Using a pony bottle (or bail-out bottle) is a safety choice scuba divers can decide to employ. I personally use two different bottles depending on the depth of dive I am planning.
A friend of mine put together the attached Pony Bottle Gas Use Calculator which is a spread sheet you can download to compute how large a pony bottle you might want to use for your typical dive plans whether they be recreational (above 130 feet) or technical (below 130 feet).
The example dive I have plugged in there is to 200 feet with hypothetical safety stop at 15 feet for 8 minutes. In that scenario a 19 cube pony would work with little margin for error. But, please play with the calculator by changing the various assumptions.
Here is my friend's write up about pony bottle use:
Looking up at the silver, shiny, mirror-like bottom of the atmosphere from 100 feet under water and realizing I just exhaled my last breath of air is something I have visualized happening to me. What would I do? They say drowning is a relatively pleasant way to die compared to other avenues. Would I panic and expend every last bit of energy in my body in a mad rush trying to swim up? At what point would the uncontrollable reflex to breathe in the water overcome my willpower to avoid flooding my lungs? These are some unwanted thoughts that pop into my head once in a while. However, my mind is eased somewhat by knowing I have a “bail-out system” in place.
Do you scuba dive deep enough to need a pony bottle? I do. So why is it that I see so many spearfishers without one? I’ve not researched any statistics on this, but from personal observations my educated guess is that probably less than 30% of southern U.S. divers use them. I believe if I did a poll of recreational northern U.S. wreck divers who commonly dive in 80 to 110 feet of water, a pony bottle (or doubles) is pretty much considered standard equipment. Why has having an independent redundant system not become common practice everywhere?
Based on many years of scuba diving experience, which unfortunately includes witnessing more than one scuba diving death, seeing many close calls and being present when scuba dives have resulted in debilitating lifelong injuries, I can honestly say that diving scares me a little. I dive for fun. Carrying a pony bottle makes me feel a lot more comfortable and I hope it
improves my safety too.
If a diver has been well trained, uses good judgment and maintains the equipment, the likelihood of really needing a pony bottle is not very high. Personally, I’ve never really needed my pony bottle. However, I’ve not yet used the airbag in my car either. Hopefully the air bag and pony bottle will continue to be superfluous.
I have tried to convince many other divers to take a pony bottle and I’ve heard an awful lot of excuses for not getting one. Let’s address some of the common misconceptions and arguments against pony bottles. It is my sincere hope that you will be carrying a pony bottle if you ever need it.
The first issue is probably the idea that “I can make it up if anything happens, I don’t dive that deep and besides, I will not run out of air.” We all know that spearfishers can get stressed and out of breath during a dive. Of course, this is not supposed to happen, but I think some of us will skip breath a little to try to get within range and get the shot off. I know that I find myself in situations where I’m swimming high off the bottom trying to breathe as little as possible in my attempt to get within range of a spooked fish. The entire time, I’m slowly building an oxygen debt and then when I finally do make the shot, the fish will sometimes take off with a shaft. Then I find myself sprinting after it and breathing harder than I should. (If pushed to the extreme, this type of behavior will surely result in a CO2 headache and may result in much worse consequences). My point is that spearfishers are probably more prone to over-exertion than other types of recreational divers and if you convince yourself that you will never get your heart rate up or be out of breath on the bottom, you are not being realistic.
If a diver should experience a complete failure in his scuba unit under these conditions, the surface may be very far away. I’ve heard experienced divers argue that if you can freedive to depth X, you can surely make an emergency ascent from a depth of 2X. I’m not sure what the basis for this argument is, but it is ridiculous in my experience. Trying to equate the swimming ability (or oxygen consumption) of a scuba diver who is out of breath and has accumulated an oxygen debt to a freediver who is much more streamlined and has rested adequately and “breathed-up” before a dive is nonsensical. I can comfortably function without breathing (i,e., hold my breath) for about a minute if I’m rested, however trying to hold my breath for 15 seconds while jogging or swimming laps is difficult. If a scuba diver should experience a complete loss of air after chasing down a fish, I doubt that consciousness would be maintained for even a minute. Having a pony bottle within easy reach, can turn a potentially tragic situation into a minor inconvenience.
Let’s look at another typical situation, where you experience a total failure at a depth of 100 feet or so. Let’s assume you are not out of breath and are just nearing the end of the third or fourth dive of the day. Should you perform an emergency swimming and/or buoyant ascent, the ascent rate required to maintain consciousness might easily be 100 feet per minute or
more. If you forget to breathe out in the upward frenzy, you could risk having an air embolism.
Even if you should reach the surface, the likelihood of being bent like a pretzel is high. There is an increasing body of knowledge indicating that in order to avoid decompression sickness, deep stops and/or a slow ascent is necessary. It is a mystery to me as to how so many divers can convince themselves that should any of the above situations occur, that they will safely reach the surface without a pony bottle.
Maybe some of the pony-less scuba spearfishers are assuming that their buddy will be there to bail them out of such situations. It has been my experience that spearfishers (especially if the fish are plentiful) are some of the worst buddy divers in the sea. For diving to be fun, I need to feel confident that should I blow a hose while extracting a fi sh or lobster from under a ledge, I have an adequate back-up system and it has nothing to do with another diver.
I’ve heard other complaints about pony bottles along the lines of: “the diver will become dependent on the pony and will push the limits, fail to monitor the pressure gauge and is therefore more likely to run out of air in his primary tank.” I would agree that anyone who uses a pony bottle to deliberately extend their bottom time is less than responsible. My advice is that every scuba diver should dive as if he has no pony bottle. After all, do you intentionally drive through red lights while quietly mumbling about the safety of side-impact airbags?
Another common complaint is that the pony is too big, too heavy and has too much drag; especially for spearfishing in “only” 75 or 100 feet of water. I’ve seen new divers buy a 30 or even 40 cu-ft pony bottle, and then choose to leave it on the boat or at home because it is “too damn heavy.” If you are normally doing no-deco dives (or decompression of less than a few minutes) you may not need a big pony. An aluminum 13 cu ft pony bottle is not very cumbersome and is probably adequate for a diver to initiate an immediate ascent from a depth of 130 ft and complete a few minutes of a safety stop.
Some divers are beginning to use the smaller 6 cu-ft pony bottles. Certainly the weight and drag of this set-up is negligible and even this tiny bottle should get you from a depth of 100 feet to the surface at a reasonable ascent rate (although there may not be much air left for a safety stop). But, having any pony is better than no pony at all.
Commonly available aluminum pony bottles include the 6, 13, 14, 19, 30 and 40 cubic feet capacities.
Attached to this post is a downloadable and very simple excel spreadsheet which may help you to determine your gas consumption during an ascent from various depths. You do need to know your Surface Air Consumption (SAC) Rate in volume (cu-ft per minute breathing rate at the surface) and there is a calculator for that on the spreadsheet. The NOAA Diving Manual shows typical SAC Rates as follows in cubic feet per minute: Slow swimming = .60; Moderate swimming =.92; Heavy swimming =1.2; and Severe swimming = 1.9. Imagine that!
Severe swimming can use almost 2 cubic feet per minute, so size matters.
The appropriate rigging of a pony bottle is another topic entirely. I personally dive with the pony mounted behind me and attached to the primary tank. I have the tank valve open and the bottle is filled with air. Some people advocate filling the pony with nitrox, but I would never recommend filling the pony with a mix that is too rich for the maximum planned depth. Using a pony as a stage bottle to perform accelerated deco, negates the primary function of redundancy.
In conclusion, take some time to think about the way you dive. Visualize an out-of-air experience for yourself and figure out how you will respond. Do you really want to increase the risk of an embolism, decompression sickness or worse, just because you were too cheap to slap down $300 to $500? How much is your life worth? Choose the right size pony, and don’t leave it on the boat! And, above all, don’t think it will never happen to me.
Attached Files
Pony Bottle Gas Use Calculator.xls (37.5 KB)