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PART 2: Table Limits - - -

Discussion in 'Ask Dr. Decompression' started by Dr Deco, Oct 31, 2000.

  1. Dr Deco

    Dr Deco Loggerhead Turtle

    # of Dives: I just don't log dives
    Location: Issaquah [20 miles east of Seattle], Washington.
    To continue the question from BillP - - - - -

    When tables are designed, everyone starts from the same initial curve, the No-Decompression Limits (NDLs). This defines essentially the remainder of the table. The latitude in design is to decide what will be the actual limits? These are based on physics and physiology and do not include “fun” as a floating parameter. I am certain that no one will be surprised by this(!).

    The US Navy tables started with a set of limits derived by testing on US Navy divers. These were young men and in very good physical condition. [We can discuss this aspect of physical fitness in another FORUM.] Many table designers today will back off with respect to time at depth with the idea that recreational divers are older and not in as good shape as US Navy men. Probably this is a correct assumption in most cases.

    A way of knowinghow much to reduce the USN NDLs is to look at the data collected by Doppler bubble detection methods by Dr. Merrill Spencer of the Institute of Applied Physiology and Medicine in Seattle, Washington. His measures of bubble formation can be considered an objective measure of the limits of incipient DCS.

    Using these Bubble measurements as the initial point for NDLs, tables can be calculated and then tested with Doppler devices. This will demonstrate that they work under the conditions for which they were devised. In the case of the DSAT Recreational Dive Planner, the concept was that they would work:

    [sp] with multilevel and repetitive dives and,
    [sp] with the allowance of repetition based on the 60-minute compartment (“tissue”).

    This set of tables was tested in a chamber and then in the open water.

    Again, these tables are to be considered as a ROADMAP telling you of gas loadings in the body. There is no implication that dives should be made to the limits in all cases.

    To these caveats must be added what has been said in previous postings, viz,
    • try not to climb ladders with your full gear,
    • watch about straining movements,
    • do not lift heavy objects such as tanks,
    • move around somewhat following each dive (but not strenuous activity), and
    • sleeping between dives is a good way to slow blood flow and reduce gas washout to its minimum - - you do not wish to do this.

  2. Dr Deco

    Dr Deco Loggerhead Turtle

    # of Dives: I just don't log dives
    Location: Issaquah [20 miles east of Seattle], Washington.
    This will be continued in Part Three concerning Missed Decompression - - - -

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