I just bought an Oceanic VT-4.0 computer, my current puck will now be my BU. My usual dive buddy uses an Atomic Cobalt, which uses "Recreational RGBM (Reduced Gradient Bubble Model) created exclusively for Atomic Aquatics". Of the two Oceanic decompression algorithms, "PELAGIC DSAT* (SPENCER/POWELL DATA BASIS) OR PELAGIC Z+ (BUHLMAN ZHL-16C DATA BASIS)"; which would come closer to what the Atomic uses so we will be diving essentially the same decompression profiles?
That's not going to be an easy question to answer. As a general rule you would expect the RGBM model to start giving short stops at a deeper depth than the Buhlmann decompression model. It seems to me that testing of the Spencer/Powell algorithm was done with a much higher ascent rate than would typically be recommended now but how that is implemented on any particular computer is anyone's guess. Dr. Powell is a member here on Scubaboard so you can ask him these kinds of questions directly, if you want. Also, some of what was put in the market under the name "RGBM" was really 'bubble wrapped Haldane" so the underlying mechanics and calculated no deco times using various ascent strategies will all fit in a narrow bandwidth. I *suspect* although I cannot be sure, that the Atomic computer is one of these.
Either way both the RGBM and Buhlmann algorithms will give you safe ascent profiles. Since the RGBM computer will probably protest if you "skip" a deep stop then I would suggest to follow that one if you insert a deep stop (or series of deep stops) into an ascent using a Haldane based model then your computer will accept that just fine. In fact, with my computer (a Suunto) I never bother to strictly follow it's ascent profile, or even really check what it said. Any ceiling it gives is useless information to me. It's invariably shallower than where I want to start making stops so I just form the ascent to my own liking and when I get to the ceiling it indicated on the bottom then it's always perfectly happy. I hope this doesn't sound arrogant but it would seem odd to me to engage in technical diving if one needed to ride the computer to get a clean ascent profile anyway.....
From personal experience what I always did when diving with a buddy who used an RGBM computer (I was using the Suunto) was to follow the deeper ascent profile and you'll find that you buddy will want to start making stops deeper than you might and his computer will probably clear shallow stops (especially the last one) earlier. So you use his deep stops and your shallow stops, especially the 4,5m (or 3 metre) stops. There's some evidence to suggest that RGBM breaks down on heavy technical dives and that one should extend shallow stops anyway, so it won't hurt your buddy to wait until your computer clears the last stop so you can surface together. In any event, any stop at 5m or shallower can be extended for as long as you like, regardless of algorithm.
In a recreational setting, it's all 6 of one and 1/2 dozen of the other. The very definition of a no-deco dive means that you can ascend (at the prescribed ascent rate) all the way to surface without getting DCS. The rest of whatever you do is just a buffer, whether it be making a deep stop or 2 or extending the 5m stop to whatever length you like. Some agencies are now teaching to ascend to 1/2 of maximum depth and put in a 1 minute stop as a matter of course for recreational dives. In my opinion, as long as you put in the necessary stops to not exceed 10m/min ascent rate until reaching your 5m stop and you were at no point over your NDL, then on a no-deco dive the 1min stop at 1/2 maximum depth can't hurt but probably has more to do with being aware of your ascent profile than with anything in particular to do with safety.
R..
