There is often a tendency to go with the flow when it comes to choosing diving equipment. For example every new trainee in our club goes out and buys a 15 liter cylinder as soon as they are ready to have their first sea dive as a qualified diver. I did the same many years ago, but since then sold it because I was either using much less gas on simple pleasure dives – or needed more gas and redundancy on extended range diving.
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!!