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Thread: Design of a deep sea water sampler

  1. #1
    Associate Engineer
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    Jun 2013
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    Design of a deep sea water sampler

    Hi,

    I'm currently designing a device that collects water samples in the ocean. It must withstand 16000 psi. I am having a dispute with my partners.

    I believe that if the device is open all the way to the bottom of the ocean, the pressure will remain the same on the inside and outside of the device (No dispute there). Once it is sealed, the pressure will still remain the same. As it rises to the surface, the pressure inside will decrease with the pressure outside, regardless of being sealed. The reason I believe this is because pressure caused by a fluid is due to the displacement of that fluid. Since water is incompressible, not only does it reinforce the walls, but as it rises to the top, it has no affect on the device.

    My partners believe either one of the two following options:

    Option 1: Once it is sealed, the pressure outside will be greater than the pressure inside.

    Option 2: Once it is sealed, the pressure will remain the same inside and outside, but as it rises, the pressure inside remains at 16000 psi, but the pressure outside will decrease as the depth decreases.

    The only reason I don't believe option 2 is because water is incompressible. PV = mRT is the only equation regarding the relationship between Pressure, Volume, and Temperature. Unfortunately, it doesn't work on water since as I stated several times, water is incompressible.

    Can some one give me any insight on this?

  2. #2
    Project Engineer
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    Oct 2013
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    I think water is not compressible as you say it isn't. but sea water is not just water. there is all sorts of stuff that might be compressible and whatever percentage of that is to the water is going to exert a lot of pressure to the inside of the container when you bring it up.

  3. #3
    Lead Engineer RWOLFEJR's Avatar
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    Water is indeed compressable... Not much... but it is compressable. The empty container that is open will be fine on its trip down but once closed and it begins to rise you will lose pressure on the outside of the container. The compressed water inside will no longer be held in that state of compression by the sea above it but rather by the container walls. Your container needs to be something that will expand a little without breaking.

    This is a blurp I found out on the www ...

    Compressibility
    The compressibility of water is a function of pressure and temperature. At 0 C, at the limit of zero pressure, the compressibility is 5.110−10 Pa−1. At the zero-pressure limit, the compressibility reaches a minimum of 4.410−10 Pa−1 around 45 C before increasing again with increasing temperature. As the pressure is increased, the compressibility decreases, being 3.910−10 Pa−1 at 0 C and 100 MPa.

    The bulk modulus of water is 2.2 GPa. The low compressibility of non-gases, and of water in particular, leads to their often being assumed as incompressible. The low compressibility of water means that even in the deep oceans at 4 km depth, where pressures are 40 MPa, there is only a 1.8% decrease in volume.

  4. #4
    Lead Engineer
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    I am afraid your friends are correct water's compressibility is very small and for most practical applications water can be considered "incompressible" but in the case of extreme pressures this is no longer true. Water's compressibility is a function of both pressure and temperature. By going to the below website you will find a table of water volume vs pressure and temperature that is identical to one referenced in the "Standard Handbook for Mechanical Engineers" by Baumeister & Marks.

    https://van.physics.illinois.edu/qa/listing.php?id=2251

    Of course, you will also need to accommodate the water’s thermal expansion from the deep sea location to the surface as well.

    Ironically, while we generally think of pressure vessels needing to have high levels of strength relative to their internal pressure, in your case, due to the small change in water volume with pressure, the lower elastic modulus and the higher the thermal coefficient of your container material the better the container will accommodate the water sample's expansion due to the decreased pressure and increased temperature as it is raised to the surface.

  5. #5
    Associate Engineer
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    Feb 2014
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    Hi,
    I think your idea is very good, you can go for it. Water is much heavier than air; this pressure increases as we venture into the water and depend on it. For every 33 feet down us travel, one more atmosphere (14.7 psi) pushes down on us (basic criteria). For example, at 66 feet, the pressure equals 44.1 psi, and at 99 feet, the pressure equals 58.8 psi.

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