Meet the twins

This year I have pretty much changed my entire diving kit, and moved up to diving with a twin-set (double-tanks). Here’s why…

Up until I made the change to diving with twinned tanks I have always dived in the UK using a single fifteen-litre cylinder piggy-backed by a three-litre “pony”.

I am not sure why a “pony” is so called, but it is the name given to a secondary air supply – normally a three-litre cylinder. This extra tank can be treated as a fully redundant air supply. It has its own regulator and pressure gauge. Treated in this manner a pony makes an excellent safety device.

And it would be my personal recommendation that all Sports Divers consider adopting this kind of set-up.

Why carry a pony?

I treated my pony as a completely redundant air supply. That is to say that I would never plan to use the air it contained on any dive. I would simply take it on each dive, confident that if a problem occurred at depth that left me no access to my main air supply, I could simply switch to this alternative and make my ascent. Very reassuring.


Having said all this, since qualifying as Dive Leader my diving has progressed, I am now happier to extend my decompression obligations and can now dive that bit deeper than I would have in past years. With this in mind I got round to thinking about the actual level of assistance that my pony would offer in the event of a problem of the kind that would force me to switch to my redundant alternative air source.

So, on the back of an envelope I calculated that the 630(ish) litres of air offered to me by my pony could get me to the surface from a depth of thirty metres. However, I would more than likely not be able to meet any decompression commitments I might have built-up during the dive. And, when I considered that I often dive deeper than thirty metres, it was apparent that, although better than nothing, I would not be completely self-sufficient in fully resolving this kind of emergency.

If I wanted to continue to remain self-sufficient I either had to change my diving style to fit within what I considered to be the safe limits of my current kit, or change it.

Meet the twins

It goes without saying that a twin-tank set-up, utilising two twelve-litre tanks, offers the diver a greater volume of gas than a single fifteen-litre cylinder. About 66 percent more so it seems: That’s around 5,000 litres as opposed to around 3,000 litres! And, one of the hidden advantages of diving with twins is that on a typical day diving a diver doesn’t have to change tanks between the first and second dive.

More redundancy.

In a twin-tank configuration, each tank should provide both an air supply, and a means of inflating a buoyancy control device. The latter is normally achieved in the UK by connecting both to your dry-suit and also to your jacket or wing.
Redundancy using this system is achieved by one of two main methods:

  • Independent twins. In this configuration, a diver would manage his air supply by alternating his breathing between air sources. In this way he avoids running one tank until it is empty. In the event of a failure of one supply, the diver simply switches to the working tank.
  • Manifolded twins with an isolator. In this configuration, a manifold joins the two cylinders together, effectively creating one large gas volume. The diver achieves redundancy by either turning off the appropriate feeds from the problem cylinder, or, isolating both tanks, effectively re-creating an independent configuration.

Given this second option, and provided the diver can isolate the failure, the appropriate feed can then be turned off, and the isolation knob can be used to reinstate access to all the gas from both tanks through the remaining working feed.

The only disadvantage with the second configuration is that with three knobs inconveniently located behind the diver’s head, a good deal of familiarity is required to be confident that he would be able to follow the correct procedures in an emergency situation.

What did I have to buy

I chose to opt for the manifold option.

However, my venerable buoyancy control jacket cannot support a twin-set, and I now faced a bewildering array of options for a new wing style BC. And, although I technically owned two sets of regulators, the set I used on the pony cylinder was the cheapest available at the time, and I would not be confident using it in earnest. Anyway, my better half saw an opportunity and decided to adopt my pony set-up – So, a new set of regulators as well then.

For a few months towards the end of 2004, I was bombarded with opinions, not only as to what wing to buy, but also which regulators are “best” and what manifold I should use. I used the time to save-up a bit. I had the feeling this was going to be expensive.

To cut a long story short. This is what I bought in the end:

  • Halcyon Explorer 55 steel backplate wing and harness
  • Twin 12L Nitrox-clean tanks, banded with 2″ steel bands with a Scubapro isolation manifold.
  • 2 x Scubapro Mark-25 first-stages accompanied with Scubapro S660 and S550 second-stage regulators


The whole caboodle cost me around 」1,400.

What did I have to consider?

I now had completely new and unfamiliar equipment, and after many hours trying to work out what the best way to route hoses from each first stage, and interminably adjusting webbing to fit, I finally put it all together and proudly walked into Wraysbury water on a cold November day. Where, I promptly sank like a stone.
I spent a bit of time in the lake on a couple of occasions, adjusting and practicing. But overall I was really happy with the way the set felt underwater. The two tanks feel really balanced, and once my weight felt right, I was keen to dive in the sea and give the new kit a real test. However by this point it was December, and I had to wait until the diving season got underway the next year.

In planning for the new 2005 diving season, I added a pocket to my dry suit. It didn’t occur to me before I tried everything out, but my new wing has nowhere to keep all your little bits – reel and spare torch and the like.

For me, this season was really all about going diving and building up familiarity with the new equipment. Culminating with a dive on the HMS Moldavia – a wreck that lies in just shy of fifty meters of water ミ a dive that I would not have certainly not have considered using my old set-up.


Now that the UK diving year is over, I happy to report that I am really pleased that I made the move to my new twin tanks. I have felt much more confident making more adventurous dives and ultimately prefer diving with the new set. So much so that I have sold my old fifteen litre tanks, and only use my jacket BC on holidays abroad.

However, I haven’t finished fiddling yet: I think that I will add a clip or introduce some other method of loosening the webbing on my harness – there have been occasions on a crowded boat that I have found it problematic to get into set without assistance, and a clip would certainly solve this problem.

Oh, and I think I would do well to buy a trolley – these twins can be awkward and heavy when you are not wearing them!