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Blue Hole Arch of Dahab Egypt

Discussion in 'Tech Trip Reports' started by divezonescuba, Jul 16, 2019.

  1. rjack321

    rjack321 ScubaBoard Supporter ScubaBoard Supporter

    # of Dives: 1,000 - 2,499
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    I wouldn't bring a 4th gas. Its a cylinder you only need to use for a brief moment (although I agree with @beester about just dropping) and having it in the rotation on the bottom is one more opportunity to breath it below the MOD. Especially if you are coming from 1 deco gas diving and now you have 3 or 4 stages in you.

    Carrying an al80 of 21/20 instead of a larger cylinder of your deeper deco gas is especially heavy on gear while light on usefulness from my perspective. As far as I can tell from your description you didn't use the 21/20 as a deco gas on ascent. I question the total volume of gas if either of the al40s of 32% or the 100% were fail. I guess you had enough 21/20 but the contingency plan(s) here are a mystery to me.

    50% is really easy to make and covers a big chunk of your deco time, and if the 100% bottle is lost can be used almost effectively at 6 and 3m too. You can also use it as a travel gas if you really need one. I'm a big fan of 50%.
     
    helodriver87, Doby45 and RyanT like this.
  2. helodriver87

    helodriver87 DIR Practitioner

    # of Dives: 500 - 999
    Location: Alabama
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    Caveat: I don't currently do anything deeper than 200 or with more than two deco gasses, so I can't speak from direct experience, but I agree with @rjack321 on this one. No need for the 4th gas.

    14/50 is, what, 300 gas at 1.4? Firmly in the hypoxic range, so no harm in dropping FO2 a hair to lower PO2 on the bottom, but I suppose that's going to be exposure time dependent.

    40s seem really small for that kind of deco. Especially for the 32. I'm coming up with a total runtime of about 90-110 minutes based on the information provided (trying to estimate bottom time with 80s). Deco gas requirements aren't leaving any room for failures/sharing. If you used something like 12/60 for bottom gas, 35/25 for 120 gas, 50 for 70, and O2 for 20, your total runtime is similar, but with the benefit of reduced gas requirements and one less bottle. And you'd still need to do bottle rotations, so that skill still gets evaluated. You could use the 35/25 for travel, carry an 80 of 50, and a 40 of O2 and be golden. Hell, you could probably get away with 40s for all three deco gasses. I'm coming up with about 20 cubic feet required for each, which gives you a healthy margin for sharing. Not criticizing (outside of the bottle size for the plan, that seems close), just playing with the numbers and armchair QB'ing.
     
  3. rjack321

    rjack321 ScubaBoard Supporter ScubaBoard Supporter

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    I don't know if this was a 300ft dive.... But if I were doing 300ft on OC I'd use 12/65, 21/35, 50%, and 100%. The 21/35 is travel down to 100-120ft which saves some of the 12/65 and the 21/35 (190ft switch) is useful as deep deco gas to keep your rock bottom reasonable. 200ft is roughly your first stop on the way up as well - so you get a gas switch to start accelerating there instead of hanging on backgas.

    Higher helium %ages are more in line with modern density guidelines. 50% covers a much bigger chunk of the intermediate deco effectively. For me, on OC and double 80s in the tropics it would be 5 bottle dive as I'd want a bottom stage of 12/65. AL80s for the 21/35 and 50%, al40 of O2. Lots of bottle rotations in this plan, less narcosis. Pretty sure this is not the kind of dive that a Tech40/T1/AN-DP whatever (one deco gas, 150ft deco diver) should be doing even after a week of classwork and progressively deeper diving.
     
    Billy Northrup and helodriver87 like this.
  4. helodriver87

    helodriver87 DIR Practitioner

    # of Dives: 500 - 999
    Location: Alabama
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    Yeah, I was working under the assumption that there was no bottom stage, which puts a pretty hard limit on bottom time and, by extension, deco. In that scenario, 120 gas is probably better than 190 since you shouldn't have much deep deco. Throw in a stage and it changes the whole plan.
     
  5. rjack321

    rjack321 ScubaBoard Supporter ScubaBoard Supporter

    # of Dives: 1,000 - 2,499
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    Max at the outside bottom of the Arch is about 120m.
    I used 35/25 once... Nothing bad happened but it was the wrong choice for us.

    Unless someone has a really really good rationale for that gas (like the profile in the cave dictates a lot of time in this range) I'll skip it for something more useful. 35/25 doesn't help your RB much and doesn't accelerate your deco enough in it's working range to be useful to me.

    PS I've done a couple of dives like this on CCR (but not on OC) with 10/70 dil/deep BO, 21/35, 50% and O2. An al80 of 100% just feels brutally heavy.
     
    helodriver87 likes this.
  6. helodriver87

    helodriver87 DIR Practitioner

    # of Dives: 500 - 999
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    Yeah, that does seem to cut down on the bottom gas requirement a bit. Adds a few minutes to the runtime with everything else constant though. I can see why you'd opt for a deeper travel gas there. It's kind of a wash in the overall profile, but gives you a better range.
     
  7. beester

    beester DIR Practitioner

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    I think this is normal. There is no need for a 3rd stage within normal normoxic diving limits, and I'm not aware of agencies teaching 3 stage dives on normoxic level. 3-4 stage dives and bottle rotation drills are typically a full trimix ticket.

    You are totally right on training and letting students experience as many possible procedures/drills as possible during the course. I somehow overlooked that in your original post and responded accordingly. My apologies.

    Finally I fully understand you not going into the 32% topic, we don't want to trigger another standard gas vs best mix discussion which is totally of topic. But the rational of a missing 70-60ft deco gas remains. :wink:
     
  8. beester

    beester DIR Practitioner

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    Just armchairing this as well so excuse the nitpicking... :wink:

    I would use a 35/25 gas as well in this scenario. In fact I've some OC dives in this depth range with these gases. Example from the logbook (and excuse for the metrics):

    Max depth: 90m
    Avg depth: 85m
    BT: 30'
    Gases used:
    Bottom gas 12/70 (D18L filled to 280b)
    1st Deco gas: 21/35 switch at 57m, total ascend time on this gas: half an hour give or take
    2nd Deco gas: 50% switch at 21m, total ascent time on this gas: 45 min give or take
    3rd deco gas: O² switch at 6m, total time on this gas: 1 hour give or take

    The problem we faced was that we were running really low on the 21/35 (1 80 cuft tank really no reserve left), of course I'm not imaging these students running a half hour bottom time at that depth... but just saying that for longer deep dives it makes sense to introduce the 35/25, because you are doing a significant portion of your deco in this depth range.
     
  9. rjack321

    rjack321 ScubaBoard Supporter ScubaBoard Supporter

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    @beester I can't tell from your post if you used 21/35 as a deco gas or 35/25. Sounds like you recommend one but used the other?

    I don't have more than a handful of dives >80m but the ones I have planned lately (all CCR) have used an 80 of 21/35
     
  10. beester

    beester DIR Practitioner

    # of Dives: 1,000 - 2,499
    Location: Belgium / Italy
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    Heya Rjack... I'm mentioning a dive where I only used the 21/35 as deco gas, not using 35/25... but using this as an example from experience... that for longer BT dives at that depth (when you start to amass more than just minutes per 3 m in the 36-21m range) that just the 21/35 as deco gas to 50% is not sufficient... in fact when I switched to 50% I had 20b left on my 80 cuft tank.

    After that dive... I changed the setup including a 4th deco gas. ;-)
     
    rjack321 likes this.

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