The point of lowest O2 isn't just before you black out though. It's shortly after you take the first breath, regardless of whether you blacked out or completed the dive successfully. How hard you push doesn't seem to come into it?
That is correct Dave (or maybe sometimes the 2nd breath depending on the factors like quality of the first breath, O2 debt etc?) but I was answering to why we don't see a lot of the blackouts exactly 20s after surfacing (and hence what is the most crucial period to watch someone for a BO if you are safety).
The way I look at it (correct me if I'm wrong obviously) is that since O2 keeps going down throughout the dive, there are two cases:
1. O2 reaches the critical level and you BO before you manage to reach the point in time where you start reoxygenating the brain by yourself (i.e. shortly after surfacing and breathing). These will be a decent number of the BOs and they can occur at any time before you surface or shortly after e.g. if you pushed enough you can BO the split second you surface or 0.5s after etc.
In all these BOs, the time it takes from the first breath to reoxygenation is irrelevant as you never reach it so if you push enough, you will fall into this category of BOs.
2. The second 'category' of BOs are the more 'borderline' ones where you make it to the first breath(s) but because you are not reoxygenating fast enough you still BO as your O2 keeps dropping although you are breathing.
Whether he meant it like this or not, Sebastien's article implied that assuming you haven't BOed, your O2 will continue to drop up to 20s after surfacing. If this was indeed the case (I don't know) then the lowest point of O2 would be at 20s after surfacing.
To me though, even if this was the case, it wouldn't mean that the most crucial point for a BO is 20s after surfacing. Actually the BOs at exactly 20s would be quite a coincidence as it would mean that the dropping O2 reaches the critical point for BO exactly at 20s and not before but in most cases the diver would either reach it beforehand or never reach it (in which case he wouldn't BO).
Sorry Dave, probably not making much sense, it's a long-winded way of explaining something relatively straightforward.
Ultimately I still think the time into the dive when you will BO is simply depending on how much you push. Don't see how there can be a point in time (e.g. 20s after surfacing) when you are more likely to BO than say other points in time before that.
Assuming everything else is equal (ie you always perform the same recovery breaths etc), the time to BO after surfacing should purely be driven by your O2 level upon surfacing (if you make it) which in turn is driven by how much you pushed the dive right?
