Diving Nitrox to increase safety AND bottom time!

[TheDeuce]

Hey all!

I got my Nitrox card a few months ago and will finally have a chance to dive EANx in October. Since it’d been a while I grabbed my course booklet to give myself a refresher course. One of the things I recall hearing in my class was that we can use enriched air to increase our no-deco time and shorten surface intervals OR we can use it to increase our margin of safety. Indeed, I’ve seen that notion here on SB, in a copy of Dive Training magazine, and from any number of instructors and manifold divers, all more experienced than I. Hence, it never occurred to me to question it.

I’m questioning that notion now – and want to show my work. Since so many adhere to the maxim that ‘Nitrox either increases safety OR bottom time, not both’ I’ll be happy to admit my error, if I’m in error. I just don’t see where I’m erring.

Here’s my way of thinking:

Grabbing my trusty SSI EAN tables, I’ll plan two dives. Both are to fifty feet but one is on air and the other is on EAN36 (chosen because N36 and air both have 50’ laid out). Our fifty foot dive on air gives me a maximum bottom time of 70 minutes, at which time I’d be an “I” diver in terms of nitrogen loading. Doing that same fifty foot dive on N36 for 100 minutes makes me a “H” diver in terms of nitrogen loading – one full group designation less nitrogen load than I was on air. If I equate a reduced nitrogen load with ‘increased safety’ then we see that I’ve increased my bottom time, by 30 minutes, AND I’ve increased my safety – by a full group worth.

At best I thought that what was impossible was MAXIMIZING bottom time while increasing safety – I assumed that if I maxed out my NDL time on Nitrox I’d be loaded with the same nitrogen that I’d be on air if I maxed out my air NDL at that same depth. But even that’s incorrect, from what I’ve seen:

Two more hypothetical dives, this time to 40 feet. One’s on EAN32, the other is on air. If I max out my NDL time on air at 40 feet I’ll have had 130 minutes of bottom time and will be a “K” diver at the end. If I max out my NDL time on EAN32 at 40 feet I’ll have had 205 minutes of bottom time and will be a “J” diver. 75 more minutes of bottom time and a full group safer. Hence I’ve both maximized my bottom time AND still have increased my safety margin.

What am I missing???



m.

 

[redacted]

Tables produce a step function mapping rather than a continuous function mapping. This step function along with the "rounding up" could produce tjhe results you are seeing.

However, I agree that nitrox can be used to increase safety (reduce nitrogen loading), increase bottom time "or" decrease surface interval time; and the "or" really means "and/or". Again, tables my make this difficult to see due to the step functions used. It is much more apparent with the continuous functions used with a dive computer.

 

[doctormike]

This is just a question of semantics, not diving physiology... Let's take the example of EAN 32. The word "or" only applies to two options:

If you breath EAN 32 while setting your computer to EAN 21 (air), you will ongas less nitrogen compared to air, so (theoretically) you will reduce the likelihood of an "undeserved hit". There actually isn't a lot of strong data about what causes undeserved hits, but it seems reasonable that less nitrogen would be better from this point of view.

If you breathe EAN 32 and set your computer to EAN 32 (or use EAN 32 tables), you will be given a longer bottom time at any given depth (stay above the MOD!).

However, you could certainly set your computer, for example, to EAN 28 while breathing EAN 32, so you would have more bottom time than if you had set it to air, and (theoretically) more safety.

 

[TSandM]

Well, your analysis only works if the people who made the two sets of tables assigned the same nitrogen loading values to the pressure groups in both. Since one does not generally go back and forth between the two, there would really be no particular reason for them to have done that.

 

[TheDeuce]

This is just a question of semantics, not diving physiology... Let's take the example of EAN 32. The word "or" only applies to two options:

If you breath EAN 32 while setting your computer to EAN 21 (air), you will ongas less nitrogen compared to air, so (theoretically) you will reduce the likelihood of an "undeserved hit". There actually isn't a lot of strong data about what causes undeserved hits, but it seems reasonable that less nitrogen would be better from this point of view.

If you breathe EAN 32 and set your computer to EAN 32 (or use EAN 32 tables), you will be given a longer bottom time at any given depth (stay above the MOD!).

However, you could certainly set your computer, for example, to EAN 28 while breathing EAN 32, so you would have more bottom time than if you had set it to air, and (theoretically) more safety.

Entirely correct - as I see it.

Basically, once you've exceeded the NDL-air limit by 1 second, you've extended your bottom time. Between that moment and the moment you have loaded the same nitrogen you would have taken on at that NDL-air limit you've both extended your bottom time and increased your safety.



m.

 

[beaverdivers]

Good point TheDeuce. Diving Nitrox to increase safety AND bottom time!

Yes, you can do both.

On a rec. multi-level dive, you generally do not max. out your bottom time. So you do lower your risk of the bends.

I have tracked a week of rec. diving using two computers. One set on 21% and one set on 32%. I was actually diving 32%. We did about a doz. dives.

At the end of the week, my theoretical nitrogen levels were about 15% lower. In order to stay out of deco on my computer that was set to 21%, I had to either go up or do a longer safety stop to clear my air computer.

Many people say that you can only have one or the other, but you can have both if you don't max out your no stop time.

The problem with having your computer set on 21% nitrox and diving a richer mix is that your computer is not monitoring your O2 load properly.

 

[Zen Diving Inc.]

Im just wondering why you didn't do a dive on Nitrox with your course...? Or combined course's... You said you finally get too, so i'm assuming you haven't at all. I know that nowadays you cant get a card from certain online course's... Personally I think its BS but thats just IMO... Which is it, you have or haven't ever used EANx?

 

[windapp]

You dive nitrox to decrease your effective depth for nitrogen. What you do with that decrease in EAD is up to you.

 

[TheDeuce]

Im just wondering why you didn't do a dive on Nitrox with your course...? Or combined course's... You said you finally get too, so i'm assuming you haven't at all. I know that nowadays you cant get a card from certain online course's... Personally I think its BS but thats just IMO... Which is it, you have or haven't ever used EANx?

I have not yet dove Nitrox - the LDS course I took was a one-evening, two-hour session, all done in the local shop. I had picked up the SSI Nitrox booklet at Divetech a few weeks prior and was well prepped though. I'm the kind of guy who likes to know everything possible about what I'm learning and most certainly wouldn't have gotten that from the session alone (where the booklet wasn't offered).

I noted that the booklet made a reference along the lines of 'in your course you may dive Nitrox' but it didn't suggest that said dive was of particular importance. Not a sentiment I strongly disagree with since the differences between diving Nitrox and air come up in dive planning, not in the actual dive (MOD's notwithstanding).



m.

 

[windapp]

I have not yet dove Nitrox - the LDS course I took was a one-evening, two-hour session, all done in the local shop. I had picked up the SSI Nitrox booklet at Divetech a few weeks prior and was well prepped though. I'm the kind of guy who likes to know everything possible about what I'm learning and most certainly wouldn't have gotten that from the session alone (where the booklet wasn't offered).

I noted that the booklet made a reference along the lines of 'in your course you may dive Nitrox' but it didn't suggest that said dive was of particular importance. Not a sentiment I strongly disagree with since the differences between diving Nitrox and air come up in dive planning, not in the actual dive (MOD's notwithstanding).



m.

Taking a course with dives is absolutely crucial to getting value from it. It really isn't rocket science, and is something that could be easily self taught. If you have to pay $200 for the course, you might as well get a couple of free boat dives out of it.