@stuartv This is interesting, could you elaborate on why you feel the risk is too high? The optional Pelagic Z+ setting is a Buhlmann based algorith, which tech divers seem to prefer. Is the problem that we can't know the exact conservatism factor since it's a proprietary algorithm?
I have no tech training at this point, so just curious to understand the thought process.
When a computer manufacturer says their algorithm is "based on Buhlmann", that is like saying Dale Jr's "stock car" (NASCAR) is based on a Chevy Monte Carlo (or whatever he drives (drove?)). What is really on the inside might have VERY little to do with the car it is "based on".
Even though PZ+ is "based on Buhlmann", the actual implementation of PZ+ is still proprietary. Which means the only ones who can say what the algorithm will do in any particular circumstances (e.g. you're ascending at the end of a dive and switch from Air to pure Oxygen) are some people within Oceanic, and whatever software they put out as a dive planner. And there is no guarantee that what it does in one scenario will be similar and predictable in a slightly different scenario. E.g. if you overstay your NDL by 1 minute and it assigns you 1 minute of deco at 10', does that mean that you can comfortably predict that if you overstay by 2 minutes, it will assign you 2 minutes of deco, or somewhat close to it? No. We don't know. For all we know, overstaying by 1 minute might give 1 minute of deco, but overstaying by 2 minutes might give you 10 minutes of deco.
Saying "we can't know the exact conservatism" is a statement that actually implies we know far more than we really do.
For technical diving, we require a way to plan our dives in advance, so that we know what our ascent should be. I.e. how long we can stay down, how long we have to stop at each depth on the way up, what gases we're going to use, when we're going to switch to each gas, and how much of each gas we expect to need.
If we're going to use out computer to track gas switches and tell us how long we really need to stay at each depth during the ascent (versus following a pre-written plan and only using the dive computer to track depth and time), then we need the dive computer algorithm to exactly match the algorithm in the planning software we used.
You could possibly do that with software from Oceanic. I don't know. Meaning, if Oceanic has dive planning software that purports to match the PZ+ algorithm, I still don't know if it allows you to plan a decompression dive, and I don't know if it allows you to make a plan that includes gas switches, and I don't know if it does anything to calculate gas volume requirements (though you COULD do that part by hand or in other software, once you know what the actual ascent is).
But, even if it does all that, technical divers are generally mistrustful of anything that is not tried-and-true, as well as anything that can't be independently verified. The actual Buhlmann with Gradient Factors algorithm is very much "tried-and-true" and it is not proprietary, so there are a variety of ways to plan a dive using that algorithm and then independently verify that your plan is valid. For example, you could do the same plan in Subsurface and in Multi-Deco and verify that you get the same results. There is no way to do that with the Oceanic, since the algorithm is proprietary.
My final thought is that one might think, "well, I'm not going to do a tech dive. I'm just going to do an NDL dive. But, I went to switch to a richer gas at the end, in order to off-gas more quickly or more thoroughly. So, I don't need to do all that extensive, formal planning, like you would do for a technical dive." That is a valid point. But, even if you were doing that, I personally would only be comfortable with taking that approach using a computer with a non-proprietary algorithm. The reason is this:
Let's say you do an NDL dive on air. You ascend to 20' and switch to pure Oxygen. You do a gas switch on your Geo to tell it you're now breathing pure O2. You hang for your 5 minute safety stop while you breathe O2. You finish your dive and get out. You do a 1 hour surface interval and want to get back in.
At that point, your computer has some internal "memory", or model, of how much inert gas is still in your tissues. What it thinks about your status, based on your last dive, is going to affect the NDL it gives you for the next dive. And there is the rub. Being proprietary and unpredictable, I personally would not have high confidence that what it thinks is safe for the next dive is really safe. I mean, it probably is. But, it's almost the opposite of tried-and-true. There is no significant community of divers that have been using that algorithm to do that type of diving (that I know of, anyway), so, there is no big pool of historical data to look at and inform us that diving that way is reasonably safe. In contrast, we are pretty well informed that you could do that kind of diving using, for example, a Shearwater computer, and have some confidence in your own safety.
I hope that helps. Feel free to fire away if you have more questions.
