Pony bottle questions

[Genesis]

As an example, let's use a high end SCR, say 1 CFM. On a 120 ft dive, you'd need 4 minutes at an average depth of 60 FSW, or a bit less than 12 CF for the ascent. Add a five minute safety stop at 15 feet, a bit under another 7.5 CF, for a total of 19.5 CF. That'd leave over 10 CF (1000 psi) in a 30 CF pony... a comfortable margin.

Uh, let's redo that math a bit, eh?

Remember, you stop at 15', so your average depth during the ascent is 52', not 60'. So let's make that dive a 130' (maximum "recreational depth") to make the math work more-or-less.

Ok, so we have a 4 minute ascent, requiring the aforementioned 12 cf. We assume you have one minute at 4 ATM to figure out what the heck happened, for a total of 16cf. And now we're going to hang for 3 minutes @ 15', which is 4.5cf, which runs you out on the 19cf bottle

But if you draw and go, you make it with about 3.5cf left, or about 540psi (assuming you started with 3,000.)

Now this is a "worst case" scenario. If you're at 110' when you draw and go, you require 7.7 cuft for the ascent, and when you arrive you have more than half the bottle remaining. This actually gives you the ability to take that minute on the bottom to sort it out before starting up.

Alternatively, you can use the "old" ascent rules of 60fpm (remember, this is a "brown water" bailout bottle) in which case you cut your gas requirements in half for the ascent. You might be ahead of the game to do exactly that, assuming you can extend the 15' hang bigtime by doing so. For example, from 110' you would burn only 6 cubes on the ascent doing that, which would leave you with 13 cf - or about 8 minutes of air - at the 15' stop.

You can also do the 30 cube pony, but now we're talking about a tank that is almost the same size as a real bottle in terms of length and such, and its starting to get to be a PITA to transport and carry. At some point the pony becomes enough trouble that you won't take it - and thus, useless.

A redundant system that you don't take due to inconvenience is worth nothing.

 

[The Chairman]

and it sat with it's initial fill for all of that time. Really. I intended it to be a bail out and I used it just for that. I am probably the exception, as I see MOST pony divers eventually calculate that extra 19cf as part of their usuable back gas... not good.

I heartily agree with the Pugster. That uneaziness is your bestest friend. It's telling you that you are not trained and/or ready for the deeper dives.

In all the dives that I have done, including being a DM and instructor, I have never run out of gas. Ever. Of course now that I have said that, I will probably run out on my next dive. I have had a couple of my charges get way low... but they have never "run out". I have seen my share of free flows at depth. But have never experienced that catastrophic failure that some refer to. Of the relatively few people that I have heard to run out of air, not one of them cited an equipment problem... they were all the result of human error: mistakes made in haste or from lack of training.

The point is... running out of air is a training issue and not a gear issue. How can we expect to resolve a training issue by throwing more gear at it? You should learn how to use the gear you already have, before you complicate things even further. And yes, a pony bottle complicates your rig. It starts with the decision as to how and where to mount the pig, and continues as an additional snag hazard, and then it even makes regulator retrieval harder... Is that my primary, secondary or pony reg I just grabbed?

I would suggest that your money would be better spent on additional training. It's lighter than a pony. It addresses the root of the problem. It's even easier to carry on the plane with you. Most importantly, it has a far better chance of saving your life or even better... keeping you from getting in a life threatening situation. Think about it.

 

[redacted]

I use a 13 as a bail-out bottle (unused so far except for rest stop checkout). For deeper dives, it marginal in terms of making a completely normal asscent. But ease of travel was an important contraint in my case. I hard mount it and use a Mk2/R190 detuned & on a necklace with a seperate SPG clipped to my primary SPG so they both get checked each time I look.

The 13 & 19 are both about 2 lb negative so they may have a noticable effect on trim. I offset mine with a 2 lb wt on the pony strap. 30's are pretty much neutral so it should disappear in the water from a trim standpoint.

With a 13, it's important that it stay full (not much reserve). I built a crossover hose so I can fill my own and top it off every couple dives. Test breathing and a minor FF takes you to 2500 pretty fast. With a 19 you have a bit more cushion.

Pony needs to be empty for air travel with valve removed. (They've checked mine every time coming out of MEX - never in US) Save the plug to keep dirt out & put the valve in a baggy. Take a small flashlight to do a quick inspectiion before you assemble.

 

[Epinephelus]

NetDoc once bubbled...
I see MOST pony divers eventually calculate that extra 19cf as part of their usuable back gas... not good.

Rick said that this isn't acceptable in his post, NetDoc has said it here in his, and it's so important that I'm saying it again! A pony is NOT "more gas." It is redundant gas. There is a difference.
FWIW, personally I find 30CF to be the minimum size pony that's worth carrying.
E. itajara

 

[MikeFerrara]

Genesis once bubbled...


Uh, let's redo that math a bit, eh?

Remember, you stop at 15', so your average depth during the ascent is 52', not 60'. So let's make that dive a 130' (maximum "recreational depth") to make the math work more-or-less.

Might want to redo the math one more time? The average depth of the ascent to 15 ft is ....

(15 + 120)/2 = 67.5

 

[Genesis]

In any event, the point is that you can make the ascent from 130', but you have little margin. At less-deep depths you have significant margin.

I am curious what the consensus poisition is on ascending more quickly (up to the previous "recommendation" of 60 fpm) and using the gas you save for additional 15' hang time, .vs. making the 30 fpm ascent and having a much shorter hang time at 15'.

Is there any good, hard data to back up one or the other being a "better" choice under duress conditions (e.g. the pony is all the gas you have - so when its empty, you WILL be ascending )

 

[Razorblade]

Genesis once bubbled...


Ok, so we have a 4 minute ascent, requiring the aforementioned 12 cf. We assume you have one minute at 4 ATM to figure out what the heck happened, for a total of 16cf.

Wouldn't the 4 minute ascent from 132' to 16.5' require 11.375 ft^3 of air if one breathes 1 ft^3/min at the surface?
Also, wouldn't it be one minute at 5 ATM absolute?
It still comes out to about the same since both errors practically cancel each other out (I get 16.375 ft^3)

Sorry guys, I was reading the thread, had a little time to kill, so decided to go ahead and be a *beeeeep*.
Now, I've gotta go.
ttyl

 

[Genesis]

error pointed out already

The point is that the 19cf bottle will get you to the surface from 130', but you won't have any "wiggle room" at a 30fpm ascent rate.

At less than 130', it will do so handily.

Since my "typical use" is in the 80-110' range, its a perfect size for me and my intended purposes, with a margin allowing me to use it (and get back up!) from 130' if the need arises.

This is also assuming a SAC of 1.0 under "actual use" conditions, which is a LOT higher than my usual SAC, but its a good idea to plan for such consumption rates, since in a real emergency you're likely to be breathing (quite a bit) faster than normal!

 

[nikyyo]

Geez.....look what I started.

 

[MikeS]

Genesis,

One thing that your model doesn’t address is any time at depth before ascent. Since it is a bailout bottle, there has to be some sort of problem causing you to need it; running out of air due to inattention is simply not expectable. So, you have to sort out the problem, deploy the pony regulator, locate your buddy, signal him that you’re going up, wait for his acknowledgement and then start up. Don’t forget that if there’s a current you should float a surface marker also, which takes additional time. I factored in two minutes.

From 110 feet with a SAC of 1.0 cf minute my model looks like this.

Figure out problem and signal buddy 2 minutes at 110 fsw = 8.7 cu. ft.
Ascend to safety stop (17 fsw)at 30 ft/min = 9.1 cu. ft.
Three minute safety stop at 17 fsw = 4.5 cu. ft.
Ascend to surface at 30 ft/min = 0.7 cu. ft.

Total = 23.0 cu. ft.

At 130 feet, the same model indicates 27.3 cu. ft.

Once in the water, there isn’t much difference between a 19 cf and 30 cf tank. I carry the 30 for the added margin.

Mike