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Some fun facts about the “pony bottle”

With the rising popularity of sidemount diving, semi-closed and fully closed-circuit rebreathers, and of course the ubiquitous popularity of traditional North Florida Cave diver’s kits (doubles, manifold, backplate and wing), it’s sometimes easy to forget that the majority of divers still manage to have plenty of fun underwater wearing a single cylinder! A single cylinder is simple, comparatively light-weight, easy to set-up and operate and is without a doubt the most common kit configuration among scuba divers around the globe. But as popular as it is, a single cylinder does have one huge drawback, and a growing number of recreational sport divers recognize the short-coming and have opted to do something about it.

Chances are good that if you are a graduate from an SDI Solo Diver program, or if you came up through the University of Hard-Knocks, you probably already know that one huge drawback is that the diver has very limited options when Murphy tags along as a dive buddy. For example, with only one regulator first stage, the only backup life-support system is your buddy’s octo. A massive free-flow really gives little alternative but to share air and get outta Dodge.

Options are even more limited if your buddy is way over there not paying attention to anything but the critter in his viewfinder. A free-flowing regulator can empty a freshly filled cylinder in minutes, and the deeper you go, the faster it drains. Swapping regs and heading to the surface may be the only course of action open to you… unless you count reaching BEHIND your head and feathering the valve on your single tank; turning off your air to fix a free-flow is definitely not something you’d want to try as an emergency ad hoc drill anyway. The truth is that without pool practice and, at the very least, a donated octopus (backup reg) attached to your buddy’s tank in your mouth, a sport diver should never turn off his gas.

The simple alternative is to carry a redundant gas source, and the most functional and practical for the average single-tank diver is a “pony bottle.”

Time for a not-so-simple definition. Just about everyone who has lounged around the aft-decks of dive boats for a season or two will have heard the term Pony Bottle to describe a variety of small scuba cylinders – all a sort of perfect copy of a full-sized cylinder but looking as though they were put through a hot wash and dry cycle and shrunk – and used for a variety of tasks.

Other names for these mini-cylinders include sling bottles, stage bottles, buddy bottles and a half-dozen or so more equally descriptive names. As with so much that has to do with scuba (for example, what IS the definitive definition of technical diving, these days?) there are few unbendable rules when it comes to words and phrases describing pieces of dive gear. A classic example is a pony bottle. I like to tell people that it can only be used for a small cylinder used as a backup air source… exactly what we are talking about here. Of course, that is not absolutely true, but between us, let’s make it so.

Now let’s assume that we have decided that having a backup source of gas is a good plan and that the most practical way for us to carry that gas is to use a pony bottle; there are three more questions we need to answer.

The first is, “how much backup gas is enough?“

Well, the short answer is, “enough to get us back to the surface.” But how many litres or cubic feet is enough? Let’s do some basic calculations using an average consumption rate and an ascent speed that will keep our personal dive computers happy as clams. Let’s also pick a depth that is on the fringe of recreational sport diving: 40 metres or about 130 feet.

We start with a gas consumption rate of 15 litres/0.5 cubic feet per minute. (By the way, the imperial and metric measures used in this example are NOT a direct or exact conversion. Close, but rounded for convenience). Let’s also say that if we have to “bailout” to our pony bottle at depth, we are going to be a bit freaked out – Murphy does that to divers – and therefore our consumption rate is going to be doubled. So we can use 30 litres or one cubic foot per minute.

Our depth has a direct relationship to the density of the gas we breathe so at 5 bar/ata (40 meters or 130 feet) we will use about 150 litres or five cubic feet per minute!

Also, let’s make some allowance for fiddling around at depth for a couple of minutes before we start heading back to the surface. How many minutes exactly is tough to guess, but it would be a mistake to think that we would start to head up immediately we detected a problem and bailed out to our pony. It would be nice to think that’s the way things would unfold but the truth is it takes time to get our buddy’s attention, get ourselves calmed down, sort out our gear and start the swim home. Initially, let’s calculate that we stay at depth for three minutes.

Three minutes at our depth and stress adjusted consumption rate requires 450 litres or 15 cubic feet of gas. (Wow that immediately rules out one of those Barbie-sized tanks. doesn’t it?)

Now we can look at the ascent itself. In an emergency, the hard-wired, natural response that kicks in is the aptly-named flight, fight or freeze response. In diving, we have to resist flight – forget about freeze and fight – to remain controlled and panic-free. As such, our ascent rate must be unhurried and moderate. My personal computer is a fourth generation model controlled by a later version of the VPM algorithm, and as such, the controlling ascent speed is about 9 metres or 30 feet per minute. Let’s use this speed to get ourselves from 40 metres / 130 feet up to six metres or 20 feet for a five-minute safety stop, which is once again a conservative choice. This gives us a smidge less than a four-minute travel time. We can round up again and make this a full four minutes. (In fact my computer would serve up a variable ascent speed causing us to slow down to about 3 metres or ten feet per minute for the last few metres approaching the safety stop. But we can ignore that in these calculations; I will explain why later.)

To establish how much gas we will get through during that four-minute swim from depth to the safety stop, we have to know our average depth. The halfway point between 40 metres/130 feet and six/20 feet is 23 metres/75 feet which gives us 3.3 bar/ata. From this we can calculate our gas needs as: 30 litres X 3.3 bar X four minutes; or 5 cubic feet X 3.3 ata X four minutes. That’s around 400 litres (396 rounded up) or 14 cubic feet (13.2 rounded up).

So far, we need 450 litres at depth and 400 litres to swim to the stop, which adds up to 850 litres. For the imperial crowd, the required gas volume is around 30 cubic feet (actually 15 + 14 for 29 cubic feet. A note: if you are doing actual calculations to translate from imperial to SI or metric on the fly, there is some slop in the numbers quoted here because of rounding errors and soft conversion values. The differences though are moot and the principle message remains the same).

Now we have to spend five minutes at the safety stop. Using our base consumption rate as a guideline, our diver will use around 240 litres or eight cubic feet, and we can round those numbers up to cover the slow ascent from the stop to the surface. (The numbers are 1.6 bars / ata X 5 minutes X 30 litres / 1 cubic foot.)

Looking at our total gas requirements from the bailout at maximum depth then, we have:
450 litres / 15 cubic feet on the bottom; 400 litres / 14 cubic feet for the swim up; 240 litres / 8 cubic feet for the safety stop. This adds up to 1090 litres (let’s call that 1200) or 37 cubic feet.

Before moving on to touch briefly on some issue that fallout from discovering just how much gas we should think about carrying, let’s make a couple of things clear.

In the calculations used here, we have been conservative with the baseline per minute consumption figure. At least half the divers reading this article would use less than 30 litres or one cubic foot per minute as a working surface rate. However, the other half would probably use more. (And by the way, these numbers do work better if you plug in your personal SAC (Surface Air Consumption) and a factor modifying that volume to account for stress based on your abilities and needs, but frankly, our conservative baseline is a REALISTIC average).

Also, we have maintained the “high” per minute consumption rate for the whole of the swim to the safety stop as well as for the safety stop itself. In all likelihood, a diver who has him or herself under control would begin to “breathe easier” as they arrived at a shallower spot in the water column with their circumstances starting to brighten. Using a stressed consumption rate throughout the dive has resulted in a high total gas volume requirement. However, we have not factored ANY gas for a swim back to an ascent line at depth; we have factored nothing in for holdups while ascending, and nothing for blimps in procedures.

We have also opted for a slow ascent, followed by a five-minute stop at six metres or 20 feet. We could just as easily have computed a faster ascent speed and a stop at three metres or 10 feet for three minutes. The resulting consumption figure would have been slightly less. However, I believe that a controlled normal ascent and a five-minute stop provides a better edge against decompression stress in this scenario.

Finally, we have worked out all these numbers based on a dive at the very fringe of sport diving. A 40-metre or 130 foot dive is the maximum sanctioned for a sport diver with special training. Not all sport dives go this deep. However, in more than 20 years teaching divers about the basics of dive planning, — and being downright lazy –I’ve discovered that using a pinnacle dive (one that’s at the far boundary of what’s best practice for your experience and the maximum for your training) to calculate contingency needs follows perfectly the axiom of calculate once, use many times. In other words, if we follow these guidelines and then bailout from a shallower dive, we should have more than enough gas, all else being equal.

Clearly, the default sized pony bottle would be something that can hold this much gas. A decent choice in my opinion is a 6 litres / 40 cubic foot aluminum bottle. There are a couple of companies making this sized tank and they are relatively easy to find in local shops. Also, this tank has pretty good buoyancy characteristics in the water, is easy to handle with a little pool practice behind you, and is simple to carry with you in the water. The important thing is that fully charged, it carries ample gas for the purpose it’s being used for. There is the whole issue about whether to have it piggybacked on one’s main cylinder, carried as a sling bottle (classic North Florida Cave Diver rig) or as a side mounted bottle (my personal favorite because it is out of the way but accessible), but let’s leave that debate for another article. Instead, let’s look at what type of gas would be the best to carry and why.

The simplest and most straightforward choice would be to always carry in your pony bottle EXACTLY the same gas that you have in your main cylinder, but this does require us to be wary of a potentially fatal mistake. For example, last week, hypothetical diver Jillian was diving a wreck on which an EAN38 was perfectly suitable, and she had her main cylinder and pony filled with a nitrox 38. Everything on her dive was perfect and the pony stayed unused. She does not bother to drain it. This weekend, she and her buddy are going to dive a reef and intend to take a photo of an Elephant Ear Sponge at around 40 metres or 130 feet. At that depth, her pony bottle mix is hot, delivering an oxygen partial pressure of 1.9 bars / ata. This is problematic.

A simple fix is to have the pony filled with a gas that CAN be breathed on a pinnacle dive. For Jillian, or for the rest of us non-hypothetical divers for whom the specter of oxygen CNS toxicity is a real one, this would be a mix containing 28 percent oxygen, which delivers a ppO2 of 1.4 bar / ata at depth.

The principle of diving with a bailout bottle or redundant gas source is a sound one. Many divers opt to follow the practice. It gives a diver – and that diver’s buddy – options when things go pear-shaped at depth, and allows for a controlled, independent ascent (by which I mean an assent where we are not tethered to our buddy by their octopus).

As with ANY procedure that’s outside the classic stuff taught in most open-water sport programs, there are a few “good to know” knowledge nuggets focused on pony bottles:

1. Have the valve and hand wheel within reach, and practice breathing from the reg while feathering the valve.
2. Fit the regulator with a full-sized SPG and check it before every dive.
3. Pre-breathe the bailout regulator before every dive.
4. Drill bailouts often until the process becomes natural and fluid.
5. Mark the cylinder contents and check MOD before every dive.
6. Have the hose for the pony bottle second stage long enough to reach your mouth (and your buddy’s) easily. A 40-inch hose is a good start, longer is usually better.
7. At least a couple of times each season practice complete ascent breathing from your pony bottle.
8. Splurge on a good quality regulator for your pony bottle. It has to perform when you may be under stress.
9. Treat your pony bottle system as life-support. Get the components serviced and checked on exactly the same schedule as your main cylinder and reg.
10. NEVER, NEVER, NEVER use the gas volume in your bailout bottle or pony in the gas calculations for a dive. In other words, do not plan your dives around the 1200 litres or 38 cubic feet you have in the pony. That gas is a RESERVE and should be ignored in one’s principle dive plans.

Although not the law, the best general advice for ANY single-tank diver who wants the assurance and personal “cushion” that comes from carrying a pony bottle is that they would do well to get some face-to-face time with a good mentor or instructor familiar with the kit and the procedures governing its use. An excellent certification course on this score is the SDI Solo Diver Certification.

A similar version of this article was first published in issue 24 of Underwater Journal, An Underwater Adventure Magazine, the official publication of SDI™ & TDI™. To learn more visit underwater journal magazine https://www.underwaterjournal.com/

Written by Steve Lewis

Steve Lewis (doppler@techidvertraining.org) is an active instructor-trainer with TDI/SDI and has written scores of articles on dive safety and skills development and is a regular contributor to several online magazines and discussion groups. He occasionally dives “open-circuit with a single aluminum 80” but never without a pony bottle by his side filled with a lean nitrox. His best-selling book called “The Six Skills and Other Discussions” is available at select dive stores and through onLine stores such as Amazon and Create Space eStore via: https://www.createspace.com/3726246.