What size tank for a pony??

[Scoobers]

So I am getting mixed opinions from my local dive club members and LDS. I have been told 3 different sizes would make a good pony for me. First a little background, I am 6'2" and about 270lbs. My sac rate at 50ft in cold dark water at a mild paced kick was .58 cuft/min. I have been recommend a 19 cuft which seems kinda on the small side for me IMO... Next is the 30 cuft which seems like a decent option and would provide me enough time to do a stop and get to the surface. Lastly I have been recommended a 40 cuft tank because I am a bigger guy and when stressed can suck down some air. I am planning to sling this tank on my right side. So before I go out and buy a tank I am looking for some more opinions. I would like your reasoning behind why you recommend a certain size. I am also thinking in the future at some point I may want to get into tech but am currently just a recreational diver. Thanks guys.

 

[Rhone Man]

Your primary choice should be based upon how much gas you think you are ever likely to need. Unless you are going particularly deep or into overheads, a 19 ought to get you safely back to the surface. But your SAC rate is quite high, so you might want the extra comfort of a 30.

It may make a difference how you plan carry it. I like to have it behind me, affixed to the main tank, so that makes a 19 more convenient. If you are slinging it in front of you, perversely a 19 can feel a bit small relative to where your clips naturally lie, and you may prefer at least a 30. Nothing definitive, but another thought to throw into the mix.

 

[halocline]

If you think tech diving is in your future, you might want to buy a bottle now that you can use as a stage later on. That's bigger than a pony.

If you do a search you'll find that this is one of the often debated topics on SB. My personal opinion is that if you "need" more than 19cft as a bailout bottle in recreational diving, something's wrong with your approach. Tech, deep, deco, those are totally different situations.

 

[McCainiac]

Go for the 40cf so you have more options in the future with it.....

 

[Scott L]

19cf...

 

[seaducer]

I think the more important question is, how do you get your pony to breathe off a regulator?

 

[MXGratefulDiver]

So I am getting mixed opinions from my local dive club members and LDS. I have been told 3 different sizes would make a good pony for me. First a little background, I am 6'2" and about 270lbs. My sac rate at 50ft in cold dark water at a mild paced kick was .58 cuft/min. I have been recommend a 19 cuft which seems kinda on the small side for me IMO... Next is the 30 cuft which seems like a decent option and would provide me enough time to do a stop and get to the surface. Lastly I have been recommended a 40 cuft tank because I am a bigger guy and when stressed can suck down some air. I am planning to sling this tank on my right side. So before I go out and buy a tank I am looking for some more opinions. I would like your reasoning behind why you recommend a certain size. I am also thinking in the future at some point I may want to get into tech but am currently just a recreational diver. Thanks guys.

... and you thought you wouldn't get mixed opinions here?

Seriously (for a moment) ... the answer to most questions concerning SCUBA are going to be "it depends" ... with a bunch of other things to think about. That's just the nature of the activity.

First off, a .58 cfm consumption rate ain't bad for someone your size and experience level. And for typical recreational profiles, a 19 cf bottle will be all you need.

But whether that's the best choice for you depends on considerations of ...

1) how you want to mount it. For back-mount, anything bigger than a 19 cf bottle are going to impose some issues due to size and weight of the bottle. For slinging, a 30 or 40 cf cylinder are a better choice.

2) how you plan to use it. A slung bottle gives you some options you won't have with a back-mounted bottle ... such as the ability to pass it off to another diver in the event of an OOA. On the other hand, although a slung bottle virtually "disappears" if mounted properly, if you're diving in kelp frequently or tend to swim very close to the bottom, it has its own annoyances.

3) future goals ... as others have mentioned, if you plan to move into tech someday, you'll already have a handy deco bottle.

Personally, I prefer a slung 40 ... but it's partly because I'm used to carrying one anyway, and I already owned four of them. But I like the option to be able to manipulate the valve easily, readily see my pressure gauge, and hand it off in an emergency. Whether those are considerations that are important to you depends really on what kinds of dives you're doing ... and with whom.

... Bob (Grateful Diver)

 

[Dr Wu]

You don't need a pony. Just practice diving for a while.

From your post you are already confused about a few things. That's understandable of course.

It's just that a pony is not a substitute for proper knowledge and technique.

Work on gas planning and developing good buddy skills. At this point a pony will be nothing more than a distraction.

 

[AndyNZ]

Look on the bright side, at least no one suggested a 13 cu ft......!

You can do the maths yourself - pick the deepest depth you are ever going to dive, pick what you think would be a safe ascent rate and decide on how stressed you are likely to be.

An example:

Max depth = 40m (130ft)
Ascent rate = 10m/min (30ft/min)

So it should take four minutes to reach the surface. Add in one minute at depth to sort your stuff out.

I work on an average stressed RMV of 30 litres per minute... it will peak higher than that, but you should be able to get your breathing back under control at shallower depths. I'd use 30lpm for the ascent calculation, and a peak of 40 lpm for depth.

So in one minute at depth I will breathe: 40 (RMV) * 1 (time) * 5 (D in ATM) = 200 litres

For the ascent, average depth = 20m = 3ATM, so I would use 30 (RMV) * 4 (time) * 3 (D in ATM) = 360 litres

Total gas required for a direct ascent to the surface is 560 litres, assuming a fill pressure of 200 bar that's a 1.8 litre tank (roughly 13 cu ft).

In this case, a 13 cu ft tank will just about get you to the surface provided nothing goes wrong, and you will be sucking hard to get the last breaths on the way up. There is zero room for error.

I prefer to add in a margin of safety. So I will add in a one minute deep stop at half depth (30 * 1 * 3 = 60 litres of gas) and a three minute safety stop at 5m (30 * 3 * 1.5 = 135 litres of gas), which brings my total gas requirement to 555 litres, or a 2.8 litre tank (19 cu ft). That means you can play tunes on the ascent rate and the stops to get you to the surface.

Because I am also using a pony when teaching and diving with other people, I also normally factor in an extra delay - which is why I personally use a 30 cu ft.

The thing to remember is that there is no single "one circumstance" that a pny will be used in - you need to think of the variation that may occur. If in doubt, buy larger than you think.

Both 30 and 40 cu ft tanks provide you with some growth potential.

My luxfer 30 cu ft sits and carries much nicer than a 19 cu ft catalina - so size isn't always a bad thing.

And just remember, this is all opinion! Plenty of people will tell you what they think - but you need to be comfortable with your choice.

 

[deepwater]

Today is a strange day, I find myself agreeing with Dr. Wu. Practice diving, gain knowledge, and perfect your technique. After you have completed all this then sling a 19cf pony in shallow/non working dives and a bigger one for the deeper or harder dives.