Diving Deco/Stages to 1/3

[ucfdiver]

So I acquired an AL80 that needs some stripping and started reading through archives on stripping and the various methods to do so. Anyways, that's beside the point.

When searching, I ran across a few threads where people were discussing diving their stages and deco gas to 1/3. Can anyone explain the logic behind this? I found several people making such comments (mainly suggesting AL80's instead of 40's for deco because of this), but never heard the thought behind it.

 

[rjack321]

I'm not a good person to explain it although it seems most common for sidemounters and solo.

 

[Flightlead]

deco != stage

 

[DA Aquamaster]

Planning thirds for deco bottles does not really make sense. Halves is plenty as you really have to exceed the plan on a given deco dive to ever require more than twice the planned deco gas. Halves also allow you to have enough deco gas for a lost deco gas buddy as you will actually use less than two halves of your deco gas as both of you will both spend some of the time on back gas and or another deco gas.

If you need more than a 40 (ie more than 20 cu ft of a given deco mix or O2) you are probably already into a two deco gas dive and a pair of 40's each with a different deco mix is still enough and would be preferable to a singel 80 from both a mix and a lost gas perspective.

In reality, using an 80 for deco usually has a lot more to do with a diver wanting to carry only one deco tank along on a trip and/or only having access to low pressure O2 with no booster available at the fill station in question to allow a full 40 of O2 or other high O2 percentage deco mix. That limitation does not really apply to 50% as you only need 1100 psi of O2 to get a full 3000 psi bottle of 50%.

With regard to a stage: Offshore, I am more inclined to breath the stage down to nothing to increase bottom mix and keep the reserve in the back gas tanks. For example if I dive double 100's and an 80 stage with bottom mix, I'd use 77 cu ft from the stage and then keep 1/3 (92 cu ft, 1600 psi) in reserve in the doubles and turn the dive at 3200 psi (about 200 psi after emptying the stage). The reasoning would be that all the reserve is in the backgas that will stay with you and can be accessed by redundant regs.

In a cave, diving a stage with thirds is more conservative and makes some sense as you leave the stage on the way in and pick it up on the way back out and the reserve third in the stage becomes the gas needed if either you or a buddy has a gas failure, can't access one of multiple stages, etc.

 

[ucfdiver]

deco != stage

Thanks for letting me know.....??

 

[Meng_Tze]

So I acquired an AL80 that needs some stripping and started reading through archives on stripping and the various methods to do so. Anyways, that's beside the point.

When searching, I ran across a few threads where people were discussing diving their stages and deco gas to 1/3. Can anyone explain the logic behind this? I found several people making such comments (mainly suggesting AL80's instead of 40's for deco because of this), but never heard the thought behind it.

It probably has to do with them not understanding gas planning related to deco, stage diving and the environment. They probably heard about, or were even trained to dive 1/3's on BG, but not thought deco /stage gas planning through.

BTW deco gas is not the same as stage. But a stage can be used to deco......

 

[Dirkadiver]

I plan many of my dives w/ using 2/3's of my deco gas. Diving 1/3's won't give you a lot of time in deco (especailly if fills are an issue). Honestly, I've never heard running 1/3's on deco gas. It's a waste & kind of pointless. But, if you want to be ultra conservative & waste a lot of gas(tough to top off a 50% bottle that has 2000 PSI in it w/o a booster), then I guess it might be a good plan.

 

[battles2a5]

I know of one cave instructor that teaches stages that way. Once you breathe 1000psi out of an 80, it will lay flat and is very easy to manage. So he will breathe a third on the way in, drop it, and complete the dive on back gas. So you may have more of your reserves in the stage than you might want, but the bottle is a little easier to manage on the way out. This approach starts to break down pretty quickly once you start adding bottles, but seems workable otherwise. Other than that, planning to 1/3 for stage or deco gas doesn't make a whole lot of sense.

 

[DA Aquamaster]

AL 72's are neutral at 500 psi with a reg attached, so you'd get the same effect in that case, by breathing them down. And they are 7.0" in diameter rather than 7.25" in diameter making them slightly handier than an AL 80 as well.

 

[ucfdiver]

It probably has to do with them not understanding gas planning related to deco, stage diving and the environment. They probably heard about, or were even trained to dive 1/3's on BG, but not thought deco /stage gas planning through.

BTW deco gas is not the same as stage. But a stage can be used to deco......

I'm thinking that's what it is, too.

I plan many of my dives w/ using 2/3's of my deco gas. Diving 1/3's won't give you a lot of time in deco (especailly if fills are an issue). Honestly, I've never heard running 1/3's on deco gas. It's a waste & kind of pointless. But, if you want to be ultra conservative & waste a lot of gas(tough to top off a 50% bottle that has 2000 PSI in it w/o a booster), then I guess it might be a good plan.

This makes perfect sense. If a deco bottle fails, your buddy uses your o2 for 75% of his deco, you use yours for 75%. While you're on o2, your buddy is on 32%, and vice versa, which makes both of you "clear" at the exact same time.

I know of one cave instructor that teaches stages that way. Once you breathe 1000psi out of an 80, it will lay flat and is very easy to manage. So he will breathe a third on the way in, drop it, and complete the dive on back gas. So you may have more of your reserves in the stage than you might want, but the bottle is a little easier to manage on the way out. This approach starts to break down pretty quickly once you start adding bottles, but seems workable otherwise. Other than that, planning to 1/3 for stage or deco gas doesn't make a whole lot of sense.

What's his logic here? Diving to 1/3 of a stage, you can't share it with a buddy, so that extra 1/3 seems to have no use. Now if it's just so that it rides easy, maybe I get that.

The other question, is does he teach to reserve the penetration portion of the stage out of backgas to recover from a lost/stolen/failed stage bottle and get him as well as his buddy safely home? If so, that would be reserving around 25 cu/ft from backgas, breathing 25 from the stage, and having 50 left in the stage, meaning he's now diving to 1/4's, which is pretty conservative, and I would think the need for extra stage bottles would quickly outweigh the benefits of the bottles being easier to carry?