Optimal Pony Bottle Size for Failure at 100ft?

[SlugLife]

Back mounted pony bottles are a line trap and make the rig much more prone to entanglement in line while underwater. It just seems to more than double the opportunity for entanglement since line can get down between the bottles.

Any back-mounted bottle can be a line trap. Adding a pony would only slightly increase that, not double. Most cables that would get caught on your pony, would also get caught on your primary. Getting caught between the bottles is perhaps not impossible, but it seems more likely you'd get entangled on the valve or regulators.

If either tank was entangled, you'd have an extra 13cu to 40cu of air to deal with the entanglement.

 

[JonnieB]

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.

Calculations:

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.

A few assumptions that struck me as odd:

# 1 Why do you think it will take you 2-3 minutes to compose yourself after your equipment failure at 100 feet? Immediately you'll switch to your pony cylinder and begin your accent. I would guess at most 20-30 seconds after the gas switch to check your computer and get oriented to depth and situational awareness and start your accent. So less gas needed.

# 4 If I read you correctly, you're allowing five minutes to ascend 15 feet? Again this seems excessive to me...one minute is plenty of time. Again, less gas needed.

Finally, I'd say 10 percent is plenty of reserve gas in a pony bottle so as to not breathe it "dry." Again, leaving you with more gas and a smaller necessary pony bottle.

I regularly dive solo to similar depths for recreational diving and by my calculations, my 13 cu ft pony cylinder is more than adequate.

 

[grantmac]

As a newish rec diver without someone to dive with when I travel, I'm honesty curious what the better alternative is for me than using a pony for deeper rec dives? I'm not a pony zealot, just someone who's tried to think logically about the problem and a 19 cuft pony seems to be the best solution (I did the math about my estimated air usage in case of an emergency and 19 cuft worked - just as it did for @scubadada )

Honestly just learn sidemount. It's by far the most flexible travel arrangement. Especially a minimalist one like the Xdeep Classic.
Can do everything from single tank to doubles and stages with one wing.

 

[NW Dive Dawg]

how is it held on -looks like a couple of screws - can you change to a new al80 bottle for a second dive without hassles- you keep the two second stages on the primary rig?

For the Quick Draw system, it's just a simple pull of two pins. Half the bracket stays with the pony and the other half stays on the BC cam band.......so very simple and quick to swap out the primary tank.

Here's a better pic showing how the bracket basically works. The forward pin acts as a "hinge" pin and the aft pin locks it in. I personally always route the "tether" between the two pins to go under the bracket so that there is far less chance of a snag and basically zero chance of either pin being accidentally or inadvertently pulled during a dive. Hope that makes sense.

When I swap my primary tank on the boat or on the shore.....etc, I just disengage the bracket and set the pony slightly aside without removing the pony first stage. Then I remove my primary first stage, swap the tank and then reinstall my primary first. It literally adds only about 30 seconds to the normal primary tank swapping process.

 

[34109411]

You GUE guys need to come to terms that people are going to continue to use pony bottles for redundancy. It is a perfectly valid configuration for recreational diving. Just let it go.

depending of the type of diving you do there is no specific need to use double. Using a pony bottle is perfectly find if your respect the basic training principale and common sens.

 

[34109411]

I had an interesting experience solo diving a couple of weeks ago: I had two failures within a minute of each other. It's the first dive I've had with any kind of stressful situation since I've been able to monitor and log my SAC rate / RMV. My drysuit neck seal tore and my bladder failed (not MY bladder - you know what I mean).

Long story short - my gas consumption averaged 2.5 times normal for about 90 seconds, then settled back down over the next 2 minutes. Some of that was pumping gas into the ocean before I realized the bladder wasn't holding it, but still, gas consumption is gas consumption.

When doing calculations for pony volume I had been assuming that my consumption rate would double under stress, which I thought was conservative enough. Now I realize it could probably easily triple in a real, serious emergency situation.

Theory is great, calculations are useful, reality is the teacher.

 

[rjack321]

What kind of problems does having redundant air create? How common and severe is each problem? And can the problem be easily avoided?

The problems I'm aware of are:

1. The diver uses the wrong regulator
2. The diver cannot find their other regulator.
3. The extra bottle and regs are extra weight/clutter.
4. Having redundant air may encourage a diver to engage in dangerous behavior.
5. The pony bottle may confuse an inexperienced dive-buddy needing air.
6. Anything not GUE is a death trap. Don't get me started ranting about those insane sidemount divers with dangling tanks! Magnets how do they work?

...and....

1. To my knowledge uncommon, not severe (just switch to the other), and easily avoided with practice.
2. This is no different than the dangling octo problem in standard configuration. Easily avoided.
3. Outside penetration diving (perhaps usually not wise solo), I consider this an excuse by lazy divers.
4. An idiot behind the diving-mask, is an idiot behind the diving mask. I've called spare-air's (3cu or less) dangerous, but 6cu and up is almost always better than nothing. It's perhaps a strawman to talk about the spare-air diver, in threads where someone is asking what size of pony they need.
5. Not severe, and we're talking about solo.
6. ...

Most of the above is remedied by some very simple and basic training, practice, and discipline.

1) More than one person has started a dive on their pony and run OOA with varying degrees of drama/downstream problems with that (up to and including drowning)
2) More than a few people have switched to a pony and run that OOA either because it wasn't as full as they planned, their consumption wasn't as planned, or they lingered at the bottom etc longer than planned
3) OOA divers have been donated a pony reg by a buddy and run OOA or it was never turned on in the first place.

All of these (and more) have led to AGE injuries or drowning fatalities in the past. There are a few pony related near miss incidents reported here on SB too. So no, its not just super very simple backup extra air - there is no such thing in scuba and to imply there is does a disservice to those who have been injured or died even with a pony and sometimes because of it.

Part of the problem with ponies in general is the lack of standardization both size and configuration: where is the 2nd stowed, is the valve on/off, is the valve even accessible, how big is it, is it full, is it staying full or leaking out, what consumption rate is used for its sizing, what gas is in it, is it donated or not, is the regulator going to be oriented, purged and working for the intended user.

I dislike ponies mostly because there are nearly as many configurations as there are pony users. As a buddy I try my hardest to be predictable, I put my light beam in a in predictable spot, I position my body in predictable places, I use equipment which has predictable characteristics, I have predictable resources to contribute to resolving problems. Ponies, above all else, are highly unpredictable in their configuration, sizing and utility.

 

[34109411]

1) More than one person has started a dive on their pony and run OOA with varying degrees of drama/downstream problems with that (up to and including drowning)
2) More than a few people have switched to a pony and run that OOA either because it wasn't as full as they planned, their consumption wasn't as planned, or they lingered at the bottom etc longer than planned
3) OOA divers have been donated a pony reg by a buddy and run OOA or it was never turned on in the first place.

All of these (and more) have led to AGE injuries or drowning fatalities in the past. There are a few pony related near miss incidents reported here on SB too. So no, its not just super very simple backup extra air - there is no such thing in scuba and to imply there is does a disservice to those who have been injured or died even with a pony and sometimes because of it.

Part of the problem with ponies in general is the lack of standardization both size and configuration: where is the 2nd stowed, is the valve on/off, is the valve even accessible, how big is it, is it full, is it staying full or leaking out, what consumption rate is used for its sizing, what gas is in it, is it donated or not, is the regulator going to be oriented, purged and working for the intended user.

I dislike ponies mostly because there are nearly as many configurations as there are pony users. As a buddy I try my hardest to be predictable, I put my light beam in a in predictable spot, I position my body in predictable places, I use equipment which has predictable characteristics, I have predictable resources to contribute to resolving problems. Ponies, above all else, are highly unpredictable in their configuration, sizing and utilityNot severe, and we're talking about solo.

1. Not severe, and we're talking about solo. Sorry for misunderstanding of english what do you mean exactly by that phrase, i do know what "solo diving is" just clarify my mind for me.

 

[SlugLife]

1) More than one person has started a dive on their pony and run OOA with varying degrees of drama/downstream problems with that (up to and including drowning)
2) More than a few people have switched to a pony and run that OOA either because it wasn't as full as they planned, their consumption wasn't as planned, or they lingered at the bottom etc longer than planned
3) OOA divers have been donated a pony reg by a buddy and run OOA or it was never turned on in the first place.

I don't see how a person is in more danger, having a pony versus not having one, and especially if used properly.

1) The solution is dead simple: switch regulators. That's not a problem for anyone with very basic training or practice with their pony bottle, it's little different from using an octo in your Open Water class. It doesn't do anyone any good to have a piece of safety equipment, that they can't use in an emergency. Ideally, one wants to switch from their main air to the pony, but it also works the other way.

This one qualifies as an example increased danger in one area, but goes back to being safer once we assume some basic competence, training, or practice.

2) A person who would have ran OOA once, runs OOA twice? I can't see how no pony would have made them safer.

3) This is a solo thread. If the OOA diver had their own pony, would they have been safer?

 

[jborg]

1) The solution is dead simple: switch regulators. That's not a problem for anyone with very basic training or practice with their pony bottle, it's little different from using an octo in your Open Water class.

I mean, it seems that way. But the problem is when they do as they trained and try to switch to the pony regulator, but it's nowhere to be found. Likewise, the other regulator that they should have been breathing is going to be ... where? This is not a long hose setup so it won't be clipped off somewhere predictable or hanging on a necklace, the reason they're not breathing it is that they messed something up earlier and it might as well be dangling somewhere behind them. Panic in 3, 2, 1, ...