This is a short extract from a book on risk management that we hope to have finished next month.
I would guess that most dive instructors, especially those who teach technical programs, get regular requests from divers to explain how to “use” a pony bottle, how to configure it so it’s not in the way, and which size pony bottle is “right” for them.
These are great questions because any diver who intends to dive deeper than 30 metres /100 feet should carry a redundant source of gas. A dive buddy is supposed to represent the first line of backup, and a well-trained and well-practiced buddy is a great resource in the event of some major gas emergency. However, the best strategy is that whenever practical strive to have a backup for your backup. In this regard, redundant air via a redundant delivery system offers a huge cushion.
The question of size is perhaps the first question to answer because how to rig and use a pony bottle depends to a large extent on its size.
When we consider using a pony bottle as a bailout or as a backup in the event of a massive gas failure with our “primary system” (the normal tank and regulator), we factor in a full minute at maximum depth to get things sorted and to gather our wits before starting the ascent. With this in mind, let’s revisit the table for SAC adjusted for depth. Since we are still talking about recreational sport diving, the limit for maximum depth is around 40 metres or 132 feet. The ambient pressure at this depth is five bar or ata and therefore the average per minute consumption will be 70 litres or 2.5 cubic feet.
Let’s also apply a realistic dive factor. Since a pony bottle is only deployed in times of stress, we need to use a DF for that first minute that reflects high-stress. The norm for this application is a DF of 2.5, which translates into 175 litres or 6.25 cubic feet for that critical first minute!
(If at this point you are beginning to question the veracity of ads extoling the virtues of those tiny emergency cylinders of “spare” compressed air, please read on.)
After the first minute, we calculate a normal ascent rate (9 metres or 30 feet per minute) up to a safety stop. That journey – about 35 metres/ 117 feet – will take about four minutes. Once again, to help simplify the calculations, we use the ambient pressure at the midpoint between maximum depth and the safety stop, which in this case will be 3.22 bar or ata. We also drop the DF to 2.0. So we have ascent time X SAC X ambient pressure X DF, which equals 360 litres or about 13 cubic feet of gas.
Now for the safety stop. Even when a dive is within the no decompression limits, there is a strong suggestion from most experts that a five-minute stop is indicated after a dive to maximum depth. So the consumption for a five-minute stop at 4.5 metres or 15 feet with a mild DF of 1.2 adds up to a total of 122 litres or 4.35 cubic feet. Finally we have to factor in a little gas for the last part of the ascent to the surface. Therefore, the best estimate is that a controlled ascent following an emergency at depth will require at least 680 litres or close to 25 cubic feet of breathable gas!
It’s the considered opinion of most divers who have experienced a real gas emergency at depth in real-world dive conditions that these numbers are neither exaggerated nor inflated. When something bad happens at great depth, there is no such thing as a plan that is too conservative or too careful. The risks of drowning, embolism, decompression sickness and various other ailments that can result from stark panic and ballistic ascents are very real and totally unforgiving. The alternative to a controlled normal ascent are simply not worth considering.
Clearly then, the “right” pony is one that holds at least 680 litres or 25 cubic feet. Because of its general usefulness, buoyancy characteristics, ease of deployment, and attractive cost compared to smaller tanks, many divers invest in an aluminum 40 (nominal capacity 40 cubic feet / 1200 litres) as the best “emergency” pony bottle.
Two final words on the topic of pony bottles before we move on to gas volume management for more advanced diving. The gas carried in a pony bottle is contingency gas. It should never be factored into the gas volume requirements for a dive. It is there for emergency use only. If the dive plan calls for more gas than can be carried in a regular primary scuba cylinder – an aluminum 80 for example – then the total kit configuration for the dive needs to be reconsidered and calls for an additional primary cylinder or a high-volume primary cylinder such as a steel 15 litre / 120 cubic-foot tank.
A bailout/pony bottle is useless if it does not deliver breathable gas faultlessly. The valve, regulator and SPG must be tested before every dive. Do not take for granted that it is filled and in working order. Analyze and label its contents, check the pressure and wet-breathe the regulator at the start of each dive.
Let’s leave this topic with one last thought. As we were editing this chapter, I read about yet another incident where a diver “ran out of air.” This time a pair of brothers and a friend were hunting crayfish in about 30 metres / 100 feet of water off the coast of New Zealand. Calm conditions at a site familiar to all three divers. Describing the victim, his brother said: He was a competent diver with several years’ experience.
I would suggest an edit… a small change but something that I hope will speak volumes to you. He was USUALLY a competent diver, but not this time. Even several years’ experience cannot compensate for serious oversight.
Plan your dive, dive your plan.