda bottle at depth. Unless of course it was mostly filled with a solid object. While it's unlikely an empty housing will implode, it could crack, the seal could be broken, the housing could deform slightly in some way, etc.
     
  5. MaxBottomtime

    MaxBottomtime Divemaster

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  6. tursiops

    tursiops Marine Scientist and Master Instructor ScubaBoard Supporter

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    A lot of things *could* happen, but they don 't because the housing is rigid. Your soda bottle would not deform either if it were rigid. A scuba tank does not deform. The point is, taking a (reasonably well-built) camera housing to depth does NOT affect the pressure inside, and does NOT cause it to leak. Your original post on this, which you are trying to back away from a little bit at a time, is quite wrong. Just say so, and move on. It's OK.
     
    Diving Dubai and giffenk like this.
  7. Chris Ross

    Chris Ross Barracuda

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    The pressure inside a housing does not change when you submerge it, there's no room for argument or discussion, gases obey the gas laws. Once you seal a volume of air inside a housing be it a small volume because space is taken up by a camera or a large because it is empty, if the o-ring is sealing no water gets in and no gas gets out. So the number of molecules of gas inside stays constant.

    The only way pressure can change inside the housing is for the volume to change or to add or remove molecules or change the temperature. The Gas law can be expressed (P x V) / (n x T) = constant. P= pressure, V = volume, n= number of molecules. For pressure to change, one of the other variables need to change, the only one likely to change a little bit is temperature and because you use absolute temperature in this equation even if you jump into 15deg C water from air at 30 deg C you only change the pressure marginally (it would go from 1 to 0.95 bar) and the pressure change is the same whether the housing has a camera in or not.

    Back to the original question I couldn't imagine having to dive with an empty housing for the first dive and agree that things may change when you open the housing to put the camera in. You might get a drop of water inside causing the housing to fog later on, you might get some grit or a hair over the o-ring when you open the housing, basically a false sense of security.

    When you say bucket tested cameras have leaked - were they bucket tested with a camera inside? O- rings are actually more likely to leak at the surface they require water pressing on the o-ring to push it against the walls of the groove to provide the seal.

    The answer to my mind is meticulous preparation of the o-rings, examining the o-ring and the groove to be sure they are perfectly clean, examine it in bright light and with magnification so you can hairs etc. If you really want insurance a vacuum system is the only thing that allows you to test your o-ring without disturbing that seal to replace the camera. I don't believe there is a risk of cracking the housing should be capable of resisting water pressure on its own, I suspect the quoted go-pro housings are cheap knock offs, and may indeed rely on the camera for support.
     
    Johnoly likes this.
  8. caruso

    caruso Banned

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    A soda bottle is in fact rigid. Until it succumbs to the negative internal pressure at depth that creates a force greater than the soda bottle can withstand, and it crumples up into a flattened piece of plastic.
     
  9. caruso

    caruso Banned

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    Even if a container remains sealed, the same number of gas molecules in a sealed container does not always mean equal pressure.
     
  10. giffenk

    giffenk Great White

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    Can you provide a link that would help us understand what you mean by "negative pressure"?

    And NO, a soda bottle is not rigid. They are quite flimsy. Even a full capped bottle can easily be distorted with just hand pressure.
     

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