Nitrox Rule of Thumb 32 vs 40

[The Chairman]

I would definitely go with @Dr Simon Mitchell over me on anything to do with physiology.

 

[tomfcrist]

Hopefully Dr Mitchell will forgive me for reposting ...

Note the last line of his post.

 

[uncfnp]

Note the last line of his post.

I did as I am sure you noted the first. One does not negate the other.

 

[IncreaseMyT]

1.4 on 36 is 95 feet.

To your second part, it depends on your profile. Say for example you're diving the Florida Middle Grounds using 32%. These ledges can be 30 feet tall. So you drop down off a 100 ft. ledge to 130ft./1.6ppo2 to grab a fish, lobster or take a pic and then swim back up on the ledge. Even repetitive dives, for 99% of divers, you haven't had enough exposure to cause concern.

Yea but my thing is, I don’t think it will give you enough extra bottom time based on your air. I think you would still run out of air first.

 

[CuzzA]

Yea but my thing is, I don’t think it will give you enough extra bottom time based on your air. I think you would still run out of air first.

Depends on what kind of tank you're using. Last time out I could just barely squeeze three NDL dives on the grounds from a HP100. Most of the time I could only get two while my infinitely more experienced buddies were getting three.

 

[rick00001967]

Who recommends 100 for recreational divers?

SSI does......until the diver gains the confidence and comfort level along with proper training, it is "recommended" to stay shallower than 100 feet.

 

[Lorenzoid]

"Many Nitrox divers make life even easier. They pick EAN40 for all dives shallower than 30 m/99 ft as this gives the most bottom time and pick EAN32 for dives right up to the maximum sport diving depth limit of 40 m/132 ft."

Most dives don't involve a diver getting to "pick" their mix at all. At a dive resort or on a liveaboard, you will pick up a pre-filled tank of whatever they bank, and it's almost always going to be EAN32. If you do happen across a dive shop that will offer EAN40, and that's what you are seeking, it probably won't make your "life even easier" because you will have to wait for them to custom mix it for you. John mentioned some dive shop in FL banking EAN40, but that has to be extremely unusual among all the dive shops worldwide. I am what you would call a typical traveling recreational diver, and in something approaching 500 dives I can count on one hand the number of times I have been offered anything but EAN32 without making myself into a special case to be dealt with separately from all the other divers.

Fine question from an academic perspective, though.

 

[HamTrainChickenLaser]

OK I glanced over this thread and SSI is mentioned many times, but nobody seemed to catch that even though we are in the SSI forum, this is based on SDI material. I know because I just finished the SDI nitrox course tonight, and I was also under the impression that 132 feet is OK for EAN32 and that PPO2 of 1.6 is OK. I found this thread with a google search because I was watching this video and she was talking about shallower limits for EAN32 and I wanted to double-check:


So I guess I should be treating EAN32 as 111 MOD? The OP started this thread in 2018 and here we are many years later and apparently SDI isn't having issues teaching this material the way they do, but is a PPO2 of 1.6 a big no-no?

 

[HamTrainChickenLaser]

From SSI


My question : Why the higher EAN for shallower dives vs low EAN for deeper dives?


Seeking to understand



View attachment 471762

Thank you

OP says "from SSI", but if you look at the graphic it is clearly SDI.

 

[inquis]

PPO2 of 1.6 is OK

It's fine on a mandatory deco stop, but a PO2 of 1.4 is the recommended limit during the bottom portion. This avoids a cascade of issues.

When "working" (e.g., kicking against a current), CO2 buildup is a consideration. (It's more difficult to exhale gas as you go deeper, and studies have shown about a 50% reduction in that ability at 100 ft.) In turn, the additional CO2 significantly increases the likelihood of an oxygen toxicity event when combined with a PO2 of 1.6.

As a general rule, the point is to avoid this possibility, and to put it simply, a MOD based on 1.4 has a larger safety margin against toxing in the presence of CO2.