using a pony bottle to extend bottom times?

[KevinNM]

OK, I have a question though, what if someone showed up with a set of doubles like twin 72's but only had one air outlet like a center post and treated the set like a big single. But let's say they like the balance of the twin 72's and buoyant characteristics better that say a modern mega single tank the size of a water heater. Would you consider that to fit the recreational criteria?
Because if they were filled to 2250 then that would be basically like a 130.

They don't fit in the tank mounts on the spree.

 

[DaleC]

I am one of those divers that uses his Al40 "pony" to extend dives. Call it a stage/pony if you want but the net effect is the same. I do this for a couple of reasons. First, I own all steel 72 tanks so using a larger tank is not an option unless I borrow one. Second, I can double my tanks up true but sometimes I want a single/pony combo, like when there is a long walk or difficult access.

As Lynne said, the most important thing to do is to understand how to use them correctly. Just as you would if you used doubles or even a large single in regards to deco limits. There is nothing magical about a pony tank, it's just a cylinder of gas, a resource.

First, I determine what my rockbottom or bailout reserve volume has to be for the dive. For me, at 100ft that would be 20cuft/diver.

Second, I determine what PSI in my tank represents that volume. For an Al80/Al40 combo that would be 800psi/1500psi respectively.

At max depth/time I need at least 20cuft in each tank before I begin my ascent. This is the same whether I solo or am with a buddy.

During the dive I breath 20cuft off the pony, down to 1500psi, then switch to backgas. The pony is now reserve only. I then breath backgas until 800psi (or sooner if NDL's become an issue) and begin ascent. I now have 20cuft backgas/20cuft reserve gas.

 

[Basking Ridge Diver]

I am one of those divers that uses his Al40 "pony" to extend dives.

I happen to use a 19 cu ft as a pony - and my view is the smaller the better for the style of diving that I do. I guess when I think of AL40 - I don't think pony.

I barely enjoy carrying the 19 cu ft - so if you are essentially carrying half your back gas - perhaps it is reasonable for someone to use it. But for me if I am going to carry that much gas - I think I would go to doubles bm or sm...

 

[LeadTurn_SD]

I am one of those divers that uses his Al40 "pony" to extend dives....

Dale, I do the same with my Al30, using the same procedure you do. I don't do this terribly often, but I'll do it without hesitation when I know I want to spend a bit more time on the reefs here than my Al80 allows.

It does take a basic understanding and then some planning and forethought, and using a "pony" as a stage results in a smidge more task loading during the dive, so I agree with the general consensus that "get a bigger single tank" is probably the best / safest solution for the OP... but for shallow shore dives, with proper planning, I don't see a big issue.

Best wishes.

 

[TSandM]

Why would you leave rock bottom in both bottles? It seems to me that you need your safety reserve somewhere, but not in both places. By leaving the reserve in the 40, in essence, you are using it as a combination stage and smaller pony, which is kind of weird. But I guess it does keep your redundancy.

DumpsterDiver, why do you have a problem with starting on the stage? It doesn't make any difference to your descent, which regulator you are breathing, and there is no gear futzing to be done on the way down. If you're sucking gas to the point where you have to do a gas switch on DESCENT, then I'd say all the people were right who said you need to work on your gas consumption. Going through 40 cf before even getting to the bottom would be quite the SAC rate!

 

[DaleC]

Lynne, I split my rock bottom between the two bottles, mostly because I solo a lot and use small tanks. For solo I need RB in primary and bailout sources. Remember my 20cuft RB is for one diver. If I left double rockbottom (for two divers) in my backgas when buddy diving I would have to begin ascent at roughly 1400 psi for my 2250psi St72 (or 1600psi in an Al80). That makes for a pretty short dive.

LeadTurn makes a pretty good point in this regard. At some point the pros outweight the cons in buying a larger tank. This is true when diving as a team and not using a pony. The amount of reserve you need in backgas pretty quickly makes a smaller tank not as feasable.

I just have too many things on my shopping list as it is to buy another steel tank, so I make do with what I have and that's why I employ the single/pony/stage idea.

Here's a diagram of how different tank configurations might look:

 

[LeadTurn_SD]

Why would you leave rock bottom in both bottles? It seems to me that you need your safety reserve somewhere, but not in both places. By leaving the reserve in the 40, in essence, you are using it as a combination stage and smaller pony, which is kind of weird. But I guess it does keep your redundancy...

Hi Lynne,

In my case, I usually only sling the AL30 when I solo. And only then if I plan to dip below 60 feet or so during the dive.

I want the capability to safely surface using either back gas or my Al30... so if I am using it as a "poor man's doubles" to extend the dive a little, I'm careful to never breath it down below what I need to get to the surface from my deepest depth... just in case it need's to become a "pony" again

My dives are relatively shallow and within NDL, so a direct ascent is safe. Leaving 1/2 (15 cu ft) is more than enough for a safe ascent.

Again, this is a less than ideal solution, but in my case an AL80 is almost always plenty, I don't want to hump doubles over the lava and the rugged entries at some sites here on the Big Island, and even slinging an Al 63 stage would be "overkill" (usually).

Best wishes.

 

[kwinter]

Over 50 posts in less than 12 hours. What is it about pony bottle threads?

we don't allow stages or doubles on recreational trips. We do have trips where stages and doubles are welcome. Single tank trips are not those trips.

What if someone shows up with a very small set of doubles, like maybe 3 liters, with a scrubber canister in between?

 

[JohnnyC]

Wookie, this is a purely hypothetical, but what would you consider someone diving mini doubles? I'm starting to lean that direction even for recreational dives as well as I enjoy the redundancy and similarity in setup to my normal doubles rig, From an "advanced" diver standpoint, a set of twin 40's or 50's makes more sense to me than a single 80. Not that your answer would prevent me from diving with you if I ever make it down that way, just curious what the answer from a captain's perspective wold be. I totally understand if a blanket "no doubles" policy exists on your recreational charters.

 

[Wookie]

Over 50 posts in less than 12 hours. What is it about pony bottle threads?




What if someone shows up with a very small set of doubles, like maybe 3 liters, with a scrubber canister in between?

No worries. Are you planning to use your 40 bailout to "extend your dive"? Then we might open a further discussion.