Mark with his tanks Photo courtesy of Mark Ellyatt

On this mix, my equivalent narcosis value was 60mish. I had a stop here for a minute and used the time to signal to the surface I was trapped at 120m with twelve pulls of the rope. I got a response asking if I was OK. I wasn't and definitely needed support diver Brian to descend earlier than planned, bringing the spare gas. There was no rope signal for this, so It didn't happen. I'd asked Brian and Christina not to attempt giving assistance below 60m because of the restriction dangers at this depth.

Dumping the gas from my wing and suit, I got free and headed up. With the free flowing reg still going it didn't last as long as it should and by 100m the tank stopped breathing. I closed it and switched to my trimix 20/30. Every few breaths I would swap to my back gas to average out the high p02. All the stops scheduled to be over 30 secs were reduced to 30 secs to reduce gas consumption. Also, the planned max depth wasn't reached. By the time I reached 60m I was ahead of schedule by almost 10 minutes and Brian wouldn't be coming for a while. It left me breathing whatever was left during the stops. At 60m I settled on top of the restriction here. I dumped all my wing gas and replaced it with exhaled trimix 20/30. This might be useful soon!

Brian showed up by the time I got to 40m. I ascended up with him still breathing my 20/30 and back gas untill 21 metres. The first tank on a rope appeared at 21m and was nitrox 50. This stop at 21m I increased from six minutes to 40 minutes, an ounce of prevention here could save some trouble later on. During this stop Brian went up and Christina came down with some more nitrox 50. The rest of the stops went to schedule. The warm drinks she brought down with her were very welcome.

At six metres I moved onto oxygen for 30 minutes, then at four metres for 20 minutes and at three metres for a further 30 minutes. Every 15 minutes I would have an air break for five minutes. I chose back gas switching here but this was a mistake because of the really hypoxic trimix 5/76. After less than a minute of breathing this, I felt my brain and vision shutting down, so it was quickly back to the oxygen (note to self - only normoxic mixes for back switching). For the further air breaks I used some trimix 20/30 (the theory behind this is to attempt to prevent Pulmonary related decompression problems and not simply buffer the CNS clock). A further ten minutes at 2m and ten minutes at 1m served to relax my body's gas tissue tensions further, a useful technique I'd used before when forced to decompress on back gas.

After all this extra decompression, I felt confident that the bends were not coming. With all the extra decompression stops, it meant close on 200 minutes in the cold water. I was still warm and dry as my Otter dry suit worked perfectly. I had spoken with Otter a couple of weeks before and they mentioned a new Artic 300 under suit. I got one of these and was very pleased I did. My support divers had "mere mortal" under suits and felt the cold pretty much throughout.

Using the RGBM algorithm on this dive was totally in order (I've since changed my mind on RGBM nonsense!), but in a low gas situation would be easy to compromise. The RGBM deep stops necessary to allow shorter shallower stops are mandatory. To have missed out any of the deep stops would radically increase the decompression schedule. When carrying out any accelerated decompression profile such as this it is necessary to have a back up "traditional" profile and make provisions for the extra breathing gas that would be needed.