Coming back with TOO MUCH air??

[GramsciBeat]

As a recovering air hog the OP has provoked a serious case of SAC envy on my part. But I am also a little confused around what the issue is; if I understand correctly the OP comes up after one dive with 2/3 of a tank left and is loath to pay for that tank to be filled.

If he does not want to refill the tank, what prevents the OP from using the same tank on the second dive, and planning the dive so that he uses another third of the original air supply, which will still result in him surfacing with a third left. That will leave him well within the recreational standard of starting an ascent with a minimum of 50bar in the cylinder*. If he is unable to plan his gas usage so as to ensure he does that, then he should definitely not be incorporating multiple cylinders into his dive plan - if he can do the plan, then he does not need to use a pony or doubles.

I really think utilising a pony as part of a standard gas plan at such an early stage is making matters unnecessarily complicated. &, personally, I would only ever use a pony as a redundant air-supply for use in an emergency situation.

*Where I dive, we don't believe in Alis - but I assume they would be filled to an equivalent of 200 Bar?

 

[spectrum]

GramsciBeat,

The catch is that things don't always go to plan and as you drive down the total air budget you can quickly put chinks in the armor. Little things like a regulator blowing off when fumbled on the surface are dismissed when it takes a bite out of 80 CF. The same glitch on 40 CF means something.

When you dive the full tank intending to come out with at least 500 PSI or a determined volume you have a cushion to absorb gauge inaccuracies. When you dive the short end of the fill some of that air must be reserved to avoid sucking the system dry and letting water in and those last few hundred PSI on your gauge may not even be real.

If you realistically plan to have enough air for you and a buddy to exit at anytime, such as Rock Bottom the dive plan quickly gets constrained.

All in all you would be setting yourself of for a lot of misery and maybe worse. There is an old axiom that say's never go in with less than a full tank. There are exceptions to every rule but overall and for general use it's well founded.

Pete

 

[jar546]

It is not all about you. It is about your buddy too. You should have enough air to safely get you and your buddy to the surface in the event of an emergency such as OOA. I doubt if you are diving SFlo that you will be limiting your dives to 30' reefs. $6 for a fill is not a big deal. So you are thinking about slinging a pony just because you want to take a 2nd dive with a less than full tank? Think about that. What if there is current and you have to work a little harder to get to where you want to go? I have been on drift dives in SFlo that required some swimming just to get aligned with a particular reef, trench, etc. There are too many unpredictable situations that can occur diving in the ocean. Save you partially full tank for the pool or a dive in a more controlled environment. But hey, if what you want to do works for you, great. I would just not do it on a boat or in the ocean for that matter. Up north we routinely do 2 or 3 dives on a single tank but that is in a quarry with no current and it is a much more controlled environment. We just do shorter dives. If I am paying to be on a boat, I am going in with a full tank. You never know what is going to happen.........

 

[gsk3]

I think the plan is fine as long as you:
-Dive 1/3 of your tank the first dive.
-Dive 1/3 of your tank the second dive.
-Never plan on using the pony to extend your gas.

The pony is there as backup. Lots of experienced folks have tried to make it clear to you that breathing down the pony as part of the plan is not a good idea. It's just not a sufficiently robust setup to be counted on. The problem with your plan is that you expect to use the pony 1 in 5 or 6 times. That's just way too much. If you can plan your gas properly, you will never have to use it. There are people on this board with thousands of dives with a pony who have never used it except specifically to practice with it. Aim to be one of those people.

 

[spectrum]

I think the plan is fine as long as you:
-Dive 1/3 of your tank the first dive.
-Dive 1/3 of your tank the second dive.
-Never plan on using the pony to extend your gas.

The pony is there as backup. Lots of experienced folks have tried to make it clear to you that breathing down the pony as part of the plan is not a good idea. It's just not a sufficiently robust setup to be counted on. The problem with your plan is that you expect to use the pony 1 in 5 or 6 times. That's just way too much. If you can plan your gas properly, you will never have to use it. There are people on this board with thousands of dives with a pony who have never used it except specifically to practice with it. Aim to be one of those people.

In the end I think the OP is heading down a road of risk with very little reward. However let's call the pony a stage for this discussion. Redundancy and additional gas are two objectives that can be accomplished with the same or similar gear.

 

[gcarter]

Maybe I am missing something.

If the OP breathes 2/3 of the tank and surfaces with 1/3 left on a single dive...

How is there greater risk in diving dive 1 using 1/3 surfacing with 2/3 then dive 2 using 1/3 surfacing with 1/3?

In both scenarios there is 1/3 tank minimum left in reserve. There is no increased risk to OP or buddy, with or without a pony.

If in the 1st scenario something happens during the portion of the dive where the 2nd third is being breathed, it is no different than if in the second scenario something happens while the 2nd third is being breathed on the second dive.

I see no additional risk here for OP or buddy. Same gas available.

 

[AfterDark]

I understand all that. And, I agree with you, although I would probably suggest he just buy an adiditonal AL80 if I was going that route.

But, in the specific context of the OP's original post - he already has an AL 80, he already has a steel 40 - why offer a solution that involves buying two additional tanks, and a set of bands, to someone who doesn't want to waste money filling an AL80 that isn't empty?

Because IMO his air consumption would propably allow him to get 2 dives from that set, have a safety margin and possibly be carrying a lighter load. There's no reason to keep the 80 and the 40, those could be sold for $$ towards a different setup. However after reading one of his later posts he seems to have money issues so he probably has to work with what he's got. OP you've received lots of good advise the best is about not putting yourself in danger over an airfill!!
By the way back in 1970 I was paying .75 for a fill today it's $7.00 still cheaper than a funeral.

 

[gsk3]

Maybe I am missing something.

If the OP breathes 2/3 of the tank and surfaces with 1/3 left on a single dive...

How is there greater risk in diving dive 1 using 1/3 surfacing with 2/3 then dive 2 using 1/3 surfacing with 1/3?

In both scenarios there is 1/3 tank minimum left in reserve. There is no increased risk to OP or buddy, with or without a pony.

If in the 1st scenario something happens during the portion of the dive where the 2nd third is being breathed, it is no different than if in the second scenario something happens while the 2nd third is being breathed on the second dive.

I see no additional risk here for OP or buddy. Same gas available.

If he/she always keeps 1/3 in reserve, then cool. I should have been more clear (or less precise with the wrong numbers!). He is talking about breathing the 80 down to near-empty on the second dive, then using the pony to get back to the boat.

 

[gcarter]

If he/she always keeps 1/3 in reserve, then cool. I should have been more clear (or less precise with the wrong numbers!). He is talking about breathing the 80 down to near-empty on the second dive, then using the pony to get back to the boat.

OK, so that's what I missed then.

 

[gsk3]

OK, so that's what I missed then.

Yeah . It's the using it as a reserve plus a stage that makes me nervous. As Lynne pointed out, if he's going to use it as a stage, then it should get breathed first. Gas in that bottle is the least flexible option (only one reg, possible to lose the bottle, etc.).