Pony size

[upton75]

Hello. Well i have been considering getting a pony bottle. I have been pricing them and was looking at the 19cf bottles. When I started to talk to a rep they said that a 19cf was not sufficient for dives down to 100 feet. MY question is what size pony bottle should i get if i plan to do deep dives of depths up to 130. I am not sure if i want to go deeper in the future. Should i go with a 30 or what the shop suggested for a deep ocean dive and buy the 40cf bottle. They told me that is want all the people in the store dive with.

Thanks.

 

[Indian Valley Scuba]

There is a lot to consider when sizing up a pony bottle. First and foremost, what would be your 'worst case scenario' in the event that you suffered a catastrophic failure of your main gas supply (such as blowing a tank neck o-ring). Would you be deep inside a wreck or other overhead environment? Or would you have any sort of deco obligation?

If neither is true, then you should be able to make a normal ascent from 100 plus feet with a small bottle, far smaller than the 19 CF one you are considering. If you might find yourself being delayed, perhaps for some of the reasons mentioned above, then you really need to learn to calculate your gas consumption rate, figure how far in you might be, and how many minutes you might need to be breathing on this redundant air supply.

Feel free to give me a call at 800-825-2452 and I will be more than glad to help you work through your dilemna!

 

[upton75]

As of right now I will not be doing any of the above, i am just looking for a bail out bottle if something goes wrong. Id rather be safe than sorry, but in the future i do want to do cave and or wreck dives. But i have other things i want to do first. So it is a just in case bottle and if i run out ot take me to the surface.

 

[NeedABiggerBoat]

Dont waste time and money, just get the 40cf. Its still a very manageable size and if you sling it properly, you wont even notice it while diving.

 

[Indian Valley Scuba]

Then a 19 should be more than sufficient, or even a 12 or 14. There's no need to hump a huge pony rig around on every dive, especially if you aren't in an overhead environment, as you indicated you won't be. Larger bottles, such as the 30's or 40's you mentioned, are generally used more as stage bottles and incorporated into the dive plan (meaning you actually plan on using them) versus a pony bottle which is planned to never use (except in an emergency).

So whatever deal you can get on a 12,14 or 19 should work for you!

Check out the prices at ScubaGearPlus for aluminum pony bottles with Free Lifetime Inspections! and more!

 

[halocline]

It's a little bit of a controversial subject, and if you search you'll find all sorts of opinions. If I understood what your LDS said, they told you to get a bigger bottle than 19cf? I think that's very bad advice.

My opinion (everyone's got one on this subject) is that if you need more bailout gas than that, you're probably diving with the wrong tank(s) for that dive to begin with, and/or you are diving inappropriately for your level of training and general gear configuration.

First, you should define exactly what you want this bottle to do for you, and why you might need it. I assume you're staying in NDL, OW situations, right? If not, that's technical diving and you need technical training and gear to be safe. So if you're looking for a pony because the diving you want to do might stretch the limits of a single tank, you could actually be adding risk by carrying it. Get some training and a set of doubles for that.

If all you need is something to get you to the surface at a relaxed pace in the event of total regulator failure, or some unexpected drain on your gas such as an OOA diver, AND you're always staying in OW, with no deco obligation, 19ft is plenty. Think of it this way, it's about 25% of an entire AL80, which is probably what you're currently diving.

Carrying a lot of extra gas has potential to allow you to get in situations that you might be better avoiding. That's the controversy.

 

[mikemill]

To determine the size you need to look at the situation that you would need it. Lets say the worse of it is 100ft dive with no overhead (including deco).

Now the general rule of thumb is to use a SAC rate of at least 1 for stressed situation (plus using 1 makes the calculation easier). Now determine what is an acceptable ascent rate. To make the example easier lets say you are doing a 50ft / min ascent.

So from 100ft to surface your average depth would be 50ft and take 2 minutes. 50ft is ~ 2.5 atm.
So: 2.5 * 1 * 2 is 5 cuft needed to get you from depth to the surface.

Now lets say you wanted to do your safety stop for 3 minutes at 15 feet. So you have:
1 [SAC] * (15/33 + 1) [atm] * 3 = 4.4 cuft (total 9.4 cuft)

Now lets say that you need some time at depth to address any problems. This could be something like untangling yourself, navigating back to the anchor line, or whatever. This is where gas really gets eaten up. Lets say that you want 2 minutes to deal with "stuff" at depth

100 ft is ~4atm. So 4 * 1 * 2 = 8 cuft (total 17.4 cuft)

Now assuming a true capacity of 19 cuft that'd leave you with 253 psi at the end.

 

[WD8CDH]

Your backup gas should be enough to get either you or your buddy (who ever has the highest air consumption) safely back to the surface. That could be as little as a 6cf tank even it you are diving double 130s at 20 feet with no overhead to as much as half of your regular air supply (1/3 of your total) when caving. You need to work out how much air would be needed if you or your buddy has a failure at the end of the dive just at the moment you are heading to the surface. Personally, I rarely have less than a 30 as a pony. Better to have extra that you don't use than to run out.

 

[TheScubaBOB]

I just added a 30ft pony to my rig and properly mounted on my main tank it isn't even an issue.

It adds less than 10 lbs to the setup and by removing trim weights from that side I maintain good balance & weighting.

 

[Scared Silly]

Do a search as this topic is a