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Safety Stops

Discussion in 'Ask Dr. Decompression' started by Ladydiver, Oct 5, 2000.

  1. Ladydiver

    Ladydiver Founder

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    Hi Dr. Deco and Company,

    I have another question for you.

    Are Safety Stops really that useful? I know that they are recommended, but what can you say about them?

    Thanks for you very useful input.

    Cheers,
     
  2. Walter

    Walter Instructor, Scuba

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    I'm eager to hear Dr. Deco's answer. Personally, I make extra stops and increease my hang time depending on how close to the NDL I am. On a typical 100 ft dive, I'll stop at 50 ft for 1 min, 40 ft for 1 min, 30 ft for 3 minutes and 15 ft for a minimum of 5 minutes. As I approach the NDL's, I'll increase my hang time, especially at 30 & 15 ft.

    Walter
     
  3. Dr Deco

    Dr Deco Medical Moderator Staff Member

    # of Dives: I just don't log dives
    Location: Issaquah [20 miles east of Seattle], Washington.
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    Offgassing and Surface Tension Effects

    The use of safety stops is one of the most valuable tools that a recreational diver has to guard against decompression sickness. The valuable studies of Dr. Andrew Pilmanis (currently at the High Altitude Protection Laboratory at Brooks AF Base) are very indicative of the reduction in Doppler bubbles in actual in-water decompressions. These bubble detection studies were performed when he was at the USC laboratory on Catalina Island. The results definitely show that the “safety stop” dramatically reduces gas bubble growth [Pilmanis, A.(1990). In Lang and Egstrom (eds.), Biomechanics of Safe Ascents Workshop , American Academy of Underwater Sciences. AAUSDSP-BSA-01-90]. Hard data, not speculation.

    These reductions come about by two different mechanisms. The first is obviously the reduction in dissolved inert gas that occurs because of off-gassing at the stop depth. It can be shown with gas loading analysis that the loss of gas is considerable. For example, in a dive of 100 fsw for 25 minutes, we find that the “apparent” bottom time is reduced. A stop at 15 fsw for 3 minutes makes this 100/25, upon surfacing, equivalent to a 100/20 dive. If the stop had been for 5 minutes at 15 fsw, the dive would have been equivalent to 100/15. It is obvious that the reduction in gas loading is considerable.

    The second reduction arises from consideration of the microbubbles that can grow as the diver surfaces. In the Haldane model, phase separation (= gas bubbles growing in tissue fluid) does not occur. However in“phase models,” it is necessary to consider the properties of the bubbles themselves. One consideration is the surface tension that is conceptually equivalent to a “rubber skin” on the bubble that causes it to contract and increases the gas pressure inside the bubble. {It is this surface tension that limits the life of bubbles since they all must eventually shrink away. This contraction raises the pressure inside the bubble and limits their lifetime.} The pressure P
    is termed the “Laplace pressure” after the man who derived the relationship between pressure and surface tension (Pierre Simon de LaPlace). The internal pressure of the bubble is thus the outside pressure P(hydrostatic pressure) and the internal, Laplace pressure P
    . When a decompression is performed, the bubble expands and this P
    is decreased as the radius grows since

    [sp][sp] P
    = 2 (surface tension)/(bubble radius).

    When the ascent is fast, the bubble grows before the gas can diffuse out, the internal gas pressure decreases because P
    decreases, and the gradient is reduced such that more supersaturated dissolved gas diffuses into the bubble. When the ascent is slow, the expansion is not great as equilibrium is maintain between the dissolved and gaseous nitrogen, and the P
    stays large, which prevents the bubble from growing.

    Thus we have a two-fold protective mechanism from safety stops and/or slow ascents (a) off gassing and (b) surface tension effects. The stops given by Walter are somewhat arbitrary but definitely increase the safety of the dive.
     
  4. Walter

    Walter Instructor, Scuba

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    Somewhat arbitrary? I thought they were totally arbitrary.

    Thanks for the info.

    Walter
     
  5. Dr Deco

    Dr Deco Medical Moderator Staff Member

    # of Dives: I just don't log dives
    Location: Issaquah [20 miles east of Seattle], Washington.
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    Walter:

    They did appear to be arbitrary, since the calculation programs are not generally available. If one were going on the basis of gas loading alone, a similar result could be obtain by stopping at 15 fsw for 8 minutes. The deep stops are probably better, however - - - along with slow ascents.
     
  6. Walter

    Walter Instructor, Scuba

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    Thanks for the feed back.

    Walter
     

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