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Descent Rate Limitation of 75 ft/min

Discussion in 'Advanced Scuba' started by azhar, Sep 11, 2013.

  1. azhar

    azhar Registered

    # of Dives: 25 - 49
    Location: Karachi, Pakistan
    Curious to why is descent rate limited to 75ft/min max in US Navy and NOAA manuals?
    What is the physics involved behind this (besides equalization)?
    What will happen in case of rapid descent?

  2. dumpsterDiver

    dumpsterDiver Banned

    # of Dives: 2,500 - 4,999
    I do 160 fpm descents when I am in a hurry.
    nimoh and Jim-SAR like this.
  3. scubadada

    scubadada Diver Staff Member ScubaBoard Supporter

    # of Dives: 1,000 - 2,499
    Location: Philadelphia and Boynton Beach
    My descent rate is limited by how fast I can swim down
    bracko likes this.
  4. aprivetera

    aprivetera Instructor, Scuba

    # of Dives: I just don't log dives
    Location: New Jersey
    Depends on the gas your using. If you are using only air, there is a possibility you could get narced, forget what you are doing and never stop decending. Taking a bit of time to go down is prudent.

    Tech divers might tell you a different story. As far as I understand, once you switch to different blends of gases, the objective is to shoot down to your bottom depth as fast as possible. Sure more people here on SB can explain that better than I can.
  5. Jim-SAR

    Jim-SAR Instructor, Scuba

    # of Dives: 100 - 199
    Location: Kenosha, WI
    I have nothing to back this up, so take it for what it's worth, but I've heard...

    There are risks with the difference between the gas partial pressures and the tissue compartment pressures. At high differential pressures, this results in a higher than normal absorption rate that may, or may not be, accounted for with current algorithms. Tissue compartments might reach their saturation sooner than expected, and then during ascent the off gassing might be at a higher rate than expected which could result in a higher risk of DCS.

    Then again, I could just be full of it. It's been known to happen.

    Sent from my iPhone using Tapatalk - now Free
    mark01 likes this.
  6. KD8NPB

    KD8NPB Contributor

    # of Dives: 100 - 199
    Location: Summerville, SC
    Isn't the Navy descent rate based on safe winching practices for lowering a Mk5 helmet diver on a platform?
  7. Nirvana

    Nirvana ScubaBoard Supporter ScubaBoard Supporter

    # of Dives: 200 - 499
    Location: Rio de Janeiro, Brazil
    I don't believe there is one specific issue that is key to setting a maximum descent speed; this speed is set as a general measure of caution. When descending fastly, you are probably going deep. The deeper you go, more gas is needed to inflate the bcd, so it becomes more difficult to precisely control the stop. In addition, as has been pointed out, there is the matter of how suddenly narcosis may become pronounced. Finally, limiting the descent speed may also have to do with controlling decompression obligation. A fast descent implies more time spent at the bottom, for square dives of same total time and maximum depth. Some maximumvalue, then, becomes necessary when elaborating a decompression table. These are my guesses.
  8. fdog

    fdog ScubaBoard Supporter ScubaBoard Supporter

    During surface-supplied classes back in the late 70's, it was passed on that excessive descent rates could lead to the uglies.

    This was described as a fast, hard-hitting narc complete with feelings of dread and dark thoughts. Dropping into darkness, or low vis, or breathing a low He mix made it worse.

    When I asked how fast this descent rate to avoid was, I was told "it depends". But generally faster than 120 fpm. Of course, depending on your physiology, it could be slower I suppose, so "it depends".

    I haven't experienced this personally, although last week we hauled butt to 200' in about 75 seconds, but I certainly can see how it would happen.

    All the best, James
    tracydr and Jax like this.
  9. TSandM

    TSandM Missed and loved by many. Rest in Peace ScubaBoard Supporter

    I don't know that this has anything to do with the Navy recommendations, but I believe there is some correlation between HPNS (high pressure nervous syndrome) and descent rate. This would only apply to divers breathing very high helium mixes at very significant depths, though.
    Brewone0to likes this.
  10. TMHeimer

    TMHeimer Instructor, Scuba

    # of Dives: 500 - 999
    Location: Dartmouth,NS,Canada(Eastern Passage-Atlantic)
    Interesting, I've never heard of a descent rate. Always took scubadada's approach.

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