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Pony bottle ascent schedule?

Discussion in 'Advanced Scuba' started by dstrout, Mar 30, 2010.

  1. dstrout

    dstrout Registered

    # of Dives: 50 - 99
    Location: Edgewater, MD
    56
    0
    I'd be interested to hear peoples thoughts on a hypothetical situation I was considering on the way to work this morning.

    Imagine I'm cruising along at the end of my 130' dive on 32%. I'm within my NDL and dive plan as I reach the ascent line. Just then, Neptune appears, decides he hates me, and puts his trident through my back gas. Luckily, I have my handy 30 cu ft pony full of air, so I start my ascent using that.

    The question is, what is my ascent schedule? I was planning on using nitrox to ascend, but now I'm on air at 130 ft.. My computer doesn't know what I'm breathing (nor even if I'm breathing). So I'm breathing more nitrogen and less oxygen than I had planned on as I ascend.

    My instinct, based on no information or knowledge, would be something like this:

    130 -> 70 @ 30 ft/min
    70 - 1 min stop
    70 -> 50 @ 30 ft / min
    50 - 1 min stop
    40 - 1 min stop
    30 - 1 min stop
    20 - 5 min stop or breathe it down to about 300 psi, whichever comes first.
    Done diving for the day

    So, do I have a lovely case of DCS, or shall I crack open a beer and have a cigar?

    thanks,
    dave.
     
  2. elan

    elan DIR Practitioner

    # of Dives: 200 - 499
    Location: Toronto, Ontario, Canada
    3,316
    378
    His trident is pretty big so if he manages to put it through the back gas it would most likely go through the pony as well. :) Check your LDS they should have anti-trident stickers :)

    BTW: they might ague but 130' on 32 is a bit of a stretch
     
  3. udtfire

    udtfire Instructor, Scuba

    246
    13
    depending on your sac rate the gas you have is enough for the return and then some as for the dcs who knows its all theory but extend your safety stop as long as possiable the go have a drink because you out of the water for 24 hr but if your pony matched your back gas well no worries maybe thats your answer in a ndl profile let me know what you think
     
  4. Blackwood

    Blackwood DIR Practitioner

    # of Dives: None - Not Certified
    Location: Southern California
    5,535
    318
    Using air instead of 32% wouldn't concern me in the slightest. That's more than enough deco.
     
  5. bfisher

    bfisher Solo Diver

    # of Dives: I just don't log dives
    Location: Middletown, Pa
    422
    0
    I know you're just thinking hypothetically, and I've never pushed this, but if you'e at 130' on 32% EAN you're already dead. Or am I wrong on this, too.

    I know this has nothing to do with ascent rate, but hey.........
     
  6. SkimFisher

    SkimFisher Contributor

    831
    33
    You're not dead. But you're pretty close. I get a PPO2 of 1.54 at 130'. Well beyond the MOD.

    ...and one more reason to invest in a computer that handles gas-switching. Or work out the schedules prior to diving. Or dive the same mix in your pony.
     
  7. GrumpyOldGuy

    GrumpyOldGuy Solo Diver

    # of Dives: 200 - 499
    Location: West Texas/NH/CA
    4,579
    2,444
    I think you have 2 logical choices for ascent schedules. Both based on KISS.

    1) You have plenty of gas and can monitor the SPG, so you use your standard ascent schedule. You are used to it, don't change anything. You already fff'ed up, no need to complicate things and go for seconds.

    2) You are not sure you have enough gas, you do a basic PADI style ascent at 30FPM to your s/s and then hang out as long as possible. It is an adequate (not ideal) protocol and reduces the chance of running OOG a second time and drowning.

    In both cases, your diving is done for the day, because you screwed the pooch, not Neptune.
     
  8. Blackwood

    Blackwood DIR Practitioner

    # of Dives: None - Not Certified
    Location: Southern California
    5,535
    318
    Depends on what you consider MOD.

    Personally, I won't plan to dive 32% @ 130 feet, but I'm not going to suggest that you die at 1.6ATM (which is slightly less than the PO[sub]2[/sub] I'm exposing myself to when I switch to the bottle on the bottom in my current avatar).

    In any case, that's not really the point of this thread as I understand it. I believe the OP is asking if one needs to account for using a different gas (with a lower FO[sub]2[/sub]) on and sub-NDL emergency ascent.

    My take is: no. A slightly lower FO[sub]2[/sub] given this profile is unlikely to cause any problems, but even if it did, not drowning is the paramount concern.

    Wait a minute, so I run out of backgas and now I'm going to fumble with switching gases on my computer at depth while sucking down a pony bottle? No thanks, I'm going up.
     
  9. Peter Guy

    Peter Guy Divemaster

    # of Dives: 1,000 - 2,499
    Location: Olympia, WA
    4,296
    1,911
    Just for kicks, I ran this profile through iDeco, a Buhlman profile, and came up with what I expected. You are within your NDL when you lose your back-gas so it is merely a normal ascent -- 30 fpm to 40 ft and then 1 minute stops to the surface.

    With 32%, your NDL at 130 fsw is 15 minutes (+/-) so you only have fast tissues which have come close to their M values which means, of course, that they off gas just as fast as they on gas (again, more or less).

    The switch from 32% to 21% isn't all that important since you have now started your ascent and won't spend very much time at all at depth. In 2 minutes you'll be at 70 feet, assuming the 30 fpm ascent -- just not much time to on gas.

    End of the day? Well, if you have more tanks and/or new regs, why stop diving?

    This is really one of those that show how "fragile" deco theory may be.

    YMMV
     
  10. Blackwood

    Blackwood DIR Practitioner

    # of Dives: None - Not Certified
    Location: Southern California
    5,535
    318
    fixed
     

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