Planning multi-level deco dive?

[packrat12]

what other deco gas did you use with EAN75? Is that just badly mixed 80%? lol

You are missing the point, it still got me out of the water without any real issues or time delays. I do not have multi deco at work but I would be a penalty of less than 5 minutes... Big WOOP.

The other mix was 32. So yes, 32 and 75%. Again - It Works and I have used that mix on other dives including single deco.

There is no magic gas mix that is the 'best'. It is what ever works for the diver. I find I use a dwindling mix even now. I try not to go through the hassle of remixing when not needed. I will often do this even for my dil. We can target gasses for many reasons, PPO2 of 1.4, 1.5, 1.6 etc, CNS penalty or just availability. Many mixes work well. A lower T bottle will give 70's or so without a booster. Why, if not needed, am I going to make a big deal about it with the dive shop??? If it works and I am happy, I will use it!

 

[rjack321]

I think I already said that 80% scales up by using 32 or 36 as a deeper gas... Did you miss that post?

70% does not scale to much of anything. Misfilling 80% as 75% does not make 70% a good deco gas choice for a novice. Recommending anything cause you can in theory dive anything is not much of a recommendation at all.

 

[packrat12]

rjack321 - I'll bite. Why other than stating "it does not scale up" does 70% make a bad choice? It is still a rich deco gas. What parameters are you using? Does availability have input? Does CNS concerns have input? Does runtime have an input? Does the fact that it just does not matter in the long run have input? Does just topping off a lightly used richer gas have input? I just do not see any issue with a gas choice that works for the diver. BTW, if you dive OC, your deco times are normally horrible compared to CCR, but I still deco with OC divers. Who cares that I am done and just hanging around.

There are many parameter to determine the proper gas. Just using PPO2 or any other single parameter is not necessarily sufficient. As I had stated earlier, my final stop will still be PPO2 of 1.12 using 70% (Only 1.28 on 80% - just over 10% improvement is all) - Not bad and a lot better then air at a PPO2 of .34. Maybe I just do not want a PPO2 higher then 1.2 for deco today who knows. hmmm seems that the a good choice for my gas mix is the one that works for me.

I would call a richer gas mix worse then a leaner mix for a novice btw. Not a good time to find how they handle a wing failure, a stop depth hold, a bad gas switch - any other of a number of things while deco'ing at near PPO of 1.6 (for 100%) or 1.5 (for 80% @ 30'). You actually have more buffer with a leaner mix! I am commenting this directly on your novice statement and not a characterization of the OP.

 

[stuartv]

Strange thing is that I am looking at one of my logs (I only have 2 at work) for a 180' dive with 2 deco gasses, one being 75%. Seems that it does work and can work well. Just depends on the dive and the person. Almost any gas will get you out of the water, it is only a matter of how much time and how much CNS risk the diver wants! Personally I find 100% too risky and not necessary for ocean deco especially with my use of last stop at 20'. Maybe in the +200' dives it does not get you out of the water fast enough, but even in basic trimix, it is quite usable. We can disagree but it does get divers out of the water and many use 80% which is only a little bit more O2!

I also think the OP's gas selection is quite correct and would not cause any issues in completing the planned dive. Mixes are a personal choice.

Thanks. I have only/always used 80%, for my deco dives so far. That is mainly because, I gather like yourself, a number of my deco dives so far have been in the ocean and we elected to deco on 80% at 30' in order to stay out of the surface surge at 20' or higher.

For planning for the Oriskany, I was basing my plan on using on AL40 for deco gas (as that is what I have). With planning to only use half (and reserve half for my buddy, in a lost deco gas scenario), the volume of deco gas becomes my limiting factor in the plan. Thus, using 100% maximizes the "deco value", if you will, of my gas.

However, after further consideration, (and to go slightly OT here) I have revised my planning parameters a bit. Mainly, I have decided that, for this particular dive, I am going to plan it so that I can complete my deco on back gas - and then carry whatever deco gas I choose as an "all-usable" gas. IOW, no reserve for a buddy with lost deco gas. I think this will make my back gas my limiting factor (I haven't actually run any numbers yet), and would not be my normal approach. But, in this case I will be going to do this dive with no buddy. Not diving solo. I mean that I will have an insta-buddy or possibly have to hire a guide. Either way, I'll be diving with a buddy that I don't know.

Because of that, I am choosing to plan as if I am going solo. I want to be sure I am carrying adequate gas to take care of myself, no matter what my buddy does. That means I can plan to use all the gas in whatever deco cylinder I carry, so maybe a 40 will be plenty. Or maybe I'll still end up carrying an AL80. Like I said, I haven't run the numbers yet.

Anyway, thank you all for sharing your thoughts. It is definitely helping me think through the most challenging dive plan I've done to date (because of all the vertical relief that is available).

 

[stuartv]

You actually have more buffer with a leaner mix! I am commenting this directly on your novice statement and not a characterization of the OP.

Thank you for saying that. But, I have no problem calling myself out. I AM a novice. Since I finished my AN/DP cert I've done something like 6 deco dives. I would TOTALLY prefer to make plans that keep me far away from having to hang with a ppO2 of 1.6!

Anyway, I digress... I started this thread to understand how you experienced folks deal with planning and managing a dive with a lot of vertical relief. Not so much about how to plan my gases - although I understand it's related and I have definitely gotten some good ideas from that part of the discussion!

 

[RainPilot]

Ummm.... at the risk of opening a whole new can of worms.... the current research seems to indicate that for a resting diver on deco the CNS clock and risk of oxtox for transient PPO2 spikes is pretty low indeed. I would suspect that minor depth fluctuations at 20' / 6m with 100% wouldn't be a large factor. Military O2 CCR divers go up to 2.0 for periods, Im NOT advocating that but we need to remember that 1.6 is not a magic number. The stats show that above 1.6 for ALL phases of the dive the odds of tox go up and for 1.4 and below it drops dramatically. If 1.6 was magic then we would have a lot more divers toxing since I would hazard that the bulk of accelerated deco in tech diving is being done on 50% from 21m and 100% from 6m.

 

[stuartv]

Ummm.... at the risk of opening a whole new can of worms.... the current research seems to indicate that for a resting diver on deco the CNS clock and risk of oxtox for transient PPO2 spikes is pretty low indeed. I would suspect that minor depth fluctuations at 20' / 6m with 100% wouldn't be a large factor. Military O2 CCR divers go up to 2.0 for periods, Im NOT advocating that but we need to remember that 1.6 is not a magic number.

I do understand that. I don't think that if I accidentally were to drop to 25' even for a whole minute during deco on 100% I would just suddenly go into convulsions. But, I also prefer to PLAN as if that will happen. At least until I am a lot more confident in my skills.

 

[stuartv]

Ummm.... at the risk of opening a whole new can of worms.... the current research seems to indicate that for a resting diver on deco the CNS clock and risk of oxtox for transient PPO2 spikes is pretty low indeed. I would suspect that minor depth fluctuations at 20' / 6m with 100% wouldn't be a large factor. Military O2 CCR divers go up to 2.0 for periods, Im NOT advocating that but we need to remember that 1.6 is not a magic number.

I do understand that. I don't think that if I accidentally were to drop to 25' even for a whole minute during deco on 100% I would just suddenly go into convulsions. But, I also prefer to PLAN as if that will happen. At least until I am a lot more confident in my skills.

 

[KenGordon]

Thanks. I have only/always used 80%, for my deco dives so far. That is mainly because, I gather like yourself, a number of my deco dives so far have been in the ocean and we elected to deco on 80% at 30' in order to stay out of the surface surge at 20' or higher.

For planning for the Oriskany, I was basing my plan on using on AL40 for deco gas (as that is what I have). With planning to only use half (and reserve half for my buddy, in a lost deco gas scenario), the volume of deco gas becomes my limiting factor in the plan. Thus, using 100% maximizes the "deco value", if you will, of my gas.

However, after further consideration, (and to go slightly OT here) I have revised my planning parameters a bit. Mainly, I have decided that, for this particular dive, I am going to plan it so that I can complete my deco on back gas - and then carry whatever deco gas I choose as an "all-usable" gas. IOW, no reserve for a buddy with lost deco gas. I think this will make my back gas my limiting factor (I haven't actually run any numbers yet), and would not be my normal approach. But, in this case I will be going to do this dive with no buddy. Not diving solo. I mean that I will have an insta-buddy or possibly have to hire a guide. Either way, I'll be diving with a buddy that I don't know.

Because of that, I am choosing to plan as if I am going solo. I want to be sure I am carrying adequate gas to take care of myself, no matter what my buddy does. That means I can plan to use all the gas in whatever deco cylinder I carry, so maybe a 40 will be plenty. Or maybe I'll still end up carrying an AL80. Like I said, I haven't run the numbers yet.

Anyway, thank you all for sharing your thoughts. It is definitely helping me think through the most challenging dive plan I've done to date (because of all the vertical relief that is available).

Do not do this dive with an insta buddy. Even if you did you should still follow the practice taught to you on your course. That is what a responsible person would do, saying 'I don't know you, you can sing for deco gas...' Isn't very nice. I find dives like this are limited by the back gas already, so I like to plan to be off it sooner rather than later.

If a 40 isn't big enough get one which is.

Are you saying you have done 6 deco dives altogether, or 6 acceleratd deco dives? What is the longest deco you have done? Are you prepared to do 30 or 40 minutes of stops? Have you done it before when it didn't matter?

Ken

Ps I did some numbers and it looked like you might have an hour at your final stop if you lost your deco gas. That is a proper commitment. I did some other numbers and 70% gets you out 4 minutes sooner than 100% as you can use it from the 12m stop on. (Making some random assumptions about levels, time and SAC)

 

[stuartv]

Do not do this dive with an insta buddy. Even if you did you should still follow the practice taught to you on your course. That is what a responsible person would do, saying 'I don't know you, you can sing for deco gas...' Isn't very nice.

That sounds like an assumption that I wouldn't discuss the dive plan and agree on it with whoever I end up buddied with. And that would be a false assumption. I would absolutely make sure my buddy is aware of what I'm carrying and is also diving to be self-sufficient.

Planning to be self-sufficient may be limiting on dive time. But, I would rather have that than put so much faith in someone I don't know. Hopefully, whoever I'm diving with will feel the same way. Someone who is willing to trust me with their contingency gas without even knowing me seems like someone I should be wary of diving with in the first place.

As far as following my training, I am following it. My training was to either plan to have enough back gas to deco on OR carry enough deco gas for me and my buddy and have my buddy do the same. In this case, I'm choosing to go with the former.

The only other option is to get someone I know and trust to come along as my buddy. And that is not feasible for this trip.

So, maybe this is my inexperience and naivety showing, but I feel like this plan is adequately safe. At MOST, I might do some very cautious light zone penetration, but I expect the majority of the dive will simply be swimming around the outside, looking in where I can. As far as the actual dive goes, it seems like it would be less challenging than the deco dive I did on the JB King in the Saint Lawrence River (max of 160', but not that long in time because it's a drift dive on a relatively small wreck site). I've done cavern dives in Dos Ojos (cenote in Mexico) and plenty of swim-throughs in NC and other wrecks, including swim-throughs in the Sea Tiger (105', off Waikiki) during a night dive. Just about all my training to date has been in cold (40f more or less) water with viz usually in the 5 - 10' range. I don't get anxious from being in an overhead and/or low viz.

I've been reading about diving the O. It doesn't seem particularly hazardous unless you start doing penetration in the areas below the flight deck. Am I missing something?

Also, I should note, from running a quick plan, if I'm diving to be able to do deco on back gas, it does mean that I could end up (if I lose my deco gas) having over an hour of deco. But, if that were to happen, one, I carry a jon line and could attach myself to the anchor line and hang as long as I have to with ease. And, two, I'll be carrying an extra deco cylinder on the boat. If I do lose mine, I would have a plan in place with my buddy and the boat captain for someone to bring me my other deco cylinder while I'm hanging.

And, really, how likely is it that either of us will actually lose our deco gas? Assuming we don't, it just means having maybe a 30 - 45 minute dive (depending on the whole multi-level thing) with probably 30 minutes or less of deco. And a solid plan in place for if either of us DOES lose our deco gas.