A couple observations:
1. Nitrox and improved gas consumption.
There is some evidence that divers can improve their gas consumption on Nitrox. But it poses significant risks. The primary trigger for a diver to inhale is elevated CO2 levels. If a diver has developed a tolerance to higher CO 2 levels, they can go longer between breathes and can theoretically use more of the O2 in a given gas mix. So under certain working conditions where O2 levels on a 21% O2 mix would otherwise be excessively depleted, enriched air Nitrox would still retain a comparatively higher PP02 and be much more useable to the diver which would conceiveably allow longer pauses between beathes and in turn allow better gas consumption. But at the cost of elevated CO2 levels.
The US Navy experimented with O2 enriched mixtures in the 50's and abandoned the idea as they found the effects of oxygen toxicity were too unpredictable. (This became one of the major objections to recreational use of nitrox 30 years later.) In retrospect it appears that one of the issues was the sample of divers they used in their research. Most were accomplished hard hat divers and the helmets of the day were, by modern standards, poorly ventilated and consequently those divers in the experimental group all had a very high CO2 tolerance and probably breathed accordingly on open circuit scuba. Research since that time has correlated elevated CO2 levels with a greater risk of oxtox as well as increased effects from nitrogen narcosis and that largely explains the US Navy's observations.
So in short, trying to improve your gas consumption by intentionally beathing less and in turn elevating your CO 2 levels is a really bad idea. Nitrox just makes it easier for less than perfectly fit divers to do this. If you are wanting to improve gas consumption, do it by reducing all extraneous movement of appendanges and the torso and by locomoting only when you have to and no faster than you have to. This reduces both O2 consumption and CO2 production.
2. Nitrox and improved safety.
It makes sense that if you do the same dive to the same depth on a gas that results in less N2 loading, that it may be safer from a DCS standpoint. However if you do a longer dive to the same depth on nitrox and more fully load one or more of the theoretical tissue compartments, you should not be expecting any greater degree of safety.
The other side of the coin is O2 exposure. At the recommended PPO2 of 1.4 for the working portion of the dive and 1.6 for the deco portion, it's virtually impossible for the majority of divers to oxtox themselves. But people and diving practices vary, so being conservative is a good idea. If you aren't nitrox does pose additional risks.
3. Feeling better on Nitrox
In practice, I found that when diving a lot (3-4 dives per day over a 2-3 day weekend) that I felt a lot better on Monday morning after diving Nitrox than air. But in these cases, the volume of gas consumed was the same and dive times were no longer. (I often deco dive, so NDL's were not a factor and dive times were gas limited, not NDL limited and times were the same regardless of the mix.)
However, I also found that when I switched to a more conservative set of tables/software (DPLan) using a bubble gradient model and incorporating deep stops and generally longer ascents, that I felt even better on Monday even if the dives were done on air. So there is more than one way to skin a cat when it comes to feeling better.
4. The $1500 restrictor analyzer.
I bought an Oxycheck ECII do it your self O2 analyzer kit for about $130.00 with a free upgrade to their expedition type restrictor and sensor chamber (the basic model was out of stock). Even if you pay for the upgrade, you could build 9 or 10 of them for $1500.00.
5. To own or not to own an analyzser
Owning your own analyzer gives you a bit more confidence in your gas. Depending how it is mixed it is not unheard of to get a reading right after mixing and to get another significnatly different reading several hours later. If filled "properly" (very slowly) partial pressure mixed gases do not mix well and the tank needs to be rolled, etc to agitage and mix the gas completely. Personally I do it "wrong" by putting the last 200 psi in pretty fast to create turbulence in the tank to mix the gases and I get very accurate initial readings with no need to roll the tank.
So if your shop uses partial pressure mixing, you definitely want your own analyzer.
Another advantage of owing your own can be safely getting additional mileage out of the left over nitrox in your tank. For example with a 3000 psi tank, if I have 760 psi of Nitrox 36 left, I can top the tank to 3000 psi with air and obtain a 25% mix that is the suitable for a deep dive to 150 ft or is at a minimum a bit more conservative than air at recreational depths.
Some shops will not let you do this due to fears of the gas mix moving up stream, etc, but those shops should also be operating the valves to prevent that from occuring anyway and should be filling slowly as it is a safe bet that at least some nitrox divers are having their left over nitrox tanks topped anyway either intentionally or by omitting to let the shop know there is nitrox in the tank.
In any case if you are refilling a less than completely empty nitrox tank with air, it really needs to be analyzed and then properly labled with the final mix.
