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Implications of diving below sea level

Discussion in 'Decompression Theory' started by Necklinsberg, Sep 14, 2015.

  1. Necklinsberg

    Necklinsberg Divemaster Candidate

    # of Dives: 500 - 999
    Location: Lancaster County, PA
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    Was watching a show on the Dead Sea and got to thinking about the effects of diving there (& any other below sea level locations).

    Online sources talk about full face masks for protection against salt in eyes, need to rinse gear extremely well, etc. But none address the effect of change in ambient air pressure.

    For altitude, one must adjust procedures for the reduced pressure from sea level starting at +1000 ft. Since he Dead Sea is -1400 ft, are there deco-theory implications?

    I would think that there would be a special concern since the diver would probably be coming from would be qualify as an altitude for diving purposes and descending to the floor of the rift valley.

    Or am I mis-understanding the situation?
     
  2. knotical

    knotical perpetual student

    # of Dives: 1,000 - 2,499
    Location: Ka'u
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    Interesting question.

    At altitude above sea level the adjustments are to dive more conservatively.
    At altitude below sea level, one could perhaps adjust to dive less conservatively, but probably better not to, since you're likely to climb to altitude after diving.
    I'd ignore the -1400 feet and treat the negative altitude as an automatic bit of greater conservtism.
     
    cool_hardware52 likes this.
  3. rx7diver

    rx7diver Solo Diver

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    A serious consideration with the Dead Sea is its salinity, which is extremely high. So, 33 or 34 ft depth there is like diving 3,428,952 ft elsewhere. (Well, not really this much, but you get the idea. Google to get the correct answer.)

    Safe Diving,

    rx7diver
     
  4. knotical

    knotical perpetual student

    # of Dives: 1,000 - 2,499
    Location: Ka'u
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    Excellent point. But since depth gauges actually measure pressure, not depth - wouldn't this be accommodated by the depth gauge giving a deeper reading than actual?
     
  5. rx7diver

    rx7diver Solo Diver

    1,117
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    Yes, excellent point!

    Hmmm, so this suggests a diver might breathe down a tank *really* quickly even if he is actually relatively shallow (actual depth)? And he would reach NDL really quickly even if he is actually relatively shallow (actual depth)? And it should take the diver a relatively long time to ascend from even a shallow depth (actual depth)?

    BTW, I just googled diving the Dead Sea. There are several sites with links to YouTube videos. Interesting. For example, people in light wetsuits need to wear tons of weight to submerge. And divers need to wear full face masks (evidently because the extreme salinity of the water means any water in the eyes would be extremely painful).
     
  6. NAM001

    NAM001 Contributor

    # of Dives: 500 - 999
    Location: the moon
    5,002
    1,316
    I don't think there is anything to worry about. You are not diving 2000 ft below sea level. (like diving 1400 above sea level. You are only looking at the effect of additional atmosphere below sea level. so if he dead sea is -30 ft. then what does 30 ft of air weigh. nothing since the entire air column at sea level is 14.7 lb. I seriously doubt that the difference is any where near the difference of fresh or salt water. I will say this . differences in atmospheric barometric pressure changes is more severe than diving in areas below sea level. our local group had a presentation on the area. There was no special considerations in that regard. Politically diving in the notrthern regions is preferred and you will need a lot of weight because of the salinity.
     
  7. Altamira

    Altamira ScubaBoard Supporter ScubaBoard Supporter

    # of Dives: 200 - 499
    Location: TX
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    Do I have this right?
    The barometric pressure at sea level(standard day) is 29.92 inches of mercury, and the Dead Sea is approximately 31.46 inches of mercury, which is approximately a 5% difference in pressure. So a depth gauge would likely read 5% lower than actual depth.

    The air being denser at the Dead Sea has a higher percentage of oxygen (+3.3% summer, +4.8% winter), so if you spent some time there before diving, you would start the dive with less nitrogen in your blood. Then if you could fill your tank at the Dead Sea, you would be breathing an air mixture with a higher oxygen/lower nitrogen content. That is a good thing.

    All that sounds good, until we get to the requirement to carry a lot more lead to get down, and then when you did get down, you would get a good view of salt crystals. That does not sound like my kind of dive!
     
  8. NAM001

    NAM001 Contributor

    # of Dives: 500 - 999
    Location: the moon
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    Good thought but todays computers sample the surrounding pressure to determine what barometric pressure is at time of dive. That is why it is good to turn on your puter on the shore before entering the water. The depth sensor is also acting as an altimeter till it gets wet. I was unaware of the higher O2 content. Curious,,, are you saying that the O2 level 3% higher 24 instead of 21 or is it 3% of 21 higher,,,, 21.5%. I suspect the latter.

     
  9. knotical

    knotical perpetual student

    # of Dives: 1,000 - 2,499
    Location: Ka'u
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    It's not a higher percentage of oxygen. The partial pressure is slightly higher because of the higher ambient pressure, just like when you submerge. The Nitrogen partial pressure is similarly higher.
    While you are acclimating at Dead Sea level, you are essentially doing a long, very shallow (~18 inches?) dive at normal sea level.
     
  10. NAM001

    NAM001 Contributor

    # of Dives: 500 - 999
    Location: the moon
    5,002
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    Thanks that explaination makes scense. I have heard of up to 1% O2 differences but it was geographical. Forrest areas. We had a speeker at our last dive meeting that was talking about the dead sea or maybe the red sea. Either way i sounded like an atm was about every 32 ft instead of the norm 33 and 34 because of salt content. I remember wanting to ask if they did any computer changes to comp for this till they said it was mostly shallow diving.

     

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