Planning multi-level deco dive?

[stuartv]

an al80 of EAN50 is perfect for maxing out your capacity while still staying within both your backgas and deco gas reserves.

I haven't done it. Do you think it's easier to dive with one slung AL80 than two slung AL40s?

 

[packrat12]

A slung 80 is not really any different then a 40 once in the water. I find getting the 80 on and of the boat a pain only because of the weight difference. You need the larger stage rigging. I carry mine when doing penetration or deep. My 80 is setup as a deep rig and not a rich rig.

 

[stuartv]

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

My training was that, in a lost deco gas scenario, I do my deco obligation using my deco bottle, then pass it to my buddy, who lost his deco gas, so he can use the other half of the bottle to do his deco. In reality, probably pass the bottle a little early, as I would finish my deco on back gas while my buddy is breathing off the deco gas bottle.

 

[rjack321]

I haven't done it. Do you think it's easier to dive with one slung AL80 than two slung AL40s?

other than the dry land weight, 1 80 is easier than 2 40s.
You can use the same rigging (its close enough) just with a larger band clamp.

 

[rjack321]

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.

Yea there's usually some custom EAN that works out slightly better than 50, 80 or 100% ala the best mix concept.
Once you can add 2 deco gases this idea pretty much goes out the window for the simplicity and familiarity of standard deco gases. Most people settle on 50 and 100. 80% is not that common compared to those 2. In part because there aren't that many stops to use the 50% on if you are getting off it at 40ft for a backgas break and then switching to EAN80 at 30ft. 80% is more commonly paired with 32 or 36% as a "deep" deco gas - still the minority of dives compared to the 50/100 pair.

 

[victorzamora]

If I start flying my computer, then I could get into a scenario where the computer says my TTS is some number that looks okay, but it is now based on a shorter amount of time to get to 20' (say, because I'm now at a shallower depth than where my "planned" ascent started, and then a longer time at 20'. So, I'm now using more deco gas and not leaving enough for my buddy (if he loses his deco). Or it might not be as simple as that. My actual profile may result in tissue compartments being loaded differently than "as planned" and result in an ascent time with a different distribution (because of different controlling compartments) that results in less time to get to 20' and more time at 20' than I planned for - which means using an unacceptable amount of deco gas (i.e. cutting into my buddy's gas).

You missed the part about "first stop depth." If both of those parameters are the same (or more shallower/shorter) when you start your ascent, you'll have a safe ascent. You may be deeper/run longer than your preferred plan and it certainly wouldn't be "optimal" but it would get you to the surface safely every time. Using a Shearwater with @+5 will fix a lot of things as well, because you could even make that your turn point and add additional conservatism there.

The argument could be made that with two Shearwaters (or Perdix+H3 or whatever two high-end tech computers you're using) that a pre-cut strict plan isn't really necessary with MinGas implemented.

 

[rjack321]

One thing that seems to be missing here is that you can do backgas deco in lieu of a lost deco gas. Assuming you leave enough for rock bottom and have no buddy failure that reserve ends up being enough to provide decent coverage for one lost deco gas. But for this is work you can't put too many eggs in the (single) deco gas basket. 45mins of deco on only one deco gas is too much for instance. That's gonna be ~90mins on backgas and its hard to keep that much in reserve. If you limit yourself to 25-30 mins of deco with the one gas then your rock bottom can handle the ~50mins of backgas deco at 20 and 10ft. Or at least a significant chunk of that while your buddy shares deco as for the rest.

 

[stuartv]

You missed the part about "first stop depth." If both of those parameters are the same (or more shallower/shorter) when you start your ascent, you'll have a safe ascent. You may be deeper/run longer than your preferred plan and it certainly wouldn't be "optimal" but it would get you to the surface safely every time.

I did not miss the part about first stop depth. My response was that, if you assume one dive profile and it gives you, say a first stop of 60' and a TTS of 30 minutes (just to make up an example), you could possibly end up doing a much different dive profile (with, for example, more ups and downs versus the plan of always ascending) that gives you a very different shape to the TLBG and which has the same first stop and same TTS, but with a different distribution of where the ascent time is spent. For example, 5 minutes less time at the deeper stops and 5 minutes more at the 20' stop. That's my theory, anyway, just based on the fact that on-gassing is not linear and occurs at different rates in different compartments.

 

[stuartv]

One thing that seems to be missing here is that you can do backgas deco in lieu of a lost deco gas.

I am totally aware of that. I had not run those numbers (yet) because past plans have usually worked out better to just plan for saving 1/2 the deco gas for my buddy. But, I will run that and see. A 40 of 70% that is all-usable would probably be just the ticket, if I have enough back gas to do all my deco on back gas. If...

 

[rjack321]

I did not miss the part about first stop depth. My response was that, if you assume one dive profile and it gives you, say a first stop of 60' and a TTS of 30 minutes (just to make up an example), you could possibly end up doing a much different dive profile (with, for example, more ups and downs versus the plan of always ascending) that gives you a very different shape to the TLBG and which has the same first stop and same TTS, but with a different distribution of where the ascent time is spent. For example, 5 minutes less time at the deeper stops and 5 minutes more at the 20' stop. That's my theory, anyway, just based on the fact that on-gassing is not linear and occurs at different rates in different compartments.

Unless you are doing a GF low of 10 or something like that, a significant 60ft first stop from a multileveling 145 to 85ft dive is going to be you way too much deco for one gas.

within reason shallow depth can be "traded" for more time. 10ft shallower will be 5mins more time. Over longer time spans deco is non-linear. Over the realistic BTs you are considering, its linear enough.