Appeared in DIVER January 2007

COMPUTER Uwatec Aladin Tec 2G
IVE UPSET A LOT OF PEOPLE by carrying my deco gas on my back, rather than side-slung. This appears to be because its a method that has never been written up in any technical diving manual.
Its a pity really, because this simple technique for adding a bit more safety to ordinary leisure dives was first shown to me by Rob Palmer, who wrote one of the first technical diving training programmes.
Unfortunately, Rob died before he could formalise this particular way of doing things, but switching to a richer mix of nitrox as soon as I get shallower than its maximum operating depth (MOD) during ordinary leisure dives seems pretty sensible to me, however its done.
Tom Mount worked with Fabio Amaral, the Big Banana-Eater at Bikini Atoll, to devise a method by which divers could visit the local 55m-deep wrecks twice a day using air, and still get the divers out of the water in time for both lunch and dinner.
They now use a surface supply of nitrox 80 hanging down on lines to a three-level trapeze suspended under the dive boat. This doesnt add safety but it does reduce hang time. The problem was to keep track of the actual deco required.

THE BIKINI ATOLL DIVERS WEBSITE lists those computers it prefers its visitors not to use - those that are either too optimistic about the deco requirements or tend to leave the diver in the water until he is well crinkly.
Of course, doing it this way does not add safety regarding possible decompression illness, but it adds a degree of safety in that you will spend less time under water.
It is not so long since I visited Bikini, but at that time only a couple of computers allowed you to switch nitrox-mix settings to correspond with what you were breathing under water. Today many are available, with Suunto and Scubapro products prominent, along with computers made by Seiko in Japan and marketed under different brand labels.
The Aladin Tec 2G (two gases) is the latest in the range of hockey-puck style computers with a user-replaceable battery made by Scubapros Swiss subsidiary Uwatec. If you want to reads our recent review of the Aladin Tec, on which the Tec 2G is based, visit shtmlaladin
The instrument uses the same two-button system for navigating in and out and around the many options available.
You can set a different maximum allowable ppO2 for each of the two nitrox mixes you intend to use (1.2 to 1.6 bar) and, most importantly, can choose to set one of five MB (reduced microbubble) levels.
This adds degrees of extra safety to deco calculations, or you can opt for the good old-fashioned Buhlmann L16 ADT program, which has seen plenty of divers safely through their dives. Many divers will do this simply because they cant be bothered to wade through the instructions.
I found the L16ADT a lot less cautious than the Suunto RGBM 100 program of the D6 computer I was using alongside the Tec 2G. Setting the minimum microbubble level (L1) at first brought the Tec 2G into line with the Suunto, but it gradually became more cautious as my weeks diving progressed.
Setting the most cautious level (L5) will be suitable for those who do one dive a month and spend the rest of their time at the bar.
When using MB levels, level-stops rather than deco-stops are displayed at first. The significant difference is that, should you choose to ignore a level stop, the computer simply readjusts or cascades to the next MB level setting. Its an optional safety setting that you can take advantage of or ignore as you choose.
If you ascend from a dive ignoring the level stops and undertake a subsequent repetitive dive before fully off-gassing, the Tec 2G might request level stops immediately.
When the MB level is initially set in this way, it is mandatory to complete all the level stops during the subsequent ascent. Deco stops are mandatory as usual.

The two-gas-mix setting is similar to that of the Tec 2Gs more expensive sibling, the Smart Tec. Nitrox mix No 1 must be of a lower fraction of O2 than mix D (deco mix) for the computer to accept it. For deco mixes of 80% O2 or greater, the ppO2 is fixed at 1.6bar.
During the ascent, the computer sounds a warning and flashes its display to remind you to change mixes and confirm that you have done so. You choose this depth to match the MOD of the mix set and maximum ppO2 set for it.
If you fail to confirm a switch, it continues as if you are still using the bottom gas (gas mix No 1). You can, of course, make a belated gas-switch confirmation.
Should you decide to re-descend and find yourself going beyond the MOD of mix D, the maximum ppO2 warning will be displayed and you should change back to mix No 1 or start ascending.
During the dive, you can also set bookmarks at points that you might want to examine closely, should you care to download the details of the dive onto your PC using SmartTRAK.
Using the Aladin Tec 2G on a dive was straightforward, and the display was as clear and legible as any Uwatec computers. But setting it up needed concentration, and I found the operation a little daunting at first.
The Tec 2G instruction booklet has been improved vastly over that of the Aladin Sport and Tec in that it is dedicated to this single model. However, the writer obviously knows what hes writing about and probably cant understand why I do not. Half an hour with the computer in my hand, pressing the buttons experimentally, solved the problem.
I often twin up two independent cylinders while diving from Red Sea liveaboards and Ive noticed others starting to do the same. Its better than using a pony in that the second tank holds a lot more gas, and you dont have to drag it all the way out with you from the UK.
Now before my bete noire of Internet forums, Mark Papp, rushes for his keyboard, I stress that I am doing the same dives that thousands of other divers habitually do with only a single tank strapped to their backs. I enjoy doing extended decompression stops in the shallows of the Red Sea, breathing nitrox, because there is plenty to look at. Im old, not bold.
Its almost as if I get two dives, one deeper and one in the shallows, for the same money!
Keeping track of your actual deco obligation when using two mixes this way gives me the option to break the surface either sooner or later, depending on circumstances. If I am accompanied by a petite female dive-guide, whose standard 12-litre cylinder would be equivalent to a 24-litre cylinder on my tall frame, I can still stay with her for the whole dive.
If the current has picked up and I feel the urgent need to find the cover boat, rather than risk getting lost at the surface, I have that option too. I will have off-gassed quicker than I would having breathed only a single mix or air, and the Tec 2G will have accounted for that.
Whether the supply of deco gas is in a tank strapped to my back, in a sling-tank clipped to two D-rings on my BC, hung down on an extremely long hose from a surface supply, or indeed held between my knees, is neither here nor there.
I believe its what I breathe that counts, and how I keep track of it, and the Aladin Tec 2G helps me to do that.
The Scubapro-Uwatec Aladin Tec 2G costs 339.
Scubapro UK.

Divernet Divernet
+ Keeps track of deco for two nitrox mixes on one dive

- Slightly daunting to understand how to set