Comments on another thread got me thinking about just how important the old mantra "plan your dive, dive your plan" is and how much more discipline is needed once you move into decompression diving. OK I admit on shallow dives I jump in the water, bimble around keeping an eye on my computer to ensure I don't exceed the NDL (or the MOD of my gas mix), monitor my contents guage and surface when I'm getting low(ish) on air. Dive planning is minimal as my "bailout" should anything go seriously wrong is to abort the dive and head for the surface. Moving into techy diving and suddenly my dive plan is written out on a wrist slate, giving a minute by minute account of what depth I should be at every point of the dive. I use the PC to calculate my deco schedule and gas consuption, carefully ensuring I have at least a third of my backgas in reserve. Yes I dive with a computer but I also carry a D-timer and wrist slate as backup, simply aborting a dive is no longer an option as sometimes by the time I get to the bottom of the shotline I am already into deco. Personally I would be looking to use Trimix on cold water dives deeper than 35m, I have been to 40m on air and didn't enjoy the experience. When using Trimix I set the narcotic depth (how narked I am) to the equivalent of diving to 25m on air, as by 30m I can feel the effects of narcosis and with all the other pressures of a deep dive I don't want the added complication of being narked. Once in the water exceeding my planned depth will totally ****up my deco schedule so is an strict no no, and the same goes for over staying my planned bottom time. Messing up my deco schedule not only makes my carefully calculated wrist slate useless but also (very importantly) means that I may not have sufficient gas to complete the dive. In any form of Nitrox/Trimix diving there is the ever present concern of exceeding the MOD of your gas mix and getting an O2 hit. But with techy diving I am normally carrying 3 different gas mixes and switching to/from at the correct depth is vital in order to ensure I decompress correctly (in line with the dive plan) on the ascent. Breathe the "wrong" gas at a given depth and you could get an O2 hit, or find youself breathing a hypoxic mix with insufficent O2 to support you!