Similarly when I started extending my diving range, I went out and bought a twin 12 liter set – why? Because everybody used a twin 12 liter! However, again I found that it did not suit my needs. Either I was getting three simple pleasure dives out of it (in which case it was too heavy and uncomfortable for pleasure diving) or my deeper longer dives still resulted in surfacing with much more than  third of my gas left.

I realised that only a handful of my dives each year would warrant the use of a twin 12 liter tank set up. I considered replacing them with a set of twin 10s.  I can get a single club dive to say 20 meters out of a 10 liter tank so twin 10s would cover me for a twin dive trip as well as the extended diving I was planning to do. However, these tanks are very short. Too short to fit my excellent Halcyon Evolve wing and back plate which I wanted to keep using - and also much too short to rest the tanks on a boat’s bench while waiting to dive!

Twin sevens were decided upon – but I paused when my friendly dive shop owner told me twin 7s were for girls! Back to the drawing board – I didn’t care about the friendly teasing – if the tanks were right for my diving I would get them. But I realised that 14 liters of gas was too little. I ran some diving software on a range of dives that I was planning to do over the next 1 to 2 years – typically a week in Scapa Flow in the Orkneys diving 35 – 40 meter wrecks.

My conclusions? I needed 17 or 18 liters of back gas in addition to decompression gas (in either a 5 or 7 liter tank). I ran the scenario of my buddy (on twin 12s) having a bale out at the worst point in the dive (i.e. on the point of starting the ascent) and found that I had plenty of back gas for a range of escape options that assumed at least one 7 liter decompression stage was always available – at 50% from 18 – 22 metres upwards (i.e. ideally 1.4 ppO2 max).

My problem – 10s too short – 7s longer but too small in volume. If only they did long 10s!! I searched everywhere, and was giving up when I happened to look at some foreign specifications being used abroad. Entering “twin 8.5 cylinders” into Google I was presented with a page from Silent Planet’s web site along the lines of “…are twin 12s too heavy…are twin 7s too small…”

It seemed that Silent Planet were just starting to import 8.5 liter Euro cylinders into the UK. They were sized to exactly fit my wing as they were as long as the 12 liter cylinders, but a lot less weight. A couple of telephone calls to Silent Planet and my friendly dive shop owner - and I was hooked – an order made!

I will dive twin 12s when the dive calls for the gas – a single 10 when I am instructing a new trainee who will use up their 15 liter even quicker than me. All else I am looking forward to using the twin 8.5s this summer – watch this space for my report and hopefully not eBay to see them on sale!!

I always found the standard 3 litre pony cylinder to be a clumsy addition to my single 12 litre tank, and my buddy with his 15 litre set was even more burdened. I was finding that it was hard to balance the lop sided effect of the additional tank, even by putting counterweights on the other side. So we decided top move to a twin set.

For anybody who has dived using a twin set I am sure they will agree that they are marvelously balanced and feel superbly comfortable in the water. I did and my buddy is equally happy with his, even though we both bought different makes. Me a Halcyon rig and him a Frog wing with Fabers. I on the other hand found that carting 40KG of gear on my back along a quay and into a boat to remove some of the enjoyment from my diving. Furthermore, I was using the twinset for two and often three dives, as I am pretty efficient with my breathing. I also saw that if I was going to do a typical dec dive, say to 40 metres for 20 minutes, I still did not need a full 24 litres of gas if I was taking some higher O2 nitrox down also to decompress with.

I think that it is very important to be comfortable with all aspects of your diving. We find that when we start out, using ill fitting club gear. The difference when we find a BC or a mask that actually fits properly is noticable. The same goes with the size of the cylinders you carry. I started with a 15 litre because that is what everybody had in my club. I found that I was always surfacing with 120 plus bar. So I moved to a 12 litre whjich is fine for most pleasure dives. In fact I use a 10 litre that I blagged from my daughter when she gave up diving – particularly for second dives of the day (I also use it for instructing as it is a doddle moving it around the pool).

So now I have taken to paper and calculator and worked out my likley gas consumption for the sort of dives that I will be doing in the forseeable future. Guess what…twin 7 litre tanks will fit the bill. No, I am unable to get two dives out of them so I will need some provision for the times when I go out on a day boat for two dives (e.g. to the Farne Islands of our North East Coast) but I will be comfortable (and happy) during 90% of my diving.

I considered 300bar cylinders, twin 10 litres, non-manifolded sevens etc etc. Why is there such a choice. In the end I was only able to choose by working out my air needs for the actual diving I was going to do – not what I might or might not be doing in five years time. Now all I need to work out is what computer I will upgrade to!