Pony vs larger tank?

[AndyNZ]

Well the guy doesn't have pony bottles in stock (not at the size I was looking for) and told me I should just buy a larger tank.

You've got a whole bunch of decisions to make, but the option of buying a larger tank isn't really one of them.

It is incredibly rare, but let's say that the tank valve burst disk ruptures or the neck o-ring goes through a dive. Your reg is not going to free-flow, but you are going to lose air from your tank pretty quick!

A pony is a good option for a reasonable level of redundancy. But , you've got some choices:

1. Where do you keep it? The two options are really mounting it alongside the main tank, or carrying clipped off to the BCD in some manner. Each has their advntages and disadvantages. I won't go into detail, but for example, if you back mount it you want to think about whether the tank valve is left open, or whether you mount is such that you can reach the valve. Personally, for many reasons, I prefer carrying as a slung tank.

2. Size - how big is big enough? Gut feel is that 13 cu ft (2 litres?) is not big enough, or that you would quickly find that it becomes limiting. A 19 cu ft would be the minimum. A 6 cu ft ascent bottle has it's uses - maybe a paperweight?! You'll probably find the difference between a 19 and a 30 cu ft negligable in terms of carrying it. You'll also find that different brands have different buoyancy characteristics. My Luxfer 30 cu ft tank sits much less unobtrusively than the better halfs Catalina 19 cu fit. At the end of the day, you need to calculate how much gas you need in your worst case ascent - search SB for "gas management" or "SAC" which will help.

 

[muddiver]

A 13 cu ft will get you to the surface from 120'. No, you will not get a safety stop with that, it is an emergency, I'm out of breathing gas ascent to safety, period. Safety stops are for normal dive ascents to allow a little cushion to your off-gassing.

If anyone tells you a 13 cu ft cyl is too small, they are speaking from mis-information, or adding more to the issue than "I need to go to the surface".

 

[Jupiter31]

Really poor advice - sounds like he wanted to sell you what he had in stock vs answering your question

It is a fairly simple point to grasp - however, he answered a question that was not posed - I'm lucky b/c the LDS' I deal with in S. FL are so much better than what i see posted on SB - Starting to understand why some posters are so vehiment about not liking thir LDS and so gung ho about buying on line.

If you want a pony bottle, thats a safety issue and it makes sense - limited sense based on the diving you describe you do, but go for it if you are more comfortable - having a larger tank is great, but its not going to acomplish your original goal-

Also, larger tanks, steel 100's are great - a little more gas capacity, but you get to shed some weight b/c the tanks weigh more - I went to steel 3 years ago and love it

Find another LDS - this guy sounds like a

 

[mikeb663]

If you are looking for redundancy then I would definitely recommend the pony bottle. I take a pony, or doubles, on almost every dive. If you are looking to get longer times then I would go with a larger tank, but the larger tank does in no way replace a redundant air supply. As far as size goes I would go with a 30 or 40 cuft tank, preferably a Luxfer 40. This will give you plenty of gas for ascent and stop if you have an issue. This is also a tank that can grow with you should you gain any interest in moving toward tech diving. Just my 2 cents

 

[OldNSalty]

If you want a pony bottle, thats a safety issue and it makes sense - limited sense based on the diving you describe you do, but go for it if you are more comfortable

Are you suggesting that it isn't really needed? I have struggled with rather I need it or not but I often find myself alone in the ocean. Usually because I am straddled with a tourist diver who hasn't gotten wet in 12 months and this is their first ocean dive. I still struggle with the question.

 

[Jupiter31]

The op states mostly all his diving is semi tropical; 40 - 60 ft - he's not exactly doing tech diving, etc

His liklihood of eq failure is very small - still, redundency is not a bad thing - mosly I was reacting to what I would be very pissed at regarding what described taking place at his LDC

I dive a lot more than the op - and I struggle with the pony or not decision too.

 

[AndyNZ]

A 13 cu ft will get you to the surface from 120'. No, you will not get a safety stop with that, it is an emergency, I'm out of breathing gas ascent to safety, period. Safety stops are for normal dive ascents to allow a little cushion to your off-gassing.

If anyone tells you a 13 cu ft cyl is too small, they are speaking from mis-information, or adding more to the issue than "I need to go to the surface".

I'd like to get to the surface safely and without getting bent. That consitutes a slow, controlled ascent. For a dive that is deeper than 20m, it includes time at the bottom to resolve a problem, deeper stops and a shallow safety stop. 13 cu ft just doesn't cut the mustard.

I can't do imperial calculations, but here it is in metric:

Assuming a stressed breathing rate of 30 litres per minute at a depth of 40m. It is likely to take me a minute to get things under control. In that minute, I will use;

30 * 1 * 5 = 150 litres of gas

Average depth on the ascent is 20m, or 3ATM. Assuming a safe ascent rate of 10m per minute it will take 4 minutes to reach the surface. In that ascent, I will use:

30 * 4 * 3 = 360 litres of gas

Like I say, for me a safe ascent from 40m includes at least a one minute stop at half my maximum depth. I don't like the idea of my faster tissues bubbling away. So I can add another 90 litres of gas.

Finally, a 3 minute stop at 5m. Let's assume I've got my breathing under control at this point and back down to 20 lpm. For my safety stop, I will use:

20 * 3 * 1.5 = 90 litres of gas


So my total gas use is 150 + 360 + 90 + 90 = 690 litres of gas

Using Google to convert from litres to cu ft - that's 24 cu ft, about twice what a 13 cu ft gives you.


Assuming that you feel comfortable enough to make a direct ascent to the surface, without resolving any issues at depth, whilst maintaining a safe ascent rate of 10 m/min, you will use 360 litres - Google gives me 12.71 cu ft. There is absolutely zero margin for error with a 13 cu ft tank.

I have measured my consumption rate in various circumstances, and it varies from around 13 litres per minute to as high as 50 litres per minute. If it were on the higher end of that, with a 13 cu ft it's not getting me to the surface. If I have a 30 cu ft cylinder, I have the option to not include my deeper stop or my safety stop.


The difference of carrying a 13, a 19 or a 30 cu ft tank on a dive is barely noticeable. Larger tanks give you options to work through your ascent safely. Just because it's an emergency, that doesn't mean you have to compromise safety.

YOU may be willing to do that, but to advise someone that they should is really not ethical for a diving professional.

I hope this "misinformation" is of use.

 

[muddiver]

Nope, no use to me. You are using a linear caculation to work a problem that works on a darivation of the change of volume with the change of pressure over time.

Second, I qualified my opinion that it is an "emergency gas source" only to provide egress to the surface.

Safety stops are for people ignoreant of the medical and testing basis of decompression theory. So for an emergency ascent a safety stop is irrellevant. You are just adding conservitive redundatcy to the original posters question and adding bulk to the system that the diver has to lift on the surface to get to and exit the water. How easy it is to sling a 200 cu ft cylinder underwater is also not as important.

For the OP, a pony is just a better way to, as the dreaded PADI agency puts it, CESA to the surface.

And yes I am comfotable enough underwater to make a direct ascent to the surface, at 30 feet per minuite, Oh wait!, 9.144 meters per minuite, safely using a 13 cubic foot (0.368119 cubic meter) cylinder if my main supply of breathing gas fails at approximately 30.480 meters depth.

What is there to sort out at depth if you have no breathing gas? Are you planning a contengency to do a couple of S-drils on the way up?

Sorry, but if you gas consumption varies by 25% than you are not a very confident, comfotable diver and therfore you should not be diving deeper than about 60 feet (18 meters). To do the kind of diving you are refering to a diver must be calm, and thoughtful underwater. Getting excited or stressed easily is not a good trait for an advanced / technical diver.

 

[AndyNZ]

Second, I qualified my opinion that it is an "emergency gas source" only to provide egress to the surface.

Safe egress...

Safety stops are for people ignoreant of the medical and testing basis of decompression theory. So for an emergency ascent a safety stop is irrellevant.

Emphasis on the word "theory". Any dive to 40m has the potential to cause a DCS incident. Given the relatively short NDLs at 40m (what ever "no decompression" is), you're essentially saying it is acceptable to bounce dive to 40m and come straight to the surface. The risk of a Type II incident is much higher under those circumstances.

You are just adding conservitive redundatcy to the original posters question and adding bulk to the system that the diver has to lift on the surface to get to and exit the water. How easy it is to sling a 200 cu ft cylinder underwater is also not as important.

Who said anything about a 200 cu ft cylinder? The difference in bulk between a 13, a 19 and a 30 cu ft cylinder is fairly negligable. Data on luxfer cylinders:


Tank Diameter Height Weight Buoyancy Full Buoyancy Empty
13 4.4 13.1 6.0 - 1.7 - 0.7
19 4.4 18.6 8.2 - 1.4 + 0.1
30 4.9 21.9 11.6 - 1.0 + 1.0


I'd agree that a 30 cu ft is significantly different in size from a 13 cu ft, but a 19 cu ft is not. It also has a marginal swing in buoyancy. Again, the difference from a 30 cu ft to a 19 cu ft is barely noticeable.

For the OP, a pony is just a better way to, as the dreaded PADI agency puts it, CESA to the surface.

"dreaded PADI agency"....? Leave your prejudices at home.

I'm not disputing that a pony is a preferable option to a CESA. I'm merely saying that it is unethical for you to say that a 13 cu ft is enough to get the OP to the surface - when you have no idea what their gas consuption rate is. It simply may not be big enough.

Let's take an example. I teach the use of pony bottles on a course, and recently I had a student (name changed), Dave.

Dave's average SAC was 30 lpm. On a stress test his SAC hit 60 lpm. Let's assume that he read your post that said "13 cu ft is enough to get you to the surface from 40m". I wonder how fast his ascent rate would be to get him there.....?

Average depth = 20m, 3ATM... so approx 180 lpm average. A 13 cu ft tank holds roughly 360 litres. That gives him 2 minutes to ascend, at a rate of 20m/min.

Increasing the size of the tank to 19 cu ft, or a total of around 540 liters. That gives him 3 minutes to ascend, at a rate of 13m/min. Still too fast for my liking, but much better than 20m/min.

And yes I am comfotable enough underwater to make a direct ascent to the surface, at 30 feet per minuite, Oh wait!, 9.144 meters per minuite, safely using a 13 cubic foot (0.368119 cubic meter) cylinder if my main supply of breathing gas fails at approximately 30.480 meters depth.

And the OP's SAC rate is the same as yours.......?

What is there to sort out at depth if you have no breathing gas?

Plenty, in some circumstances. Orientation, for example. Or maybe you can't ascend directly to the surface - one of my favourite wreck dives here is in the middle of a shipping lane on route to a major port. I'm not sure I'd want to make a direct ascent to the surface with out the option to stop at some stage and check for boat traffic.

Sorry, but if you gas consumption varies by 25% than you are not a very confident, comfotable diver and therfore you should not be diving deeper than about 60 feet (18 meters). To do the kind of diving you are refering to a diver must be calm, and thoughtful underwater. Getting excited or stressed easily is not a good trait for an advanced / technical diver.

Your arrogance is amazing. Have ever thought that maybe gas consupmtion varies with things other than stress? Something as simple as swimming against a current, for example?

Also, don't you find it odd that you are saying that a diver needs to be "calm and thoughtful", yet you are saying that they should carry the smallest possible air source that gives them no option but to make a direct ascent to the surface, with no option to stop and think? You're simply adding a stressor that doesn't need to be there. Do you also advocate not having an SPG on the pony as well?

Just because a 13 cu ft is the right option for you, doesn't mean it's the right option for everyone - give people the tools to work out what is right for them, rather than forcing your opinions on them.

 

[Doc Harry]

I like the idea of a stage bottle instead of a pony.

A pony is favored by some because it's out of the way. This disadvantage is that you can't hand a pony to someone else. That's why I prefer a stage bottle for extra gas. (Actually, I prefer to dive with doubles if I need or want more gas.)

A 13 cubic-foot may get you to the surface from 100 feet if you have a very low SAC and aren't breathing heavily on the way up and don't have to spend more than few seconds at depth to deal with a problem and you don't need a safety stop and you don't mind getting bent.

If you do the math, 20 cubic feet is a more comfortable minimum number for an egess from 100 feet, assuming you don't a buddy at your side to help out.