In cold water there are other factors to consider. Deco is not an exact science. Individual physiology has a large part to play in it. People have toxed at less than 1.4.
A diver I had been on boats with in Erie toxed at 1.3 in a cave. He felt it coming on and even tried to get higher in the cave to ward it off.
I don't doubt what you're saying, but that's new information for me. The medically-based controlled studies I've seen haven't documented a case at below 1.7. What symptoms did your acquaintance have? Was hypercapnia a possible alternative explanation for what occurred?
Steve Lewis taught me about risk factors to take into account. Assign values to them based on my comfort level and adjust my level to them. Cold water, dark, high current, my age, fitness level, deep, etc. All of those need to be considered. Depending on the dive and the task I may be ok with 1.4 in Bonaire. Inside a wreck in the Great Lakes? 1.3 or even 1.2. Depends also on how many dives I'll be doing and how much deco.
I think that makes sense.
I am trying to say there are reasons all the agencies picked these MOD numbers.
One of my questions was whether all agencies use 1.4. I know PADI does. Do GUE, TDI, SSI, etc., also use 1.4?
The reasons are that now and again someone fits and drowns. Normally that happens with long exposures or high ppO2 but not always.
I was under the impression that the lack of studies, an abundance of caution, and wildly varying and poorly understood variation from one individual to the next (and one dive to the next) also played a role. I am unaware of any confirmed cases of ox tox at 1.6 in the literature.
if you don't trust the O2 analyzers, how are you sure that you have the exact mix unless you dump the tanks before you fill every dive?
The bank analyzers use the same sensors as the handheld ones, and same as the ones in rebreathers. If you have your own, you know how it has been treated.
Trust but verify. I believe the O2 analyzers, for the most part, I just don't consider them reliable enough indicators to take precedence over other data that should also be valid. If you have two clocks and they disagree, one of them is wrong, same principle here.
If I were to always use 32% I would never have to dump the tanks, right?
Does it calibrate properly? If yes, then it's calibrated, you should calibrate against an air tank, but should note that the atmospheric air should be close to 20.9
Common failure paths. Was the shop calibrated against the same "air" tank? How do we know it was air? If it is 24% because of, say, residual O2 in the nitrox stick, then the shop's analyzer and my analyzer would both read low on the sample. For example.
Another factor that you aren't 100% sure of with the bank analyzers is if the gas is fully mixed before it got to the tanks. Remember that the sensors measure the PO2 not the FO2, so they have to analyze on the intake of the compressor. There may have been insufficient blending before it got to the sensors, and may have picked up CO in the compressor if there was a bad ring in one of the stages. Rare, but people have died because of it.
I think that we share a similar level of appropriate paranoia.
In technical diving we have to rely on the handheld analyzers because any sort of PP filling, or topping off is not analyzed by the fill station, so you have to trust the analyzers and we almost never dump gas before we fill normal backgas bottles. Good enough for that use should be good enough for you.
I'm not sure quite how to respond to that. There are plenty of other ways to control and confirm gas mixing ratios. If I were doing PP filling I would be using as accurate a pressure gauge as I could reasonably use, and would also use an accurate scale to weigh all cylinders before and after. Mass is something that can be measured cheaply and with great precision. With knowledge of the molar mass and a little math the FO2 can be determined with great precision.
