Noel,
I think I can come up with some guidelines.
First of all, among my friends and I, we find that our recreational limit is half or less of our maximum depth. For example, take a diver who has reached 150 feet on a line, with a buddy, and felt he was near his limit. As a basic rule, that same diver will have a ULTIMATE safe limit of 75 feet on a 'fun' dive. That would only be in EXTREMELY IDEAL conditions and usually his safe limit would be much less than that. My point is that if you are ever diving to more than half of your max, without a descent line & buddy, then you are risking death.
However, in my own case, the one time I ever suffered a samba on a 'fun' dive was a dive to 23m for 1'30", when I had previously done 5'02" at 20m and 3'16" at 88m in supervised conditions. My point is that on the 23m/1'30", I was at about 1/4 of my max depth, at less than 1/3rd of my max dive time. The problem was I took too short of a recovery since my previous dive, and I couldn't feel satisfied. I also had a thick suit and heavy weight belt.
From that experience, I have found a simple, but labour intensive, method of checking your risk factor. That method is to do moderate static breath-holds on the surface. If I would need to do a dive closer to my safe limit, and I was wondering if I was drained etc., I would take one breath through my snorkel and hold until I felt the first stimulus to breathe (mouth still on the snorkel!). Then, I would wait a little longer (20 seconds or so), exhale, take two or three breaths, and then hold gain until the first stimulus to breathe. For me, I know when that stimulus should come, if I am rested, recovered and in the zone. If the stimulus comes extra early (or extra late), then it is not safe to do a dive. The test must be done several times to wipe out the effect of any excessive breathing you have done after recovering from your dive.
Further, diving with a heavy weight belt, in a configuration where you sink most of the descent and do all the work on the way up, vastly increases your risk--it was another contributing factor in my 23m/1'30" samba.
With experience, you get to know your own body to the extent that you can reduce your own risk factors, along with common sense like the above suggestions. It is my personal belief that with slow, gradual experimentation, along with occasional 'max effort' dives with supervision, one can learn one's own body to the point where moderate recreational dives are far safer than sports such as skiing.
Many people criticize those who push their limits in controlled, safe conditions. But, it is my belief that the only way to learn your own signals & feelings is by pushing it under safe conditions. The highest risk dives I ever did were BEFORE I had ever done any 'max' effort dives under supervision, and they were before I had ever had a blackout. Once I had done max effort dives under safe conditions, I slowly learned to know my body far better than I did before.
However, NO internal signals or feelings can ever act as a foolproof guide DURING THE DIVE.
For example, in my case, I always pack before starting my dive. This increases the safety for me in a strange way. If I am dehydrated, or I have low blood pressure, or if I am very hot, or if I breathed too much before hand, then I will get light headed during the packing -- A SIGN THAT I SHOULDN'T MAKE THE DIVE.
For example, my first ever blackout was the worst of my diving career. I was diving with a buddy, and I was incredibly hot and dehydrated. I tried to pack my lungs, and I did, getting very light headed. I started the dive and actually blacked out a couple of seconds into the tuck dive, from the packing effect. I woke up underwater at 8 feet of depth uncertain of where I was--it was murky, so I assumed I just lost the line. I went back up, told my buddy I lost the line, and then breathed AGAIN for 3 minutes (even though I hadn't accumulated any CO2 from the 10 second dive). Then I tried packing and felt the light headedness coming, so I stopped packing and dove without any packing. I went 8m farther than my pb and blacked out badly at around 2m on the ascent. At the time I had no clue about the 'signal' of getting light headed during the packing, etc.... So, if you look at that dive, here was what went wrong:
- Extremely dehydrated
- Way too hot
- Ignored signal of light-headedness during packing
- Dove WITHOUT ANY PACKING, meaning 3L of less air in my case
- Tried to break my pb by 8m (way too much), a pb which was set WITH PACKING
- Used 3-4lbs MORE weight on my belt than I had used on the pb dive
- Ended up breathing far too much, due to the initial blackout from packing, then breathing up from the beginning again