I RECENTLY RECEIVED the following email from Jane, a friend and an experienced diver.
“Pete and I were diving on a site south of Phuket in Thailand. It was the third day of our trip and the first dive of the day. I slipped my gear on in the boat, did my usual checks and back-rolled into the water. I had a problem descending at first so I exhaled fully then flipped forward and pulled myself down for the first few metres.
“As I descended my tank-valve was banging into my head, which was a bit annoying. So when I got to the bottom at 30m, I took my gear off, adjusted my tank strap, pulled it higher and slipped back into my BC.
“I was finding it hard to breathe from the regulator but thought: “It’s OK, not a problem, I can manage.” I tried switching to my octopus but it was just the same, so I switched back to my primary.
“Then I checked my gauge, not thinking that there might be a problem, just making one of my normal occasional checks. The needle was at zero. I thought: ‘Hmm, where’s Pete?’”
“I swam over, tapped him on the shoulder and indicated that I was out of air. He looked surprised but took out his octopus, gave it to me, then double-checked my gauge and valve. “Yup,” I thought, “I told you I was out of air.”
“We exchanged let’s go up signals. Like me, Pete has an extra-long (about 1.5m) hose on his octopus, so we could ascend comfortably together side-by-side, without getting in each other’s way.
“We both felt relaxed, so there was plenty of air for us to make a slow ascent and do an extended stop at 5m before surfacing: no drama.
“I later found out that I had gone in using the almost empty cylinder from my last dive of the previous day. It had not been changed out. I thought I always checked my air before going in the water, and then again at frequent intervals during the dive. I obviously don’t!
“Nor did I pick up on a couple of obvious signs that I was low on air. What Pete calls “my legendarily low air consumption” seems to have blinded me to the possibility that I could ever have an air-supply problem early on in a dive.
“A couple of months later, we were diving Cannibal Rock in South Komodo when Pete heard and felt a massive explosion behind his head.
“At first he thought he had been fish-bombed, but when the thunderous noise didn’t stop, he realised that a catastrophic air supply failure must have just taken place somewhere behind his head.
“He looked at his pressure gauge, which was down to 100 bar and falling fast. He could actually see the needle moving. His first thought was to head up, as he was not in deco, still had some air left, at least temporarily, and was not too far from the surface. But then he decided that an air-sharing ascent would be the most relaxed and therefore the safest option and turned to look for me.
“The first diver he came across was Martin, but he was burdened by a monster video system and Pete recalled that his octopus was an inflator-hose regulator – no good going to him.
“Then he spotted me, swam over, asked very politely if he might share my air and, when I graciously agreed, we ascended comfortably together and survived to tell the tale.”

“After these two incidents, we have decided to make a significant change in how we dive. We both realise that the outcome of both events could have been very different if we had not been diving together. What if I had been solo? What if Martin had been Pete’s only option?
So now, whenever we are diving below 20m on our own or with divers we do not know well and trust, we will always take an independent air source with us.

Jane asked me what I recommended and this, in a nutshell, is what I told her.
The best option is a pony cylinder. This is usually attached by brackets to one side of the main cylinder and has its own regulator and contents gauge, so you no longer need to have an octopus second stage on your primary regulator. Your pony regulator is now the one you pass to an out-of-air diver in an emergency.
The pony regulator should have a long brightly coloured hose and be attached in the same way you used to attach your octopus.
The pony’s contents gauge should also be clearly marked, perhaps with tape the same colour as the pony’s regulator hose, and clipped off to your BC/harness in a place other than where you keep your main cylinder’s contents gauge. It’s very important that you make it impossible to get the two confused, even when at depth and narcotically challenged.
When configuring your pony regulator, put both hoses on the same side of the first stage, then turn the first stage on its side when you attach it to the valve so that the hoses point tidily downwards rather than looping out to the side, where they might snag on a reef or rock.
You can mount the pony cylinder valve-up or valve-down, fireman-style. Ensure that the valve handle is positioned so that you can easily reach it. This is so that you can turn the valve off in case the pony regulator starts to free-flow in mid-dive. Normally the pony valve should be turned on at the same time you turn your main cylinder-valve on, and left open throughout the dive.
You do this so that, if you need to use the pony, all you have to do is pop the regulator in, purge and breathe. You don’t have to hunt for the valve or worry about grabbing the regulator and taking a mouthful of water, having forgotten that the valve is off. This is something that, in an emergency, could turn minor anxiety into full-blown panic.
You may see what seems like contrary advice elsewhere, suggesting that you should keep a pony cylinder switched off until you need it.
This is the normal procedure followed by technical divers with decompression gases, but their situation is different. Their deco gas is useless to them at depth. In fact, to breathe it accidentally at depth could be fatal for them – the main reason for keeping it switched off until needed.
The gas in your pony cylinder is the same as in your main cylinder and, if you ever need it, you will want it to be available immediately.

Good choices are slim, long cylinders like Luxfer’s S019 or S030. The S019 contains 560 litres of air at 207 bar. The S030 contains 850 litres. Both are about 0.5kg negatively buoyant when full. The S019 is neutrally buoyant at 50bar, the S030 slightly positive.
Decide which one is for you based on the amount of air you calculate you may need to make a slow 10m per minute ascent from the deepest depth you usually dive to, allowing for a safety stop, and adding 50% extra to allow for the fact that you are likely to be breathing significantly more quickly than usual following an air-supply emergency.
Use the formula I described in my technique article in February (5 Things Tec Divers Do) to work out how much you will need.
Practise using your new configuration in the pool, then on easy dives first to get used to it. You may find it difficult to keep your balance with the extra bulk on your back, but a little practice and shifting your weights around a bit usually cures that. You may even find that you can lose a little lead with the pony on board.
Some divers prefer to clip their pony cylinder onto BC or harness D-rings and carry it side-slung under their arm with the regulator and SPG hoses strapped under lengths of inner tubing in the same way that technical divers configure decompression cylinders. If you really can’t get used to a back-mounted pony cylinder, try this instead.

Remember that your pony is a reserve supply. Never plan to use it to stay down longer. Always plan to bring it up full and untouched. It is only there for you or for another diver to use in the event of an air-supply emergency.
Never fill a pony cylinder with any gas other than the gas you are using in your main cylinder. Keep it simple.
You may consider a SpareAir instead of a pony cylinder. Having a SpareAir is definitely better than having no reserve at all and it is small and easy to carry, but it is not really a viable alternative.
It contains only 85 litres at 207 bar – too little air to guarantee a safe ascent from any significant depth.

Read more from Simon Pridmore in:
Scuba Confidential – An Insider’s Guide to Becoming a Better Diver
Scuba Professional – Insights into Sport Diver Training & Operations
Scuba Fundamentals – Start Diving the Right Way

All are available on Amazon in a variety of formats.