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Pony bottle ascent schedule?

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

  1. Colliam7

    Colliam7 Tech Instructor Staff Member ScubaBoard Supporter

    # of Dives: 1,000 - 2,499
    Location: Kents Store, VA
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    Ignoring the issue of particular enriched air mix and depth combination - you later amended that, and it wasn't the focus of your question anyway, even if it did generate a lot of comment - there is really no reason to fundamentally change your (presumed) previous plans for a normal ascent, just because you are ascending on air instead of enriched air. You have plenty of gas (30cf) to ascend from 130', and the fact that you are breathing air instead of a slightly enriched mix on the ascent is inconsequential, in terms of the actual additional nitrogen loading that would occur during the ascent. You could use the schedule you proposed. Or, you could ascend at your normal rate (let's assume 30 ft/min) to your 15ft S/S (or 20ft, if that is your preference) and hang out there.

    What would I do if my backgas became unavailable at 130', and I had a 30cf bottle of air, and was within NDL? I would ascend normally to 15ft, do my safety stop, and surface. If I wanted to be very conservative, I would add 5 min to my S/S. If my backgas becomes compromised, and I have a more limited reserve (e.g. a 13 cf bottle, instead of a 30), my interest would be in getting shallow sooner rather than later - but, again using a safe, normal ascent rate - and doing whatever safety stop I could with the remaining gas. If, for whatever strange reason, my limited reserve ran out (bottle wasn't full to start with, I was breathing hard looking at the tips of the trident sticking out of my tanks, I was using a Spare Air, etc) I could more easily do a CESA and surface from 15 ft than from 70 ft.
     
  2. SkimFisher

    SkimFisher Contributor

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    In this hypothetical scenario it's an unnecessary risk to dive within one body length of a contingency depth. If that's how you like to dive. So be it. I'm just illustrating a point.

    It's also poor advice to pass along to new divers. The rules are there for a reason.
     
  3. H2O 70

    H2O 70 Divemaster

    # of Dives: I'm a Fish!
    Location: Baltimore, Maryland
    822
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    Hello Dave!

    Typically they say that your pony should match your back gas. A fair amount of folks will carry a higher FO[sub]2[/sub] gas in the pony. Not a 'deco' FO[sub]2[/sub] (50%, 80%, 100%), but something like EAN40.

    At ppFO[sub]2[/sub] of 1.6, that give you an MOD of 99fsw. If you cruising at 130fsw using your EAN28 and Neptune strikes, although not ideal, you could easily make it to 100fsw and breath the pony. You're going from 5ata to 4ata so overall bubble growth shouldn't be bonkers, not like it is in that last 30fsw to surface.

    I usually carry EAN40 in my bottle and for the dives in NC this is great because most of the dives have a bottom on 115-120fsw and I'm not down there a lot. I'm cruising closer to 110-100fsw, so a 10ft bump up to 100fsw to breath my pony is no problem.

    Good luck!
     
  4. TSandM

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

    36,349
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    I'm curious about the people who would accelerate their ascent or curtail their stops because they had had a gas failure. If you have planned properly and switched to a secondary source with enough gas in it for a normal, unhurried ascent, why would you change that? Yes, you are on a single gas source, but the majority of the people who dive within recreational depths dive on a single tank without even a secondary source. Somehow I don't see any need for haste.
     
  5. SCUBA482

    SCUBA482 Public Safety Diver

    # of Dives: 200 - 499
    Location: So Cal
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    I like this plan. Even if diving on 21% O2 at that depth, this wouldn't hurt to use during any SS.

    As for the OP; I'd make a regular ascent to SS depth and hang there for at leat 3 minutes but probably longer since the 30cf tank is plenty. Longer SS's won't hurt, and some other folks mentioned the buddy factor as well. I would hope some contingencies are planned out in advance as well.
     
  6. GrumpyOldGuy

    GrumpyOldGuy Solo Diver

    # of Dives: 200 - 499
    Location: West Texas/NH/CA
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    Out of morbid curiosity, who are "they" and what agency/group recommends such a practice (carrying 40% in a pony)?

    It seems a waste of effort to carry a hot mix in a pony with the the associated risk and very little up side.
     
  7. Blackwood

    Blackwood DIR Practitioner

    # of Dives: None - Not Certified
    Location: Southern California
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    I don't think he said that any agency/group recommends it, just that "a fair amount of folks" do it.
     
  8. Meng_Tze

    Meng_Tze Homo Bonae Voluntatis ScubaBoard Supporter

    # of Dives: 500 - 999
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    My take is that at the point you loose all BG and you have a single pony with 30cft of gas, one can safely assume this will have an effect on mindset and SAC. It would be prudent to get to shallow depths as soon and safely as possible. It gives a little extra room for additional errors.

    Now, if the theoretical case says that there is no chance for panic, fear or other wise heightening SAC, and altered mindset, sure take your time within gas volume to make it to the surface.

    But we both know that many of those divers who dive single tank, single source, also would be challenged to keep a cool head coming up from 130ft on a single 30cft tank after just having seen all gas erupt from their single tank while pushing NDL......
     
  9. H2O 70

    H2O 70 Divemaster

    # of Dives: I'm a Fish!
    Location: Baltimore, Maryland
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    What Blackwood said. I don't think any agency recommends it, but I know of and seen a fair amount do it.
     
  10. GrumpyOldGuy

    GrumpyOldGuy Solo Diver

    # of Dives: 200 - 499
    Location: West Texas/NH/CA
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    Fair enough. Now I will assume you have some rational behind this besides seeing a fair amount of others do it. Care to share your logic, I am still at a loss as to the gain for NDL dives?
     

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