How big of a tank as a Pony????

[Snowbear]

Genesis - I agree. I only sling stages any more. I stopped using a pony and learned instead to carry the reserve in my back gas. I usually go ahead and use up that reserve in the 30-0' range at the end of my dives, though

 

[Snowbear]

When I do dive with my 13cf Pony it is mounted upside down to my single 80 or 104 so I can reach the valve. I also have the second stage connected by a 7' hose. If I ever needed to share air it would make it much easier

7' hose on your pony bottle? How do you stow all that hose?

 

[MikeS]

Here is my philosophy for whatever it’s worth.

In no-decompression no-overhead diving, the only time a bail out bottle (pony) makes sense is from a depth where a controlled swimming ascent is not practical. So for me below 60 feet is the point where I consider a pony practical.

With proper training doubles are a better redundancy solution but not practical for everyone.

This is apparently the only other person here who knows how to do the math properly.

Just as important if not more so, than doing the math properly is the assumptions you use to model air consumption.

You need to make assumptions for the following in order to build a model.

1. Time at depth to identify and resolve problem and start ascent.
2. Maximum depth.
3. Air consumption rate.
4. Ascent rate.
5. Safety stops if any (depth and duration).

For example, the model I used to size my pony was the following.

1. One minute at depth to resolve problem and if applicable shoot a surface marker.
2. 120 feet
3. SAC rate of 1.2 cubic feet a minute.
4. Ascent rate of 30 feet per minute.
5. Three minute safety stop at 15 feet.

So the scenario is:
One minute at 120 feet, with a SAC of 1.2 uses 5.6 cubic feet.
Ascent from 120 to 15 feet at 30 ft/min with a SAC of 1.2 uses 11.6 cubic feet
Three minutes at 15 feet with a SAC of 1.2 uses 5.2 cubic feet
Ascent from 15 feet to the surface at 30 ft/min with a SAC of 1.2 uses 0.7 cubic feet
For a total of 23 cubic feet.

That’s how I ended up with a 30 cubic feet pony. Stage slung you will not even notice it’s there once in the water.

 

[brian_dixon]

One minute at 120 feet, with a SAC of 1.2 uses 5.6 cubic feet.
Ascent from 120 to 15 feet at 30 ft/min with a SAC of 1.2 uses 11.6 cubic feet
Three minutes at 15 feet with a SAC of 1.2 uses 5.2 cubic feet
Ascent from 15 feet to the surface at 30 ft/min with a SAC of 1.2 uses 0.7 cubic feet
For a total of 23 cubic feet.

How do you calculate the ft3 on the ascents???

 

[DA Aquamaster]

The issue with a 30 or 40 is that your TOTAL weight is now quite close to that of light doubles (e.g. 72s), ESPECIALLY if you have something like an HP120 for a primary, and you can safely do two NSL dives on a set of 72s if your RMV is even halfway decent.

So from my point of view at the point where a 19 is not enough, you may as well dive light doubles. If it is enough, cool. If you're tooling around in the 30' range forget about either and just dive a single with your pony being "the atmosphere".

That was pretty much the point in going with a larger pony that will grow with you. When I found that a 15 was not enough pony for the diving I did, I went with a 30 cu ft pony but then quickly realized that:

1) There are very few things even a large 30 or 40 cu ft pony can do that a set of doubles can't do better because the depth range where any pony is both truly needed and truly useful is fairly narrow. And,

2) The lead that can be removed with the addition of a second negatively bouyant steel 72 and a set of bands offsets much of the difference in weight between a large pony and the comparatively light 2nd steel 72 meaning you can switch to a set of double steel 72's with less than a 10 lb increase in total equipment weight.

And at that point a 6, 13 or 19 cu ft pony will become an expensive doorstop while a 30 or 40 cu ft pony filled with Nitrox 50 becomes very handy for accellerated deco which in turn allows more bottom time while still maintaining a 1/3rd reserve.

 

[Genesis]

Well in that case just buy it as a stage instead of a pony in the first place

If that's the route you're on, then just get the doubles now and forget the pony entirely, then buy the deco bottle when you have a use for it.

But for the guy who doesn't want to dive doubles for whatever reason, now or ever, the 19 has enough gas and is almost transparent underwater. The 40 is a good tank too (I have one) but its definitely "there", even when slung like a stage.

I posted that picture earlier in this thread; you have to look REAL CLOSLEY to see the 19cf slung in there. Its there though - trust me. For single-tank spearfishing where the extra maneuverabiltiy is more important to me than the doubles, down to 100' or so, I still like the big single and a 19cf pony.

 

[cdennyb]

I use a 13 for my argon bottle, and a 30 for my deco or bailout bottle(s). Never can have enough gas! Maybe not for you, maybe for both of you!
db

 

[perpet1]

I'm no genius, but the way I see it, at 99' there's only 3.25 cu. ft of gas available to you in your 13cu ft bottle. (13 / (1+99/33)) = 3.25

that is less than 5.

That is an interesting way to look at it and if the tank were a balloon it would be 100% correct.

The typical way to approach this is to assume there is the volume in the tank because after all it is a tank and not a balloon (in this case we assume there is 13cf at 3000psi). Then you depth correct the consumption rate to find out how long the tank will last. Using your example would work except you would not depth correct the consumption rate. The end result would be the same time to empty a tank.

But you are correct in that is you released the contents of the tank at depth into a bag it would fill bag with 3.25cf with air.

One slight math problem...... 13/(1+(99/33)) The way you had it written the answer would have been 4.29 which would have been incorrect. But you had the correct answer so I am thinking this was just a typo.

Pete

 

[Rick Inman]

I have managed to get a hold of a nice extra reg and want use it for a Pony tank set up. I have noticed pony tanks range between 6 cubic feet and 40 cubic feet. At what point does the pain in the *** of carrying an extra tank along equal the benifit of carrying one along? I usually dive in warm water, between 70-100 fsw. I am not planning on using it to dive solo at those depths, but rather as a reduntant back up. Would the volume differ any? Thanks for the input.

For your viewing pleasure here are some pics i took on my AOW dive at the Speigel Grove.
Keep in mind I am no National Geographic Photogropher :59:

Brian,
Loved your pictures! Congrats on your OW in Jan!
I dive with a pony myself (which some would say is doing it wrong), and I solo dive as well. But in my opinion, I would not recomend someone who was just certified to use a pony bottle yet - if at all. Even though you say you will not solo, it is EASY to get a false sense of securty - and I mean FALSE - with the pony. You should get a couple hundred dives under your belt first, focusing on basic skills, buddy skills, task management, etc.
Just my 2 cents!

 

[Big-t-2538]

The typical way to approach this is to assume there is the volume in the tank because after all it is a tank and not a balloon (in this case we assume there is 13cf at 3000psi). Then you depth correct the consumption rate to find out how long the tank will last.

Agreed...there's still 13cu.ft of gas under pressure in the tank, but I find it easier to reduce the available volume you're going to consume than to mess around with the SAC.