Decrease in Buoyancy Deeper Underwater.

Discussion in 'Physics' started by SplitInfinity, Jun 5, 2013.

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  1. SplitInfinity

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    Mar 3, 2013
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    What I have experienced when I dive is how the further down I go the less buoyant my body becomes.

    Now when SCUBA diving and I have used both Compressed Air and Heliox...a Helium and Oxygen mixture used when doing deep dives as using compressed air at depths of over 140 feet...over 5 atm. of pressure...the body begins to build up Nitrogen which can cause Nitrogen Narcosis as well as the Bends.

    Our bodies encounter 1 atm. of pressure at sea level and an additional atm. every 33 feet down under water.

    As your body goes down deeper underwater the water column will increase pressure upon your body and compress your body from all directions. Thus even though I use weights to overcome the buoyancy of my wet or dry suits...the deeper I go...I will at some point have to add air or heliox into by BC...Buoyancy Control...a vest worn that can be inflated as needed to add buoyancy.

    Split Infinity
     
  2. LDC3

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    Apr 27, 2013
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    Yes, so what is your question?
     
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  3. SplitInfinity

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    Mar 3, 2013
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    No question...I just found it interesting and thought others might as well.

    Split Infinity
     
  4. SplitInfinity

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    It should be noted that the Human Body has a density that is very close to the density of Salt Water.

    A person who is muscular when exhaling as much air as possible from their lungs will sink and a person with a great deal of fat upon their body...even if they have exhaled the air from their lungs will tend to float as body fat takes up more space per Kg. than does Water.

    Muscle takes up less space per Kg. than either water or fat and thus a person with high muscle low fat body weight will have a much lower buoyancy.

    What does lower buoyancy the further a person dives down is water pressure compressing the body and specifically compressing the Chest cavity where lies the lungs.

    As a persons chest is compressed the lungs cannot fill with as much air or heliox so even though a person is wearing weights to allow them to sink...as they obtain the depth they desire...a diver will add air or heliox to their BC thus obtain neutral buoyancy...if they go down even further more air or heliox must be added to the BC to create even more buoyancy to remain neutrally buoyant.

    Split Infinity
     
  5. LDC3

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    Apr 27, 2013
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    I thought you might want an explanation. It's really simple.
     
  6. SplitInfinity

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    Mar 3, 2013
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    Explanation of what?

    Split Infinity
     
  7. Markd77

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    Sep 7, 2009
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    Clearly the explanation is incorrect. If you were diving and breathing compressed gas then the pressure at the regulator is supplied at the same pressure as the depth that you are at, which allows you to breathe normally and expand your chest cavity as much as you like.
     
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  8. t06afre

    AAC Fanatic!

    May 11, 2009
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    Your body is mostly fluids and other incompressible tissue. Then you go scuba diving. A valve in the breathing regulator will adjust the air pressure you breathe in. To the water pressure around you. If not you will not be able to breath. And your lungs would collapse. Second I would suggest reading up on Archimedes' principle and how gas behave under pressure. Boyle's law is important here. As gasses in opposite to fluids are compressible. Then you go deeper in the water. The water pressure will compress the air in your Buoyancy compensator and also air in your diving suit. Draw a graph using Boyle's law. And you will get the answers to your questions.
    And also in this case can we stick to real scinece please! No wall of text using spurious science. For the depths used by hobby scuba divers. My explination will hold water so to speak.
     
    Last edited: Jun 6, 2013
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  9. WBahn

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    Mar 31, 2012
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    To be more specific, the air in your lungs, BC, diving suit void, and anywhere else that is exposed to the submerged pressure will compress and be more dense. So even if you use compressed gas to bring them back up to the same volume, they are denser and thus don't give the same bouyant effect.

    Your body will also experience a second-tier effect due to the compression of your tissues, particularly the abdominal cavity. But this is a lower-order effect.
     
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  10. Markd77

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    Sep 7, 2009
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    I'd neglected the effect of the air in the lungs being more dense, but I don't think it makes much difference. The air in the lungs at atmospheric pressure is about 6 grams if I'm right (around 6 litres of about 1.25kg/m^3), so you would have to go pretty deep before it made much difference. The largest effect is the air in the wetsuit or drysuit being compressed.
     
  11. SplitInfinity

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    Mar 3, 2013
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    Mark...I am an expert Master Diver with close to or over 100 wreck dives to my credit and I have used Heliox to dive down to depths well over 250 feet.

    Do you honestly think I am not aware of Boyle's Law?

    The reality is even at say 99 feet of depth which is 4 atm. of pressure upon the diver and upon a tank of compressed air...air is about 20% Oxygen and 80% Nitrogen.

    Every breath I take of my regulator even though that air is still 20% Oxygen and 80% Nitrogen...because I am at 4 atmospheres of pressure...each breath it giving me 4 times the amount of Oxygen and Nitrogen such a breath would give me at sea level.

    This level of pressure will compress my body and chest cavity to the point that my lungs will not be allowed to take in anywhere near as much air as I would be able to at sea level.

    But since even a small breath is still giving me 4 times the amount of Oxygen per cc that it would at sea level...my body get's more than enough Oxygen.

    This 4 atm. of pressure compresses everything from my body and chest cavity to my dive suit and just a 4 atm. compressed dive suit alone will change my buoyancy dramatically.

    Split Infinity
     
  12. SplitInfinity

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    Mar 3, 2013
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    See my reply to Mark.

    Split Infinity...p.s..you are making assumptions upon my vast knowledge of this topic.
     
  13. SplitInfinity

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    Mar 3, 2013
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    You are incorrect about chest cavity compression being a low order effect. It is either #1 or #2...possibly behind Dive Suit compression.

    Split Infinity
     
  14. SplitInfinity

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    Mar 3, 2013
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    Just as an example...when a person sucks in their gut...they reduce the total area or volume that their body will displace a great deal.

    Now when I am diving and down just 66 feet thus 3 atm. of pressure...I can tell you that such pressure is easily compressing my bodies area or volume of displacement a great deal.

    Split Infinity
     
  15. WBahn

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    Mar 31, 2012
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    Same question. What is a "great deal"? 1%? 10%? 25%? 50%? 66%? 75%?

    Not looking for anything exact and I'm not suggesting that you have to use one of the answers I listed -- those are just to give an idea of the kind of ballpark I am thinking in terms of. Whatever you think is a reasonable guestimate will do so that we have an idea what kind of effect you have in mind.

    Same question. What is a "great deal"? 1%? 10%? 25%? 50%? 66%? 75%?

    Perhaps you can shed some light on something I observed when I was getting my Open Water certification some ~30 years ago. We were working in the pool and I had my BC adjusted for neutral bouancy at about the 6ft depth. Using a light housing as a reference, I was able to easily demonstrate the change in bouancy just by breating in or breathing out. But if I held my breath (generally not a good idea, but I was careful to constantly monitor by depth relative to that light housing just a few inches away from by face) and confirmed that I was not ascending or descending and then sucked in my gut or pushed it out I have virtually no effect on my depth.

    I thought I understood why that was so, but your much greater experience and expertise says that my explanation must have been completely wrong. So could you explain why it had so little effect?

    Changing gears.

    If you were to take a trashbag (not an elastic balloon) and fill it with air just to the point that any more would puff it taut (so that the amount of air in the bag is as great as possible while keeping the pressure inside and the pressure outside the same), how much would you expect its volume to decrease by at 66 feet? 1%? 10%? 25%? 50%? 66%? 75%?

    If you were to then put your regulator into the mouth of the bag with the regulator mouthpiece pointing up so that it would tend to let air flow out at the ambient pressure, how much of the bag's original volume would you expect to be restored? 0%, 1%, 10%, 50%, 66%, 75%, 100%?
     
  16. Georacer

    Moderator

    Nov 25, 2009
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    If you have vast knowledge of this topic and there is no question you want to ask, then I don't see any reason for this thread to stay open.

    Please take a step back and reconsider what other people can know about you and how you respond to them.
     
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