[Andrew H]

Thanks for all the input guys. I'm a former PADI OWSI and have 150+ dives including 40m no deco and nitrox dives so not exactly new, but currently without gear. I am hoping to dive wrecks while in England, have no urge to dive below 40m on air, particularly not in these waters. But appreciate the need for increased runtime for dives to 30m plus. Hence deco and most likely accelerated deco, Maybe I should have mentioned that before... I would like to progress to Trimix in couple of seasons time experience permitting. There seems to be a strong move away from deep air and towards trimix at shallower depths. Oddly some agencies require you to have a number of dives beyond 30m on air while others recommend you maintain an END of less than 30m... confusing stuff! I think the recreational agencies will start to recommend normoxic trimix for dives which were formerly extended range. Does anyone use trimix with high oxygen for dives of 20-40m or is it considered too expensive? The CCR crowd must have experienced this using trimix diluents on shallow dives after deep dives, there must be a different experience even at 30m?
From past experience I know that a dive on a wreck in 27m of water would be planned on PADI table as 30m for 20min, But much of the dive will be done at 20m on an intact wreck so diving a computer pays hugh dividends in that respect. I am wondering diving to around 40m on nitrox with deco, the difference would be similar? I am also assuming that diving deeper on mixed gas say 60m there would be less change and at 90m much less change because of the reduced relative change in ambient pressure as the dive is deeper? Seems to me then that you need a fairly compelling reason to spend hundreds of pounds on a VR3 when you could cut tables for the dive effectively for free. Any way that's what I'm driving at, don't seem to be any nearer to solving my problem so far.