Optimal Pony Bottle Size for Failure at 100ft?

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So just curious......what keeps the pony from just sliding out of that slot rail when you are in a head down, fully inverted dive?

I'm always head down but not fully inverted unless I'm going down a metre diameter vertical pipe
I dive in and dive down inverted, and sometimes even surface fins first inverted, captains love that

So but man there is always something but here it's not much just a hammer adjustable locator tab


Which does not necessarily have to be depressed to release the bottle if you give it a strategic wibble
@chrisjur, your math is correct and 40cf is a very good size pony bottle for the dive plan. However, it is possible to restate some nuances and clarify your assumptions.

When you have a failure - any failure - you may fix it fast and won't have to spend 3 minutes at depth. However, at that time you will suck a lot of air. Your heart rate will increase substantially, so in a single minute you may consume as much as as you would in 3 minutes. Therefore, your math is correct. Assume that 1 minute of failure will last you 3-5 real minutes. And this is conservative. If you don't dive trimix, there may be some narc'ing going on at that depth.

You're also missing one assumption - in the case outlined in the original post, you presume a direct vertical ascend. What if you dive a wreck and need to get back to the upline? What if you need to shoot a bag? Therefore, I'd add a 100ft swim to your assumptions just in case... You should be able to use a 40cf bottle even w those calculations. Anything smaller is just silly.
Great points. Thanks, mr_v.
I have been utilizing a 30cu ft. pony bottle that I "stole" from a friend a while back, but am evaluating the purchase of a new pony.

A bit of background: I frequently dive deep-ish wrecks where most of our time is spent between 75 and 100 feet. While I do not venture out on my own, I do often dive in teams or am separated from a buddy at a distance that would be too far for him to acknowledge my situation or provide me with emergency air. I do not perform wreck penetration alone, under any circumstances.

So, the question that I'm asking is what size pony would be optimal to address a failure that occurs at 100ft.? - which I think is a reasonable depth to work with for this exercise given my type of diving.

I've worked out the math below, but I would greatly appreciate any feedback regarding anything that I got wrong, or any poor assumptions that I am making.

More assumptions and background info:

1. This will be used as a true pony bottle by definition and standard practice - it will only be used in emergencies and never to otherwise extend bottom time
2. I am calculating this based on a hypothetical catastrophic failure at 100ft depth in a non-overhead environment
3. Used for recreational diving only (e.g. no deco obligation)
4. Assuming 20% reserve is required for the pony bottle so that I'm not breathing it dry
4. My average SAC is 0.6 cu ft./min over the past 77 dives according to my air-integrated Garmin MK2i. However, for these calculations, I will use 1 cu ft./min since I will probably be huffing and puffing my ass off after the sh*t hits the fan at 100 ft.


1. Failure occurs at 100ft

Depth: 100ft
Duration: 3 mins
Assumption: After switching to pony bottle, I will probably need 2-3 mins to compose myself and think about my situation
Gas consumption: 12 cu ft.
Formula: 4ATA x 3mins x 1.0cu ft/min SAC = 12 cu ft. gas consumed

2. Ascend to 15ft for a safety stop
Duration: ~3 mins
Assumption: 85ft of ascent at a conservative 2 seconds/ft. = 170 seconds
Average Depth: Let's say 50 ft.
Gas Consumption: 7.5 cu ft.
Formula: 2.5ATA x 3 mins x 1.0cuft/min SAC = 7.5 cu ft. gas consumed

3. Safety stop
Depth: 15ft
Duration: 3 mins
Gas consumption: 4.5 cu ft.
Formula: 1.5ATA x 3 mins x 1cu ft./min SAC = 4.5 cu ft. gas consumed

4. Final exit and swim to the boat
Depth: Surface
Duration: 5 mins
Gas consumption: 5 cu ft.
Formula: 1ATA x 5 mins x 1 cu ft./min SAC = 5 cu ft. gas consumed

Total Gas consumed from all stages above: 29 cu ft. gas consumed

Therefore, the recommended pony tank size is 40 cu ft.
(With a 40 cu ft. tank, keeping 20% reserve would equate to 32cu ft. of usable gas, which would be suitable for the above scenario, with some headroom.)

Is my logic correct here? Any other insight would be greatly appreciated.

Thanks so much in advance.
In your standard dive situations id use a 40 as it has better boyancy characteristics than a 30. There is a reason your friend let you steal his 30, they are just not nearly as good as a 40 and most people dont use them if a 40 is available. Btw try breathing that 30 down to almost empty.... you may feel like a balloon is pulling you up.... where a 40 should not feel like that and using less of the air in the 40 will also help.

however a 19cf will get you to the surface just fine as you will not be sitting around 2 or 3 minutes figuring things out with a 19 you will just head straight up and do a quick stop at 15 feet.

its an emergency, itll likely only happen once in your diving career if ever. The 19cf will keep you alive and be far far far less bulky. Consider you might do a few hundred dives never needing that 40. So in that light a 19 makes more sense.

and yes I did say if it was me id do the 40.

im not trying to make sense here just answer your post lol.
really struggling also to decide if i go with a 19 or a 30 even if i do really shallow dive i am afraid that a 19 might be not enough i know i take a lot of air while diving.
really struggling also to decide if i go with a 19 or a 30 even if i do really shallow dive i am afraid that a 19 might be not enough i know i take a lot of air while diving.
At what PSI do you normally start heading for your safety stop, on the deepest dive you intend to do with the pony-bottle?

As generic advice: If you're unsure, bigger is usually the safer route, assuming you consistently bring it with you on every dive. However, if it's big enough you sometimes leave it behind, then even a small one is better than the one you left at home.
At what PSI do you normally start heading for your safety stop, on the deepest dive you intend to do with the pony-bottle?
i start do dive solo this year i put around 10 dives. i use a 80 cf. the dive shop where i go only rent 30 CF pony. i find it a bit bulky. I didn't calculate my sac rate yet but i assume it might be on the higher end i am 6.2' and 220 pounds. I don't exceed 30 feet on my dive . Usually i will turn around at 1500 and as a rule of thumb will start ascend at 1200.

It s cold water diving in low to medium visibility depending on the weather. my season goes from the beginning of june till november. (just to give a bit of info.

I feel very comfortable diving solo and understand the pony is a redundant air source and not to extend my dive. I really do plan my dive and dive my plan.

Your suggestion are welcome i might have one or to question after if i don't bother you.
i am afraid that a 19 might be not enough i know i take a lot of air while diving
It's pretty easy to find a lower bound by just noting the time and pressure before ascent (on your normal tank), or check your log if you use AI. You will need more when stressed, but if that easy scenario consumes more than 19 cf, that size is clearly not feasible. I'd suggest that if you consumed more than 11.5 cf (which is 430 psi in an AL80 cylinder), then the 19 cf is not appropriate. (That allows for 50% increase in consumption due to stress and a couple hundred psi to keep the reg happy.)
This isn't really how pony bottles are calculated, however:

1200psi /3000 psi * 80 cu = 32 cu.

You effectively turn your dive having 32cu remaining. Typically, we also try to have 500psi or more, when we actually surface. How much PSI would you have remaining?

Just to shortcut the conversation, lets say you have 700psi remaining. That means you used (1200-700) 500psi to surface from that depth, including safety stop and whatever else you normally do.

500/3000*80 = 13.33 cu.

The above would be an empty (0psi) tank if it started at 13cu. You could still safely surface with a 13cu, but you might surface a little faster and skip the safety stop. However, a 19cu would have almost 900 psi remaining....

(19 cu - 13.3333 cu used)/19 cu * 3000 psi = 894.7 psi

It's easy to make incorrect assumptions, or things in the real world might work a little differently than mere raw math. Even incorrect gauges can throw things off. I also like to actually test those assumptions. Since you can rent a 30cu, why not do a dive to this "max depth" you intend to go, then switch to the pony-bottle, and surface normally, including safety stop. Check how much psi you have left on the 30cu. If my calculations and your measurements are correct, I'm guessing you'll have around 1700 psi remaining on the 30cu.
, then the 19 cf is not appropriate. (That allows for 50% increase in consumption due to stress and a couple hundred psi to keep the reg happy.)
^ Plus this. We have to keep in mind other things that could go wrong, including entanglements and increased breathing rate, other equipment problems, cramps, etc. Those other things that went wrong may also have lead to the OOA situation, which is why they can't be discounted.

My guess is 30cu may be an appropriate size.

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