Bottom stages + MGR

[AndyNZ]

Just curious, if planning a dive using a bottom stage (in a non-overhead environment) as well as back gas - how much of the gas is usable?

For the sake of argument, let's say the dive is at an easy site - step off the back of the boat and pootle around and surface anywhere (an "all usable" dive). Also, let's assume that this is a T1+ dive, so one bottom stage and one deco gas, max depth of 48m, using an AL80 for 50% so that the deco time isn't limiting the dive.

MGR for a 48m dive is 100bar in twin 12s = 2400 litres. I can see an argument that says that as long as you have your MGR in your backgas, that you can use all of what is in the stage. But then I'd also see that it would be prudent to leave some reserve in the stage.

So what do people do? And how would that vary for T2 dives using two deco gasses plus one or more bottom stages?

 

[Rainer]

Did you receive the T1+ endorsement (i.e. 25 T1 dives, then one day with instructor)? I thought you just took your T1 class.

In any case, kill the stage first.

 

[AndyNZ]

It's a curiosity question, more than anything else.

 

[boulderjohn]

I would kill the stage as well.

 

[AndyNZ]

Would the argument for killing the stage be that it then becomes ditchable and simplifies resolution of subsequent problems?

I certainly wouldn't be thinking about splitting MGR across the stage and backgas - realised that wasn't too clear in the OP. Any reserve in the stage would be "additional"... which, if it isn't needed, then I guess I've answered my own question.

 

[Blackwood]

Any reserve in the stage would be "additional"... which, if it isn't needed, then I guess I've answered my own question.

Pretty much

Just need to ask yourself: what's the purpose of the stage?

Is it to facilitate 2 dives on a single set of doubles (how I use them)? If so, what benefit would there be to leaving gas in it and going to backgas sooner? (IMO: there is no benefit, but there are drawbacks)

Is it to extend bottom time for a deep dive? If se, what benefit would there be to leaving gas in it and going to backgas sooner? (IMO: there is no benefit, but there are drawbacks)

The only reason in open water I'd intentionally leave gas in a stage bottle is if I am using it to travel somewhere and plan to detach it when I reach that somewhere.

 

[boulderjohn]

It's a nice moment during your ascent (assuming you have a line) when you can put your empty stage on the line and send it to the surface.

 

[rjack321]

Also, let's assume that this is a T1+ dive, so one bottom stage and one deco gas, max depth of 48m, using an AL80 for 50% so that the deco time isn't limiting the dive.

Still limited by one deco gas. IMHO 30mins is a reasonable max although GUE will give you a T1+ endorsement to 45mins of deco on one gas.

Bottom line:
You need to be able to deco out on backgas
You need to be able to get an OOA buddy up. to your next breathable gas or the surface.

Stage does not factor into either of these Min gas calculations which are in L or cf of backgas (in the doubles). With a stage you're just alot more likely to reach a time limit before you reach min gas in the doubles - since you kill the stage first as others have said.

 

[LiteHedded]

It's a nice moment during your ascent (assuming you have a line) when you can put your empty stage on the line and send it to the surface.

on deco the other day I shot my empty stages like a lift bag.
it was quite comical

 

[Blackwood]

on deco the other day I shot my empty stages like a lift bag.
it was quite comical

... until they hit the ceiling?

:mooner: