Getting round the bends
Deep-diving on trimix involves long decompression stops - or does it The new RGBM system promised to shave three hours off a 160m dive for Jersey diver Mark Ellyatt. So how does it feel to be a solitary human guinea-pig

I HAD USED COMPUTER SOFTWARE TO PLAN DEEP DIVES BEFORE - the Proplanner and Z-plan products - but last April I decided to order the Abyss dive-planning software designed for the Explorer mixed-gas computer. When it arrived, I called Chris Parrett at Abysmal Diving in the USA for my registration code and told him about the deep dives I had planned.
     It was then that he told me about Reduced Gradient Bubble Mechanics, or RGBM, an algorithm which was being developed and which the company planned to incorporate in its software. He said this new model would revolutionise my decompressions.
     It took a year before the new RGBM software arrived, but it turned out to be dramatically different to anything else around (with the exception of its counterpart, the Variable Permeability Model or VPM).
     To get a steer, I looked around for other divers who had already used RGBM for trimix diving, but outside the US caving community there seemed to be no experiences to go on.
     I contacted a number of extreme deep divers, but they were very sceptical and said they would prefer to wait and see. I read up on the theory, continued to e-mail around but in the end decided that the only way forward was to test my body against RGBM.
     I worked out a dive plan. A trip to dive the Carpathia liner, 170 miles out in the Atlantic, was on the cards, and I thought this would provide the perfect testbed. The dive boat had a recompression chamber on board, too, which I thought might be a wise precaution!
     I worked on the tables constantly, checking them with the authors. They said it all looked good. But when I checked the schedules against other software I had used in the past, all the old schedules told me that my deco time should be about three hours longer than I was planning on RGBM.
     When you deep-dive, its important to be free of doubt. It wasnt easy to feel confidence in a set of numbers generated on a supercomputer in an office in the States, with no supporting evidence.
     So far as I could make out from the RGBM technical gobbledygook, it seemed that if a diver started his deco stops very deep and these lasted for around 30 seconds at a time, you would offload the nitrogen bubbles contained in your body much faster, because they were still very small and thus easier to eliminate.
     If the theory was correct, you would then arrive in shallower water with far less nitrogen to off-gas. Traditional deco plans involved the first stops occurring at much shallower depths and lasting for longer. These were effectively letting you bend, then unbending you with the sheer amount of time spent in the shallows - in this case three hours more.
     I figured that I could always try to follow this complicated schedule as best I could and, if it all started going pear-shaped, could simply revert to a traditional bend me/unbend me deco plan. Sorted!

The Carpathia trip was curtailed by lumpy weather, and I was left disappointed because I had been unable to try out my new deco plan. So when I got back to my home in Jersey I checked the tides in the wreck alley called the Hurd Deep, off Alderney.
     The Deep runs roughly east to west, and the tide runs between south-west and north-west. It is not deep everywhere, and is quite narrow. My plan was to dive to 160m for 12 minutes bottom time plus 4 minutes descent.
     I chose as the site the wreck of the Baden, a large German battleship in 170m, because a) it was deep enough and b) I knew where it was!
     I was all set to go, but the weather wouldnt play. I could choose between good tides with bad weather, or spring tides with perfect weather. Sadly, I had to go for the latter.
     I drove up in the RIB with support diver Ben Lumsden and boatman Rob Meier, enjoying the sight of 50 miles of flat sea. After a while we found the wreck, and that was when the tide started to turn.
     I put on my doubles of trimix 8/65 with a third cylinder of trimix 14/33 on my back. On each side I had a 12 litre cylinder, one of trimix 21/30, the other nitrox 50. I sorted out the video camera and my comedy hat full of lights. Rob put me on the buoy and over the side I went.
     A plankton bloom was stifling the light below about 20m, and dropping at 40m a minute meant that the darkness was soon total. I went in breathing the trimix 20/30, and felt good. The current was worse than I had feared, but the weight of the kit helped to pull me downwards.
     At 60m I switched to the trimix 14/33 to check the regulator. All was fine, and at 100m I switched to my bottom mix, which came easily. Apeks TX 100 regulators with 67 per cent helium had meant easy breathing for me in the past down to180m, and they instilled confidence now.

The current was getting harder, and by the time I reached 140m, I estimated that it was running at 2 knots. By now I could feel the shot weight/anchor slipping below me, but with my safety becoming compromised by the strength of the current, I thought it might be easier on the hands to drift anyway.
     Holding the line with bare hands in the 3ÂC water was a chore, but my mind was elsewhere. By 150m I could feel the shot jumping off the seabed, then coming down and jamming hard on what had previously been a mainly sandy bottom.
     I had the choice of aborting and ascending the line, at the risk of coming off it, or descending, unclipping the shot and drifting. I decided to try the latter. At 158m the seabed was shooting past me like a video game. It didnt look clever.
     The shot weight was leaping around like a bouncing bomb now, and I didnt fancy an encounter with it at 160m, so out came my very sharp dive-knife. When I had cut the line I felt more relaxed, and was able to drift along for the rest of the dive.
     Why was I doing this again Because I liked the challenge, and relished the sense of accomplishment!
     Now came the tricky part, following my new deco schedule. I free-ascended up the loose line to 135m for a 30sec stop, then to 126m for another 30sec, then to 120m for another. The remaining deep stops followed fast and often, and as I felt they had been carried out OK, I stayed with the RGBM deco all the way to the surface, waiting 30 seconds here, one minute there.
     Ben arrived at 50m with backup gas. I exchanged a cylinder for my camera and bump hat. He had also brought me some gloves, which seemed to have shrunk to Action Man size, but I somehow managed to stretch them over my hands.
     Gas switches and stops took me to the final hurdle - only 35 minutes on oxygen. This is where I felt anxious, as my back-up deco schedule would have required almost 120 minutes on O2!
     I did the shorter time, took five minutes to reach the surface and, once back on the boat, waited for the worst. Every niggle and ache now concerned me - was I warming up for a knockdown bend I felt unusual, mildly dizzy perhaps for a few minutes, so went straight onto the oxygen.
     The plan was for an immediate redescent should I feel bendy, and only if that didnt work would the cavalry be called.
     In fact I felt fine minutes later, but continued on surface oxygen for 30 minutes just in case. Looking back, I think the problem might have been no more than a sinus imbalance, as I had felt snotty on the ascent.

The journey back went fine, helped by the fact that we had drifted more than eight miles towards home during the 2 hour 20 minute dive! I was able to call the chamber guy I had pre-briefed to say it didnt look as if I would be needing his services today.
     The dive left me highly impressed by all the work done by Dr Bruce Wienke of the Los Alamos National Laboratory and his colleagues to create the RGBM decompression model. I was also grateful for the Abyss dive software which enabled the table for my dive to be constructed.
     My dive was experimental and such diving should still be considered in that light. My preparation was based on years of almost daily deep dives, proper hydration and fitness, no smoking or drinking (mostly) and lots of training.
     I am convinced that when all the parameters are favourable, the RGBM/VPM deco algorithm will turn out to be one of the greatest innovations for diver decompression. I will certainly be using it to plan some deep wreck dives in the near future.

  • Mark Ellyatt is a full-time diver-trainer. Contacted him at Contact Abysmal Diving on 001 303 5307248 or visit

    Is this the shape of deep diving in the future This is the RGBM schedule Mark Ellyatt used on his 160m dive in the Hurd Deep.

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