Rock bottom calcs with pony bottle

[LG Diver]

Hi all,

I've read a number of thread here about how to calculate "rock bottom" cu ft and pressure, but none of the threads I've read make any mention of how a pony bottle would factor into the calculations (if at all). It would make sense to me that it should be taken into account somehow, as it is an available gas supply and it's intended purpose is to provide redundancy in an OOA situation (yours or your buddy's). Do I simply subtract the available cu ft of the pony from the total rock bottom cu ft calculated for me and my buddy (ensuring that I don't subtract more than 1/2 of the total so I always have enough in the primary tank for myself)?

 

[radinator]

If the purpose is to provide redundancy, you don't count it at all n planning. It is redundant.

If you use it in planning, it is not redundant, but necessary.

And I think that would be a big mistake.

 

[LG Diver]

If the purpose is to provide redundancy, you don't count it at all n planning. It is redundant.

If you use it in planning, it is not redundant, but necessary.

And I think that would be a big mistake.

Interesting. It wouldn't be necessary, except in an OOA situation- exactly it's intended purpose. By your logic, If I'm diving double Al 80's instead of a single Al 80 and a pony, would I plan the dive as if I'm diving a single Al 80?

 

[radinator]

Interesting. It wouldn't be necessary, except in an OOA situation- exactly it's intended purpose. By your logic, If I'm diving double Al 80's instead of a single Al 80 and a pony, would I plan the dive as if I'm diving a single Al 80?

Not necessarily. Depends on the setup.

If you were diving double 80s with a manifold, in the event of a reg failure you can still access all of the gas in both tanks.

If you have a reg failure in your single tank, you've lost all your gas. You now only have the pony remaining.

You see, very different.

Also, with doubles, you have a pressure gauge to tell you the air in the doubles. Does your pony have a gauge?

 

[Kevrumbo]

Ross-O & Northcoast Diver did a good write-up regarding gas consumption calculations and a recommended Pony Bottle volume:
http://diver.net/bbs/messages5/44855.shtml
Personally, I would use Double Manifolded Tanks with an Isolation Valve, or at least use a AL40 as back-up to my Single Main Tank, and sling it like a Stage/Deco Bottle. . .

 

[OE2X]

Diving with a buddy - which I assume is your premise here:

When you calculate rock bottom it is always based on you back gas. Your rock bottom is based on supplying the gas needs for two divers to get to the surface from the deepest part of your dive, making all stops.

A pony is used in the event that one person has a failure and needs to use that gas. You should always do a rock bottom calc for the size of your pony based on the needs of one stressed diver getting safely to the surface. You may find that a 9 CFT will not get you safely to the surface from 130' if your sac is 1.5

You do not factor your pony into your gas needs or your buddy's needs for a dive. It should be considered a last ditch gas solution and one to be used if your buddy is now Murphy.

 

[neil]

Not necessarily. Depends on the setup.

If you were diving double 80s with a manifold, in the event of a reg failure you can still access all of the gas in both tanks.

If you have a reg failure in your single tank, you've lost all your gas. You now only have the pony remaining.

You see, very different.

Also, with doubles, you have a pressure gauge to tell you the air in the doubles. Does your pony have a gauge?

If LG had a pony big enough to hold his rock bottom supply, and a gauge on it, and his main tank reg fails, he still has his RB supply. That seems like it ought to work, even if it seems a little more complex than just main tanks and RB planning.

 

[Halthron]

It all comes down to the purpose of the pony. If one is carried for the express purpose of something to use in an emergency situation then it shouldn't be included. If the intent is to carry extra gas that just won't fit into the tank on the back then include it. Just realize that doing so doesn't leave you with the redundancy.

IMO, I don't see a problem with the first scenario, as long as you aren't diving solo since your buddy is your reserve. The problem comes about when you now have this extra gas and, depending on the size, may go into unplanned deco and don't have the gas to back it up for both you and your buddy.

 

[lamont]

The problem with the pony, that if its backup to having bad gas management and you run out of your backgas at depth and go on the pony, then your buddy is likely out of gas as well.

If its just backup in case your buddy does a disappearing act right when you have a free-flow and need to do an ascent, then you need enough gas to get one person to the surface doing stops (Al19 is absolute minimum). You could subtract that gas out of your backgas rock bottom since the pony would be handed off to your buddy if they go OOA on you. That assumes that you'll be able to quickly sort out the CF at depth and get everyone on the right gas and ascend. I'd prefer to just ignore the pony for planning purposes and plan on having your buddy on your backgas if they have a problem. Then if you're ascending you have the option of passing off the pony bottle rather than a requirement to do so.

 

[gcbryan]

It depends on how you set things up. The problem with the question that you are asking and with some of the answers is that some have a philosophical problem with carrying a pony and therefore all answers regarding pony usage will be negative.

There is no problem with using a pony. If you want to minimize the number of second stages you carry you may decide to only have one second stage on your main tank and your backup second stage on your pony. In this case planning rock bottom for you and your buddy on your main tank makes no sense as you may only have one second stage on that tank.

In that case consider your pony to have your rock bottom gas and for your buddy's to be in the main tank. Having gas for your buddy assumes a need for redundancy and that can be either your main tank or your pony as it's there for redundancy. Be sure to use a SPG gauge on that tank as you do for your main tank.

You can mount your pony on your main tank or sling it. It's your choice. It is a little silly when advice is given that you shouldn't carry a pony but then more advice is given on how to carry it if you do use one.

I don't think people should kill each other but I don't also have a preference as to how they should kill each other should they decide to do so.