A pony is a small cylinder, totally independent from your main setup, which is carried to give a diver redundancy in the event of a problem with his primary.
There are a few things a pony ISN'T which I'd like to point out here to avoid confusion.
That's what it isn't. Now for what it IS.
In the UK, a pony is typically a 3 litre cylinder, rated to 232 bar, and is usually made of steel. Aluminium ponies are used, but are less popular. 300 bar is very rare, though apparently popular in some other countries.
Ponies have become very widespread in recent years in the UK, redundancy becoming an ever more popular concept. However, exactly how they are used varies widely, with many opinions being voiced about the best way to rig a pony setup.
The first opinion about ponies that I'd like to address is the "If you think you need a pony, you need a twinset" argument.
Now, I dive with a twinset, and I sold my single cylinder and pony setup to do it. I had a lot of reasons for the switch, but the pony being useless was not one of them.
There are two typical arguments used by the "Ponies are useless" brigade.
Firstly, there is the claim that if you can't get to the surface holding your breath, you're too deep to be using a single cylinder, and you should be on a twinset.
I fail to see the logic in this one. If a diver is at 20 meters and his main cylinder dies for some reason, which is the better scenario:
Both scenarios get the diver back to the surface, but both did not reach the surface with the same degree of safety. Obviously, having a redundant air source was of benefit to the diver.
Would a twinset have added any benefits to the above scenario? No. A twinned diver would have done exactly what the pony diver did: Switch to his backup and aborted the dive.
The other argument is "Ponies don't have enough air to get you back to the surface." A typical example of how this argument is put across is:
"You're at 30 meters with a breathing rate of 20L/min. Suddenly you go out of air. Your breathing rate doubles from the panic. You look for your buddy, deploy a dSMB, admire the scenery, faff around a bit more for some reason, and two minutes after you switched to your pony, you start to ascend at 10m/min. You fail to do a safety stop, and surface. You are therefore on your pony for five minutes. At 30m, a breathing rate of 40L/min will mean 160L/min, 5 x 160 = 800L. A pony only holds 696L of air, so you died."
Now let's apply a reality check to the scenario, shall we?
"You're at 30 meters with a breathing rate of 20L/min. Suddenly you go out of air. You immediately switch to your pony. Because you never go out of air, you don't panic and so your breathing rate only increases to 25L/min due to the sudden surprise. You look for your buddy, fail to spot him, and start an immediate ascent. You carry out a 3 min safety stop at 6 meters, then surface."
The ascent, assuming a constant ascent rate of 10m/min, had an average depth of 15 meters, therefore:
3minutes x 25L/min x 2.5 bar average pressure = 187.5L air used.
Plus, a 3 minute safety stop at 6m:
3minutes x 25L/min x 1.6 bar pressure = 120L air used.
Total amount of air used in your ascent = 307.5L - less than half of your pony's total capacity.
You therefore had a good 300 litres of air to use whilst you were at the bottom - this would give you three minutes in which you could look for your buddy, deploy an SMB, or return to the shotline. Note that if your buddy was using an SMB, and he ascends shortly after you do, he will be caught in the same currents as you, and therefore surface very close to you, so you don't necessarily have to deploy an SMB if you're diving in a current.
A pony can't get you out of every situation, but it can get you out of most in the recreational diving limits. It can even cover you for a fair amount of decompression: In the above example, the diver could have stayed at 6 meters for well over ten minutes, and most divers planning deco stops have a much lower breathing rate than our hypothetical friend above did.
Lastly, let's not forget that the above examples all assume no air from your buddy. With every training agency in the UK stressing the importance of buddy skills, a pony's main use will not be to get the diver to the surface, but only as far as his buddy, who (Especially in the low-vis conditions in the UK) should only ever be a couple of meters away. Between your pony and your buddy's reserve, you should ALWAYS be able to get to the surface at a safe, controlled rate.
So, a diver does his calculations, decides that a pony is the way he wants to go, and goes out and buys himself a pony rig. The next question is how to set up your pony rig.
To answer this, we need to ask: What must a pony do?
And the answer is, of course: A pony must deliver air when it's needed.
There are several pieces of scuba gear that can be ignored if they break during a dive (Like a strobe light). There are others which are an inconvenience if they break (A broken fin strap). And there are also some that are life-threatening if they break, and the pony is one of those.
If you ever have to resort to your pony, then it's because your main regulator has stopped giving you air. Your pony is a last resort, so if you use it, it has to work. No exceptions.
A pony should therefore be a no-frills system. A demand valve and a pressure gauge is all that should be on a pony. No direct feeds, no sirens, no air guns, nothing. The more things there are attached to the pony regulator, the more chances you have of one of them breaking.
A pony should be switched on. The valve on a pony should only be closed if you have a very good reason, and you are certain that you can cope with being out of air and being unable to use your pony until you open the cylinder valve. For typical recreational dives, you're better off with the pony open and ready to go.
A pony should be full. A pony doesn't have enough air in it for you to risk diving with it anything other than full. Nor should you waste its contents on anything other than an emergency: It should not be used for filling up mini cylinders, inflating SMBs, inflating your drysuit or BCD. The air in a pony should go to one place only, and that's your lungs. You can inflate BCDs and SMBs with exhaled air if you really must, but you should not have a direct feed running off your pony to do that job for you.
A pony should be accessible. And that doesn't just mean the demand valve. The cylinder valve as well should be in reach. If you have, say, a freeflow on your main reg due to regulator icing, then there's a high chance of your pony also freezing and freeflowing. If a pony freeflows, and you can't reach the cylinder valve, then you've had it. A freeflow will empty a pony very fast. But if the valve is in easy reach, not only can you close the cylinder and stop the gas loss, you can also continue to breathe off the pony, by opening and shutting the valve as you breathe in and out. It's a laborious way of breathing, but it's better than not breathing at all.
There are three ways a pony can be carried: Back-mounted, side-slung, or front-mounted.
Front mounting may have some advantages, but a pony isn't THAT small, and it can cause problems - if you use a harness, you loose your uncluttered chest area. If you're in a stab jacket, the pony will almost certainly obscure the clips, and may even make them come undone.
Back or side is where a pony should really be put. There are arguments for both.
If you're wearing a stab jacket, the side pockets could well be an encumbrance, making a pony awkward to carry side-slung. Stab jackets also tend to have D-rings that aren't located in the right place for carrying a pony. With a stab jacket, you're probably better off with a back-mounted pony. In order to make sure you can reach the valve, though, the pony will probably need to be mounted upside-down. You'll need a longer than normal hose on your demand valve to use this setup.
If you're using a twinset, then back-mounting is a less attractive option: the pony will either be in the middle of the two cylinders, hard to reach, or at the side, making for a fairly wide profile. Side mounting is more attractive, but may be difficult if you're using a stab jacket. Judgement call.
If you're using a wing with a backplate and harness, you'll be able to move your D-rings to wherever you like. You'll also be uncluttered at the sides. Side-slung is the way to go.
That's the location of the cylinder. Now what's the best place for the demand valve?
Because a pony is meant for your use only, you don't have to worry about it being visible or easy for your buddy to reach. It should be convenient for you and no-one else. The best place for a backup DV is under your chin, hanging off a necklace. If you ever need it, it's right where it's needed.
The next thing you need to know when you've got your pony, is what to put in it.
If you're only qualified to use air, then that's all you can put in a pony. So that's simple.
If you're Nitrox qualified, you could put the same mix in your pony as you have in your main cylinder. Which is all well and good, except it means you have to empty and refill your pony every time you use a different mix. This is an unnecessary waste of gas. You're better off having a pony content that can be used on any dive. That will probably mean air, or a fairly weak Nitrox mix. For instance, if your diving tends to be less than 35 meters, you could fill your pony with 32% Nitrox and use it on every dive. I'd be more likely to use air though, as it would save me the annual cleaning a Nitrox cylinder needs.
One thing that you should NOT do is fill a pony with a Nitrox mix which has an MOD that's shallower than your planned depth for a dive, e.g. a 50% mix for a 35 meter dive. A pony is a bailout device to give you gas when you need it. It should be useable at all points on a dive. Otherwise, you're not carrying a redundant source of air, you're carrying a decompression cylinder. Don't delude yourself into thinking that a stage cylinder is a redundant air source. The stress of knowing you're breathing off a cylinder with too much Oxygen in it drastically increases your chances of an Oxygen hit.
Finally, don't let your pony give you a false sense of security. Don't stop monitoring your cylinder contents gauge because you know you've got plenty of air. Don't plan a dive that you may not have enough air for, just because you know you can switch to your pony if you run out. A pony is there for emergency situations, not to cancel out bad planning or poor skills.
And if you're a person who frequently does run out of air due to bad planning or monitoring, don't think a pony is the answer - never use equipment to overcome your own deficiencies.
A pony is not a magical guarantee of safety. A pony has limits, and you should know exactly what those limits are before you even think of taking a pony under the water.