IT WAS A COLD AND DAMP MORNING when I turned up at Capernwray dive centre on the edge of the North Yorkshire hills. I had not brought so much as an undersuit, let alone cylinders or fins, but for once this was no oversight.
     The course on which I was about to embark would not see me putting so much as a toe in the water. No, the purpose of this class was to learn how not to poison other divers, and how to avoid blowing myself up.
     Merseyside Sub-Aqua Club (to which I belong) is situated on the banks of the Mersey. It is one of the oldest BSAC branches and, with two RIBs, a spanking new compressor and, most importantly, its own pub, it is thriving.
     At the bar one winters evening, some of us were grumbling about the cost of gas. Geoff Oldfield, our Diving Officer, suggested we set ourselves up to mix our own. But who would do the blending, and which would be the best system for us
     Only Alistair Reynolds, the clubs president and BSACs Technical Manager, had ever actually done a course. As usual, this was the real-life prompt to get my finger out and see exactly what is involved with all things mixing.
     Thats why I was now installed in Capernwrays spacious classroom, overlooking the lake.
     Teaching the class was friend and some-time dive partner Adam Hanlon, who has graced these pages more than once while taking some of the courses I have reviewed. As resident Dive School manager at Capernwray, he teaches everything from basic open water to advanced technical diving.
     Geoff Chilton, a quietly spoken Yorkshireman, was the only other person in the class. Surprisingly, although nitrox-qualified (a prerequisite), he was there for no other reason than wanting to know how it all worked.
     After Adams course overview, I started to realise how much there was to it.
     Encompassing everything from gas properties, obtaining nitrox, setting up a gas-blending system correctly, how to O2-clean components and much more, the course title Gas Blender was perhaps something of a misnomer.
     We looked at the three different ways in which nitrox can be obtained. As I was to advise the club on this subject, this part of the curriculum was of particular interest to me.

LlewocSIS Nitrox Stik
Using a nitrox blending Stik (a patented design, hence the odd spelling) to blend by continuous flow is a fairly cheap and simple approach. You blend the oxygen with air before it is fed through the high-pressure compressor, though it still involves the hiring of large J cylinders from a major gas supplier.
     However, mixing before compressing does allow all that expensive oxygen in the hire cylinders to be used.
     A bonus is that dive equipment need never come into contact with O2 percentages above 40% - the industry limit above which O2-cleaning is mandatory. Unfortunately, as it requires the flow rates to be adjusted until the desired O2 percentage is correct, it is not ideal for one-off fills but excellent for mixing and banking large quantities.
     This one was not for us, then.

Molecular sieves
Membrane systems are becoming increasingly popular. As with the Stik, the desired O2 content is set before compressing. This method de-nitrogenises ordinary air to leave a gas with a higher oxygen content, so no O2 is involved, making it particularly safe.
     The downside is that it is an expensive system to buy, and will allow mixes only up to 40% mixes to be obtained. So that wasnt for us, either.

Partial pressure blending
     Good old partial pressure blending, the quick and dirty way to mix gas, and currently the most popular. This involves decanting a predetermined amount of pure oxygen into a dive cylinder and topping up with air (and/or helium) to obtain the desired gas mix.
     It is cheap to set up and easy to do, but it requires the handling of pure oxygen and just about every component (and cylinder) will be exposed to this, making it the most hazardous of the three systems.
     Additionally, the hire cylinders containing the oxygen can be used only until a minimum pressure is reached (just above the O2 pressure required to make the desired mix), so you may find yourself sending back cylinders with 20% of the gas you paid for still inside. For all its faults, this seemed the best system for my club.
     A little later, down at Breathe Easy, Capernwrays small but fully equipped (and immaculately kept) maintenance and test centre, we were each given a well-abused and manky regulator on which to practise O2 cleaning.
     I had cunningly opted to bring in one of my own Scubapro MK10 first stages. I wanted to see what its innards looked like and hey, if I was going to O2-clean something, it may as well be mine!
     First came a full strip-down of the reg into its component parts before scraping off the visible gunk. The parts then underwent a series of rigorous cleaning processes involving an ultrasonic bath. It seemed more involved than the sterilising of medical instruments!
     Unfortunately, my cunning plan backfired. We didnt have time to re-assemble the components, so I was left with a bag full of regulator bits!
     On our way back up to the classroom, and having found this part of the syllabus particularly interesting, I asked Adam if we would now be qualified to undertake O2-cleaning.
     Er, no, he said. There isnt actually such a thing as an O2-cleaning qualification. There is a service course for each regulator, and cleaning kind of falls under this banner. This part of the class is more for you to understand the process.
     Well, can I take a regulator service course for my Apex regulators I asked.
     No, Apex only runs courses for people in the industry, working for a shop or a school. Hmm, do I detect the HSE at work here
     Back in the classroom, we worked our way though the difference between O2- compatible and in O2 service, another set of regulations that tries to marry theory with the real world.
     When you have your cylinder cleaned, it is deemed to be in O2 service. That means that if you do what you are supposed to do and have it filled only from O2-compatible compressors, 12 months later it will still be spotless.
     However, if I decide to fill it from an oily contraption made from old tractor parts, no one will know unless I tell them, and the cylinder will still get filled while the sticker says its clean.
     In essence, this rule seems to say: We trust you blindly for 12 months, and then we dont.
     I guess they have to draw the line somewhere. Perhaps a cylinder logbook would be the way to go
     We moved down to the filling room, which was full of daunting-looking knobs, pipes and a smorgasbord of warning labels. Before we touched any of that, we had to go through the procedures of what we would need to ask potential customers when they came for a fill, such as ascertaining their diving qualifications, the depth they wanted the mix for, the narcotic dose they were prepared to sustain and so on.
     We also had to ensure that the cylinders we were filling were up to the job. Here, the industry seems to have got itself in a right legislative mess, interfering with systems that have an enviable track record for safety, and seemingly concerning itself more with bureaucracy.
     For example, one label that Adam showed me read: Caution Argon Do Not Breathe which now had to be replaced with another that still read Caution Argon Do Not Breathe - but in a different colour.
     Nitrox stickers produced only a few months before were now illegal, as the colour had changed from yellow with green stripes to green with yellow stripes - or was it the other way around
     Cylinders must now be yellow with white and black hatching on the top rather than, say, just white. Perhaps it would be safest to ban all colour-blind people from diving. This is EU rules gone mad and, believe me, just the tip of the iceberg.
     Having established that the cylinder is the right colour and wont spontaneously combust due to having the wrong-size lettering, we were almost ready to move on to the blending process, but first we had to fill in the shops fill-sheet with the required information.
     The requested mix specified by the customer (me) had to be noted, along with all the other details such as maximum operating depth, cylinder number and O2-clean status. I would also be required to analyse, mark and sign the actual mix post-blending before being allowed to take the gas away.
     Cylinders connected up, and the various knobs and pipes explained to me, it turned out to be much less complicated than it looked.
     As Capernwray used the partial pressure blending method, we needed to open and use the large storage cylinders in a specific sequence, starting with the lowest pressure first.
     This is called cascading and enables the shop to get the most out of each cylinder before having it refilled.
     Because they were full of pure oxygen, everything had to be done at very slow speeds (a maximum fill rate of 5 bar per minute is advised) to avoid any frictional heat build-up. This makes the process very time-consuming.
     The quantity of each gas required had been calculated using the intuitive DSAT gas mixing program supplied with the course materials, and then recorded on a piece of tape on the cylinder being filled.
     Each step was crossed off as it was completed. In this way, should the blender be called away, his replacement would know exactly where he was up to.
     As diving-specific Palm and pocket PC programs are ever more popular, I decided to compare the mixing program downloaded to my phone with the numbers dictated by the DSAT program, and found that they matched up exactly.
     The fill complete and the mix analysed, it was back upstairs for the final exam (the usual PADI/DSAT multiple choice). My only task remaining was to figure out what to do with the newly created bag of regulator spares!

     As with diesel cars, in which you have to do a lot of miles just to break even on the increased purchase price, mixing your own gas to save on costs can be a bit of a red herring.
     In the case of a large club such as ours, a 32% fill of a 12 litre cylinder now costs about £1.30 compared to the £6-7 in a shop, so Id say we got a good deal, though not without a lot of inconvenience to a couple of hardworking members.
     If mixing at home (increasingly frowned on by the HSE, and likely to be made more difficult as time goes on) the main return would be convenience rather than cost. Which route to follow to obtain your fills largely depends on which of the following three categories you consider yourself to be in:
  • If you take your car back to the authorised dealer for servicing, for peace of mind you will probably continue to pay the extra cost of going to the shop for fills.
  • If you are more likely to use a local garage to cut costs, you may want to be part of a club that mixes its own gas.
  • If you are an avid collector of Haynes car manuals, you are always likely to want to mix for yourself and service your own regs, preferring to spend time rather than extra money.
What would be more cost-effective for clubs, where labour costs cease to become an issue, is to service and O2-clean their own equipment, but unfortunately the industry is not keen to see this happen and does not offer training or servicing parts to those not in the trade.
     Even taking a bona fide blending course on which you are shown how to O2-clean does not authorise you to do so. I can happily service the brakes on my car, so this seems a bit illogical. I am aware of the official reasons for not wanting to allow ordinary people to perform these tasks, but they seem to have more to do with safeguarding the industry than with safety.
     Gas-blending courses are generally taken out of necessity rather than for fun (I would guess Geoff Chilton is unique!).
     They are not particularly interesting, and certainly not dynamic, but if the BOCs recent stance of supplying gas only to certified clubs and shops is followed by other UK gas suppliers, then they are likely to become a lot more popular.

  • The two-day DSAT Gas-Blending course at Capernwray Dive School costs £175. Call 01524 735132 or visit www.dive-site.co. uk/divesch.htm

  • Stickers
    Stickers galore
    Capernwrays mixing panel looks more complicated than it is
    Chris takes apart his MK10, but who is going to put it back together
    Divernet Divernet Divernet
    The ultrasonic baths
    Breath Easys regulator testing machine
    Under pressure - Chris and Geoff sit the final exam
    Brand-specific tools for the regulators