Would there be anything wrong to dive independant doubles without a manifold?

[Scubaroo]

O-ring once bubbled...

Breathe the stage down first and then switch to backgas..

...because then you have full backgas and only need to do a single switch. How far are you breathing the stage down - basically using it up?

If your plan is "turn at half gas minus rockbottom", I assume that you add the amount of gas you plan to breath from the stage (eg 35cf out of 40cf) to your backgas, and calculate halves minus rockbottom from that?

Remember we're talking NDL non-overhead here, like an "out-and-back" boat dive with more bottom time.

So if we calculate rockbottom as gas needed to surface sharing air from the deepest part of the dive as being 10cf (just plucking figures here), and we have 95cf backgas, and 40cf stage, the profile would be:

usable backgas - 85cf
usable stagegas - 35cf

total gas - 120cf

dive plan:

stage - breath 35cf
switch to backgas, ignore remaining 5cf in stage
backgas - breath 25cf
halves minus rockbottom reached, turn dive
backgas - breath 60cf
dive complete with 10cf in backgas

???

humor me

 

[O-ring]

I think one camp argues that you should breathe 1/3 of your stage, then switch to backgas and breathe 1/3 of that down, then switch on the way out, etc. leaving 1/3 in each tank for reserve. I think most people are moving toward using backgas as the primary reserve since it is easily shared and won't be dropped, etc. I remember reading something about breathing a stage down to 1/2 +200psi and then dropping it and breathing it down the rest of the way on the return trip, but I think that was cave. Hopefully someone who does this type of diving will show up and steer us toward the right path....I don't dive with stages..

IMHO, in the ocean, I would breathe the thing all the way down (minus whatever reserve you decided to leave in it) and then drop it just in case you were not coming back the same way you went in for whatever reason. I think this varies by dive plan and individual circumstances though.

For your dive plan, I guess it depends on if you are diving thirds or just rock bottom. I think we were able to agree that diving rock bottom was fine for non-overhead NDL dives where you did not have to reach the point you started from (i.e. live boat or shore dive), but that we were going to dive thirds on a boat that ties into the wreck or in any other overhead situation.

 

[MikeFerrara]

I never really thought of doing a stage dive with a single tank on my back.

First off gas management is I think the problem with the configuration that's being discussed. If the extra tank is a stage then the back gas will have two second stages correct? or are we still talking independent doubles with one tank slung ?

I guess I still don't see areal benefit to such a rig.

 

[Scubaroo]

MikeFerrara once bubbled...
or are we still talking independent doubles with one tank slung ?

Single first stage on back, single first stage on sling. So yeah, independant doubles, except one is slung stage like.

This thread has gone way off course, but a scenario I can picture this in is on vacation - only AL 80 singles are available, and you've brought your single tank wing. Why not stage a second AL 80 for a longer dive?

 

[MikeFerrara]

Scubaroo once bubbled...
Single first stage on back, single first stage on sling. So yeah, independant doubles, except one is slung stage like.

This thread has gone way off course, but a scenario I can picture this in is on vacation - only AL 80 singles are available, and you've brought your single tank wing. Why not stage a second AL 80 for a longer dive?

For a no stop OW dive with this configuration...
I would breath them down evenly by alternating between them and reserve a total volume equal to my "rock bottom".

or...

I would rig the back tank as usual with two second stages and suck the stage almost dry reserving all of rock bottom in backgas. Call it a stage dive without the rule of thirds.

But...What kind of dive would require us to do this? I'm thinking for it to be a no-stop dive with this much gas we're talking about an all day 20 ft dive. LOL

 

[Scubaroo]

MikeFerrara once bubbled...
I would rig the back tank as usual with two second stages and suck the stage almost dry reserving all of rock bottom in backgas. Call it a stage dive without the rule of thirds.

But...What kind of dive would require us to do this?

Option 2 suits my thinking - rigged as normal backup necklace, long hose on backgas, and just carry a stage with 40" hose.

My usual buddy and I are finding our dives being limited by available gas rather than NDLs, and he has no interest in forking out for a new doubles rig (he would need a new BC or backplate as well as the tanks), so we are investigating this for some extra bottom time. Usual CA stuff - shore/boat diving under the kelp in anything from 50 to 120 fsw.

 

[leadweight]

Using a stage seems to be a reasonable thing to do, especially if a new wing would be required to dive doubles. When traveling, it might not be possible to find a 40 cf to rig as a stage, but most places have some tanks in the 50 to 60 something range available for small divers, or normal size divers with phenominally low air consumption. An 80 cf stage would be a drag, to say the least.

 

[metridium]

leadweight wrote...
Using a stage seems to be a reasonable thing to do, especially if a new wing would be required to dive doubles. When traveling, it might not be possible to find a 40 cf to rig as a stage, but most places have some tanks in the 50 to 60 something range available for small divers, or normal size divers with phenominally low air consumption. An 80 cf stage would be a drag, to say the least.

What sizes of aluminum tanks are there between 40 cf and 80 cf? I can't say I've noticed (m)any, but then I've never been looking for them before. I've seen some small steels, but I don't think I'd want steel stages.

 

[MikeFerrara]

There are al 50 and al 63. But an al 80 works just fine. All my stage and deco bottles are al 80's no problem even with more than one clipped on.