you can't say anything statistically, but if you are going to make that statement, what is the point of gradient factors in the first place? or NDL's for that matter, your Suunto is calculating completely different decompression profiles than your Shearwater and you say it doesn't matter, so why dive a computer at all? Why bother to choose which decompression algorithm makes the most sense to you, etc etc?
Well, my thinking is like this:
A. There is a general consensus that all algorithms (ZHL-16, RGBM, VPM-B) do provide a statistically safe decompression profile at least in the normal range. At very big depths (out of the range of what even most tech divers do) there is some debate about the validity of the results, but probably up until 100m all algorithms are good enough.
B. It was accepted that pure Buhlmann was not safe enough, so we got GF. Something like GFhi 85 was pretty much proven by practice/word of mouth to be safe enough. Some want to be safer and go to 70, some risk more and stay at 85, and probably very few go higher than 85.
C. We though in the past that deep stops are good. Now we start to think they aren't as good as we thought, so GFlow goes from 30 to maybe 40 in some cases. We still think some deep stop and slower ascent curve is good, thus we don't do a 70/70, even if some are even flirting with this idea. But overall part of the thinking that brought deep stops had some logic (even if the research raised some doubts), so as long as we accepted that logic, we find it hard to completely drop it. Even if we know that the whole logic was based on just some theoretical model, which we have no clue yet whether it is correct or not (meaning that we don't yet fully understand the biological events that lead to a DCS hit versus just some inoffensive bubbling in the body).
D. All theoretical models that we currently use do not fully mimic the real events in our body (because, as I have said already, we even don't fully understand them yet). So, the models are close to reality in some ranges, and they get too far from reality out of those ranges. So the same algorithm is safe at some depth and unsafe at other depth. Because of this, in the pure recreational range, Shearwater is confident enough to propose the values of 45/95, 40/85 and 35/75. Those settings will mimic recreational computers (at 45/95 probably close to Aeris/Oceanic, and at 35/75 very close to Suunto). Is this safe enough? Probably so, as many dives are performed like this with good results. Outside of this range, they go to 30/70 and advocating 40/70 in their blogs, based on current research, and leave the tech divers to decide if they will take more risk or not. But the baseline is that 30/70 is good enough in most conditions, as long as you can afford the length of the deco.
So my point was that there are some common values for GF that seem to be good enough for some purposes, even if we don't have accurate data to really prove it. But outside of these well known settings, I see little advantage into playing with the GF. Let's say I'm tired and dehydrated one day. Should I put 30/60 or 40/60 instead of 30/70? Is this enough? Yes, I have extended my deco to compensate for some impossible to measure factor, but I cannot tell whether I've got the needed safety margin or not. It is just something for my peace of mind, but without any hard fact behind it. I could as well leave the computer with the default 30/70 setting and just extend my last stop or my last two stops by a few minutes, as long as gas and water temperature allow it. It is exactly as empirical as changing the GF, achieves the same result, and does exactly the same.
I'm not saying adding safety to compensate for particular conditions is bad. I'm just saying that when you say you adjust your GF according to the requirements of every dive sounds very precious and high tech, while it doesn't really mean so much. You play with some numbers a little bit more, but in the end it isn't much more advanced over just staying a few more minutes in the water at the end. In both cases you just don't know how much is enough and how much more is too much when you diverge from the baseline universally accepted GF values.
Am I missing something?
