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you can calculate the best mix for your depth by using the formula:
where ATM = (planned depth/water type) +1
fresh water is 34
salt water is 33
so, for example, if you want to know the best mix for a 100 foot dive fresh, your ATM
is 3.9 and your optimum mix is .35 O2 (Nitrox 35)
given the +/- 1 Nitrox percentage marging of error, 36 is about the top you'll ever need to use. narcosis can become a real problem past 100 feet. of course, many people dive much deeper than that on air.
as for Nitrox 32 ... well... its disadvantages (time wise) compared to 36% don't kick in until about 70 feet, so ... maybe someone thought it would make a good "shallow" mix
They are special because they were chosen by NOAA to be their standard mixes.
NOAA Nitrox I (EAN32) tables were published in 1978. NOAA II (EAN36 tables) were published several years later.
With the limit of 1.6ppO2 commonly used those days (with a 45 minute exposure limit) the MOD for NOAA Nitrox I is pretty close to the 130' limit that is another old USN/NOAA standard for many types of diving.
The choice for NOAA II isn't as clear, which is perhaps why the NOAA II tables weren't published until a dozen years later in 1990. Too rich of mix will run into oxtox problems before hitting NDL limits. The extreme example is EAN100 which has MOD of 20' or less which brings zero benefit since you have essentially unlimited NDL on air at 20'.
You can repeat the oxtox and NDL calculations for progressively leaner mixes and see that somewhere around EAN40-50 is the richest mix that makes sense.
I just took my Nitrox class -- just wondering why are EANx32 and EANx36 "special" enough to have their own tables -- why not some other O2 percentage?
There are tables for blends other than Nitrox I and II.
As has been mentioned, NOAA published 32% and 36%. Maybe these were arbitrary, maybe not (read Charlie's post). However, these became the most common banked blends, so it's not surprising that many shops will only have those tables on hand.
Actually, on the last liveaboard I was on, the amount of Nitrox was variable according to the sensor. On one dive the amount might be 31.7% and on another 30.3% and another at 33%. My guess is since each tank was never emptied all the way there would continue to be different amounts in each tank and maybe someone had put just compressed air at one time in a tank and it was never emptied all the way / or maybe the sensor wasn't held in the exact same position that I held it on the previous check. What ever the reason you should realize that the reading on the sensor will probably never be exactly 32% or 36%. Maybe you've already noticed that a tank fill on a dive boat is never exactly to 3200/3100 or even 2600, it's always 3-something or less.
So while you are diving with nitrox you will also be approximating.
A lot of analysers are only accurate to within 1% anyway so anything inside that is meaningless.
Most places i go to now dont need tanks empty to nitrox fill and simply calculate whats needed to alter the mix. (within reason).
31.7% and 33% are both within probably error of analyser so fine for 32%
30.3 is a touch low but not a lot.
Remember analysers ideally need adjustment settings for altitude, humidity and temperature if not calibrating with clean dry air.
Anyone taking offence at anything in my posts - tough. It's only an internet forum. Stop being over-sensitive. The real world isn't as warm and fuzzy.
Remember, underwater only YOU are responsible for YOUR own safety. Nobody else is.