Planning multi-level deco dive?

[stuartv]

Yeah, I'd do pretty much want the others said, but would plan for some time at (say) three levels (160, 145, 100), then stay within those constraints while using the computer to tell me where I'm really at. You want to be able to fall back to the tables if necessary and know you have at least that much gas available. By the way, the O is a good dive to use a 40 of 50% and another 40 of 100%. Play with a two-deco-gas scenario in your planning....

I have thought several times about how much nicer it would be to carry two deco bottles for a dive like that. I will have my Helitrox cert by the time I do the dive, but not the full Trimix cert. Which means I don't expect that I will have any formal training in carrying 2 deco bottles at the time of this dive. And I am not comfortable "self-teaching" how to properly manage 2 deco bottles. I carried two 40s numerous times during ANDP training. But, that was always in the context of bottle passing drills. Not carrying 2 to be able to use 2.

I know I'll get training on carrying and using 2 deco bottles when I eventually take the Trimix course. I kind of feel like I should wait to attempt dealing with 2 deco bottles until after I've had that formal training. I don't want to kill myself by underestimating the chances of switching to 100% when I really meant to switch to 50%.

 

[tbone1004]

i don't plan to share deco or stage gas, you aren't carrying an extra second stage so are you planning on buddy breathing? If you plan on using no more than half of your deco gas, you can always use your 50% bottle at the 100% level and alternate with backgas to get the time.

Regarding the shape of the curve, what Victor and I were apparently trying to get at at the same time *that answer wasn't planned, but you can tell we're dive buddies because of it....*, is that you make the curve as worst case as possible. You are at a deeper depth for longer time than you are planning, that is the maximum you can stay at that depth without violating some sort of rule that you have established, in this case a gas reserve. You plan the dive based on those unrealistic maximums so your deco planning is set for you to be deeper for longer than reality and your actual deco curve will be shallower and shorter than what your tables say.

 

[RyanT]

Regardless, I gather there is no well-defined procedure for how to do this. You plan as close to your expected "reality" as you can, leave yourself a good safety margin, and then make sure you don't exceed the planned parameters, fuzzy as they may be, and apply a healthy dose of conservative judgment as you "fly" the computer.

Correct. You can plan a super rigid profile and dive it down to the second (good luck). Or plan a profile and build in some additional conservatism. FWIW, I plan my dives with a SAC that is above my rate. I then use this gas consumption value as my absolute minimum, and then ensure this gives me at least close to enough gas to finish my dive in the event of me or a buddy with a lost gas or +5' & +5 min scenario. This usually gets me back on the boat with a healthy amount of gas left, but I'd rather err on the side of too much than not enough.

 

[rjack321]

I have thought several times about how much nicer it would be to carry two deco bottles for a dive like that. I will have my Helitrox cert by the time I do the dive, but not the full Trimix cert. Which means I don't expect that I will have any formal training in carrying 2 deco bottles at the time of this dive. And I am not comfortable "self-teaching" how to properly manage 2 deco bottles. I carried two 40s numerous times during ANDP training. But, that was always in the context of bottle passing drills. Not carrying 2 to be able to use 2.

I know I'll get training on carrying and using 2 deco bottles when I eventually take the Trimix course. I kind of feel like I should wait to attempt dealing with 2 deco bottles until after I've had that formal training. I don't want to kill myself by underestimating the chances of switching to 100% when I really meant to switch to 50%.

You're going to have like 30mins of deco to do on this 45min dive. That's really the max for a single bottle (and more than most people can do on 1/2 of an al40 of O2).

You might consider using an al80 of EAN50. Reduces your rock bottom to plausible levels, gets you onto a deco gas sooner and doesn't require all those backgas stops just to get up to 20ft. An al40 is enough for 30mins of EAN50 deco but just barely - there's not enough to share. Hence the 80 gives you some ability to share.

 

[stuartv]

I said "ascent from 145' to 85' over a period of 20 minutes"

Yes there is, it's 115ft for 45mins.

I completely do not understand your response.

If you meant to say 115' for 20 minutes, then I get you. You're saying just use the average depth for the same amount of time. But, as tissue loading is non-linear, I am not convinced that that is a very accurate way of doing it. And I know that deco is not an exact science, so accuracy may be regarded as very fuzzy. But, the model used is very precise (whether it's accurate or not) and so I'm saying that I'm not sure using the average depth gives an "answer" that is necessarily close to what the model would say if it were applied correctly, instead of with a workaround of using average depth. But I digress...

I think I got your point, so thank you.

 

[PfcAJ]

Fwiw, while on a dive if you go to dive setup>plan dive (something like that) itll show you your current ascent schedule.

Since you know how much gas you have in your deco bottle (and subsequently how many minutes of gas that is) you can use that feature to ensure you don't outrun your deco gas supply.

I get what you're driving at, but its hard to plan a dive like that with tables. Two different depth levels is probably how I would do it, but sometimes discretion is the better part of valor and the best plan is to keep it simple.

 

[packrat12]

The Shearwater's TTS and +5 can be used to help plan your ending deco requirements during the dive. If the TTS is within your expected deco time, you reasonably know you have enough gas. As it pushes your high side of your expected time, you need to be more concerned. Travel time and issues are not accounted for. It is not perfect but it does give a good indication. Also you are able to use the Shearwater planning tool during the dive for more detailed information.

Personally, I would plan this dive at 145' and watch my TTS. Since, in this scenario, we have 2 computers, I do not need tables as backup. I would also plan it at 160' and look at the extreme edges of gas usage so I have a final understanding of my gas requirements at the extreme end.

 

[stuartv]

You're going to have like 30mins of deco to do on this 45min dive. That's really the max for a single bottle (and more than most people can do on 1/2 of an al40 of O2).

You might consider using an al80 of EAN50. Reduces your rock bottom to plausible levels, gets you onto a deco gas sooner and doesn't require all those backgas stops just to get up to 20ft. An al40 is enough for 30mins of EAN50 deco but just barely - there's not enough to share. Hence the 80 gives you some ability to share.

I thought about using an 80. But, I don't have rigging for an 80 and have never tried slinging one. But, you gave me an idea. Maybe I could sling 2 40s of the same gas? That would allow me to use 1 whole bottle for myself and still have a reserve for a lost gas scenario. And not have to worry about switching to the wrong gas and killing myself.

I ran a few numbers and it actually appears that using 70% for deco is the sweet spot. It gives me a shorter runtime than 50%, 80%, or 100%.

I'm looking forward to eventually being able to carry and use 2 different deco gases, but that time is still out in the future.

 

[rjack321]

Sorry I had a typo. If its 145 up to 85ft of 20mins then that is an average of 115ft for 20mins and that's almost an NDL dive on 25/25 (not sure what heliox you are planning on). Your first stop will be somewhere up around 40ft for 1 min. But you can basically go straight up and that keeps the rock bottom low. Perhaps 5 mins on O2 would be prudent though.

If you are doing a 45 min dive (which is what I would do). That's like 30mins of deco and an al80 of EAN50 is perfect for maxing out your capacity while still staying within both your backgas and deco gas reserves.

 

[stuartv]

Fwiw, while on a dive if you go to dive setup>plan dive (something like that) itll show you your current ascent schedule.

Since you know how much gas you have in your deco bottle (and subsequently how many minutes of gas that is) you can use that feature to ensure you don't outrun your deco gas supply.

I get what you're driving at, but its hard to plan a dive like that with tables. Two different depth levels is probably how I would do it, but sometimes discretion is the better part of valor and the best plan is to keep it simple.

That's exactly what occurred to me. I know how much time I have with a 40 at 20', so checking the TTS and the current ascent plan would let me ensure that I don't rack up more time there than what I can do.

Thanks.