Is there a valid reason for a pony bottle

[caruso]

I was recently on a Belize livaboard trip with my girlfriend and several members of ScubaBoard and I was the only one out of about 24 divers who carried a pony bottle. One of them asked me why I carried the bottle and my answers are also in direct response to the question posed by the Op.

1- My girlfriend doesn't always dive with me. On that trip I did every available dive (about 27) she did about 2/3 that amount. When not diving with her (or my siblings on other dive vacations) I prefer to dive alone rather than take my chances with an instabuddy. Even if the unknown buddy is a good diver, it's still inconvenient to have to keep track of where they are, let them know I want to go this way or that way to see something interesting, etc. It is without question that solo divers must have the redundancy in the form of an independent bottle. It's mandated by the solo diver certification and most dive ops will enforce if it they allow solo diving at all- which the Belize Aggressor 1V does.

2- Piece of mind. On a Cozumel trip a year or two back I didn't bring the pony bottle which is not typical for me. As I type this I forget why, perhaps I didn't expect to be doing much in the way of deeper dives, or there wasn't that much diving planned for that particular trip and I didn't want to bother lugging it around. Anyway I didn't have it. On one dive -think it was Palancar Deep-, I swam away from the group as I often do and was perhaps 30 years into a swimthrough and it suddenly occurred to me that if I had a catastropic failure of my regulator, or burst a hose I'd never make it back out and to the surface or to the group in time and it made me feel rather tense. Having that pony bottle allows me to enjoy my diving more because just knowing it's there allows me to relax when I'm off on my own.

3- I mentioned it earlier but just to round out this post and make it complete for those who didn't read the entire thread and to clarify what seems to be a confusing point- I typically "use" the pony bottle (without EVER taking a breath from it) as a reserve to my main tank gas which allows me to draw more gas from the main tank and surface with perhaps 200 psi or even less. That amounts to many hours of extra diving time over the years. Again I am not "extending the dive by using the pony", I am extending the dive because I am not surfacing with 700 psi in my main tank that will never be used.

 

[doctormike]

Yes, some divers go in the water planning to use their pony bottle to extend their bottom time. When that is done without any actual calculated plan, that is a poor practice.

As always, an excellent post. Just want to expand on the "without any actual calculated plan" idea for new divers reading this thread.

What is being described here is a stage bottle, not a pony bottle. Small tanks can be pony bottles (redundant gas supply), stage bottles (extra gas that you plan to use for the dive), deco bottles (beyond the scope of this discussion), or bailout bottles (ponies for rebreather divers).

I don't have a problem with planning to use a stage bottle. If you are diving with someone who has a much lower SAC rate than you do, and a larger tank isn't available, using a stage bottle on a dive where redundancy isn't required is reasonable.

 

[caruso]

What is being described here is a stage bottle, not a pony bottle..

I use my pony bottle for both. During most of the dive, its an emergency supply of gas. Towards the end of the dive, it becomes my primary reserve so I can draw the main tank lower. I've been told by others that my strategy is going to kill me some day because that's not how you're supposed to do it. The fact that I've got 2 completely separate supplies of gas and I almost always surface with more gas than any other diver on the boat doesn't seem to matter.

 

[doctormike]

I use my pony bottle for both. During most of the dive, its an emergency supply of gas. Towards the end of the dive, it becomes my primary reserve so I can draw the main tank lower. I've been told by others that my strategy is going to kill me some day because that's not how you're supposed to do it. The fact that I've got 2 completely separate supplies of gas and I almost always surface with more gas than any other diver on the boat doesn't seem to matter.

Totally reasonable, as long as you aren't diving in a situation where you need redundancy (i.e. most recreational diving).

What you are doing is no different (and in some ways safer) than just using a larger primary single tank. The vast majority of dives in the world are done without redundancy. Just because I have made the case for the utility of a true redundant pony bottle that isn't part of your dive plan, that doesn't mean that you can't dive safely without one!

 

[caruso]

What you are doing is no different (and in some ways safer) than just using a larger primary single tank. The vast majority of dives in the world are done without redundancy.

For most of the dive I've got redundancy of two separate cylinders. Once the main tank gas drops below what is considered to be a safe reserve I no longer have that redundancy and I tailor the dive accordingly. At at that point I'm working my way back to the line if I'm not already close to it, or even starting my ascent.

 

[doctormike]

For most of the dive I've got redundancy of two separate cylinders. Once the main tank gas drops below what is considered to be a safe reserve I no longer have that redundancy and I tailor the dive accordingly. At at that point I'm working my way back to the line if I'm not already close to it, or even starting my ascent.

Exactly. Redundancy for most of the dive, which is better than for none of the dive. And a larger total planned gas volume. As long as your SAC rate isn't low enough to go into deco, sounds good to me.

 

[Gareth J]

I use my pony bottle for both. During most of the dive, its an emergency supply of gas. Towards the end of the dive, it becomes my primary reserve so I can draw the main tank lower. I've been told by others that my strategy is going to kill me some day because that's not how you're supposed to do it. The fact that I've got 2 completely separate supplies of gas and I almost always surface with more gas than any other diver on the boat doesn't seem to matter.

A pony is generally consider to have a limited amount of gas, i.e. a 3 litre cylinder. When used as a pony, it is the redundant gas source, there to ensure you can get back to the surface complete and complete all the compulsory decompression time you have accrued.
3l x220 = 660 litres of gas.
@ 20m - 3x15l/min = 45l/min ---- 660/50 = 13 minutes
@ 30m - 4x15l/min = 60l/min ----660/60 = 11 minutes.
Thats whilst you are relaxed, when it all goes wrong the breathing rate elevates so you should half those times as a minimum. Granted, you will be going up, so you should get more time.

Just a note on elevated breathing rates.
The RN did 'tests' on clearance divers, They compromised the equipment without warning and then measured the elevated breathing rates. Initially, divers where breathing something like 5 to 6 times their normal rate. This did drop in stages. The biggest drop was when the diver reached their first perceived point of safety, for some this was the first stop, for others it was the second, or the first gas switch.

From what you are saying, you are diving independent twins. Which is a different technique!
I personally don't like independent twins, I used to dive that way until they got me into serious trouble. After that i went to an isolated manifold, which is what I should have done at the start.

 

[Fastmarc]

I prefer to call it a PONEE, Piece of Necessary Emergency Equipment.

I like this! I feel I'll be getting stickers made.

 

[MaxBottomtime]

Why did you add an octopus to your set up?

I began diving with Merry when she was a new diver. After changing all of the gear she was sold as a new student we have dived together since 2006. She now dives solo 99% of the time.

 

[caruso]

From what you are saying, you are diving independent twins. Which is a different technique!
I personally don't like independent twins, I used to dive that way until they got me into serious trouble. After that i went to an isolated manifold, which is what I should have done at the start.

I wouldn't know the difference between independent twins and two tanks with an isolation valve but I'll take your word for it that my technique with a pony bottle is similar or the same as the former.