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Driving in hills after diving?

Discussion in 'Decompression Theory' started by geoff w, Mar 23, 2015.

  1. geoff w

    geoff w Instructor, Scuba

    # of Dives: 500 - 999
    Location: malibu
    940
    255
    Most weekends I don't give any thought to driving through local canyons after a beach dive, since we don't dive deeper than about 40 feet for under 90 minutes, but the weekend before last I joined a PADI instructor deep specialty class and we did two dives - 120 feet, then 95 ft. No deco, but both on air. Later in the afternoon, several of us independently drove to the other side of the hill, which got up to about 1300 ft and we all went around to go through a canyon road with a lower maximum elevation, even though it was out of the way. Last weekend, I asked Karl Huggins what his advice would be, and he suggested treating the dives as altitude dives in terms of NDL planning, with the NDL figured based on altitude for the highest elevation to drive afterwards. Is this the recommended approach (it made sense to me), or is there a better way to plan? If this is the right way to do it, does time at sea level before driving over the canyon count as off gassing, since nitrogen should be getting out of my body, or on gassing, since I'm below the planned highest elevation for my drive?
     
  2. tbone1004

    tbone1004 Technical Instructor ScubaBoard Supporter

    # of Dives: I'm a Fish!
    Location: Greenville, South Carolina, United States
    18,776
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    i'd plan them as altitude dives, but at 1300ft you're at 95% of an atmosphere. At 120ft you're EAD at that altitude would be 126ft, and at 95ft you're at EAD of 100ft. Officially you should plan it as an altitude dive, so diving tables, you'd just treat it as a 130ft dive and then a 100ft dive instead of 120/100. Diving a computer could be a bit difficult since you can't always force it into altitude mode and many might not actually recognize 1300ft as "altitude" when you get up there. Do an extra few minutes on your P-stop and try to have a few hours before doing it.

    Again, that's the way that I would treat it, but officially I recommend diving tables and preferably getting nitrox certified as that makes your equivalent air depth that much better. I.e. going to EAN24 and diving with the computer on air would yield about the same NDL's as treating it as an altitude dive, using best mix of EAN30 it gives you that that much more.

    PADI's official stance is anything above 1000ft needs to be treated as an altitude dive and that is mainly because the analog depth gauges and any computer that doesn't compensate for atmospheric pressure at the surface will read a shallower depth than you actually are which is obviously bad. It's a tricky science, but since you're on the cuff and it is only a quick drive up to the hill likely a few hours after you surfaced I think you'd be fine.

    Any time spent at sea level is off gassing since your tissues are still loaded from depth, you only ongas when your tissues have not equalized with the PN2 you are breathing. If you lived at 2600ft and came to sea level you would be ongassing, then went up to 1300ft you'd be offgassing until it equalized. Once it equalizes, you have to change pressures to start the diffusion process again.
     
    geoff w likes this.
  3. Duke Dive Medicine

    Duke Dive Medicine Contributor Staff Member

    2,935
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    The U.S. Navy Diving Manual has a procedure for ascent to altitude after a dive or dive series. The manual is available for download here; the procedure is described beginning on page 9-57. It uses RGDs and is based on the Navy's decompression tables so you'd have to figure your dives using those tables. Karl's method is another way to do it and if he said it, it's definitely worth exploring. You could look at both procedures and see which you think is best suited for you.

    Best regards,
    DDM
     
    gert7to3 likes this.
  4. geoff w

    geoff w Instructor, Scuba

    # of Dives: 500 - 999
    Location: malibu
    940
    255
    I had two dive computers with me - Suunto Vyper and Shearwater Petrel 2 in recreational nitrox mode. I never thought of looking at tables until this discussion. Karl's suggestion on the Suunto to set it for an altitude dive made a lot of sense. There's no way to set the Petrel to a specific altitude, and in recreational nitrox mode it runs B├╝hlmann ZHL-16C with Gradient Factors, which I have set to medium conservatism (40/85). Interestingly, the Petrel popped into deco right before starting to ascend, but the Suunto didn't. I need to look more into how the gradient factors work and how that can relate to altitude. The Petrel had me do a 5 minute safety stop on the first (120 ft) dive, the Suunto only 3 minutes. Since our second dive was basically dives 3 and 4 of the PADI deep specialty, our safety stop was 8+ minutes. Nobody in our group had any issues driving over the hill later in the day, which doesn't make it right, but was certainly one of those lightbulb moments for me
     
  5. tbone1004

    tbone1004 Technical Instructor ScubaBoard Supporter

    # of Dives: I'm a Fish!
    Location: Greenville, South Carolina, United States
    18,776
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    some really good articles on GF's on the Dive Rite website actually in their blog. The second number is what you want to keep tabs on and the higher it is the faster you get out, it essentially correlates to your tissue loading when you surface. Lower they are, the less nitrogen is in your system, so theoretically less chance of getting bent, but your deco stops are longer while you offgas, the higher they are, the quicker you get out of the water but there is more residual nitrogen in your body. The first number is pretty irrelevant for recreational profiles but relates to at what depth your deco stops start.

    The Petrel automatically measures altitude at the surface and adjusts accordingly which is why it's important to turn it on before you descend, but you can't override it. Next time you do that dive, when you get to the bottom of the hill take a picture of the tissue loading graph with your phone. When you get to the top of the hill do the same, it should show that the bars of gotten bigger indicating a higher N2 loading in your body, they will go back down as you descent. Pretty neat feature.
     
  6. Dr Deco

    Dr Deco Contributor

    # of Dives: I just don't log dives
    Location: Issaquah [20 miles east of Seattle], Washington.
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    Hello Geoff:
    As Duke Dive Med says, the US Navy Diving Manual has the necessary tables for diving and then going to altitude. That said, many divers ascend small passes without much concern. Flying requires proper time constraints. If you would just as well not look up the tables, remember not to rush to drive up to altitude - time will help with off gassing. Additionally, refraining from heavy straining and lifting will prevent the enlargement of micronuclei which could later grow into DCS bubbles.
     
  7. sylverbard01

    sylverbard01 Registered

    25
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    Those must be some pretty big hills.
     
  8. tarponchik

    tarponchik Contributor

    # of Dives: 500 - 999
    Location: USA
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    WHO claims here that flying on a commercial jet is an equivalent of 1.8-2.4 km altitude. However, some hills can be higher than that.

    On Nov 28th I did a night dive, coming out at 19:25 hrs with my Archimedes II comp showing 3 bars of nitrogen. This was a shallow dive with max depth 49 ft and average depth 21 ft so nothing surprising here. A-II gives you 6 bars in the "normal range", then you can add 2 more bars in the "red zone", and then any additional bars indicate decco requirement. Next morning we went hiking off Saddle road. My altimeter showed altitudes of up to 2,845 m and this was 14-16 hrs after the dive. Around 2 pm we were at Mauna Kea visiting center and an hour later I hiked to Lake Waiau at altitudes around 3,700 m. Maui, of course, can give you the same opportunity.
     
  9. tursiops

    tursiops Marine Scientist and Master Instructor ScubaBoard Supporter

    # of Dives: 2,500 - 4,999
    Location: U.S. East Coast
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    You did one dive that counted for two?
     
  10. geoff w

    geoff w Instructor, Scuba

    # of Dives: 500 - 999
    Location: malibu
    940
    255
    I was tagging along with a course director training instructors to do the deep specialty. In general, the number of dives an instructor needs in order to teach a specialty is half what a recreational diver needs for the specialty, so the second dive for the instructors would correspond to dives 3 and 4 for when they teach the specialty
     

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