Nitrox course. What's the point?

[doctormike]

I'd put it even harder.

It's easier to cure bent than it is to cure drowned, so if you're OOG, get to that huge, nice reservoir of gas called the surface atmosphere.

Sure, but I would make it clear to new divers that being OOG isn't the only reason to surface immediately. Plenty of people have died with gas in their tanks, after making it to the surface.

So new divers staying within NDL - if FOR ANY REASON, you feel like you really need to be on the surface NOW, that's OK. That doesn't mean rocket up from the bottom, of course, you should try to make a safe slow ascent. But it's OK to forget about the safety stop if you really want to get out of the water.

 

[Gareth J]

So that’s what I’m saying if a gas is dived to its max NDL there’s no safety benefit no matter what O2 %: people on here are saying the nitrogen loading is less using nitrox when leaving the water even if it’s dived to its NDL.

I have said this numerous times through the thread.

In simplistic terms, you can either:

• dive Nitrox as if it where air and get a safety benefit

OR

• dive Nitrox as Nitrox and gain extra in water time. And get no safety benefit [1]

Addendum
[1] Technically option 2, is not strictly true. As was explained by the DDRC medical team. Having a bend on Nitrox, compared to a bend on Air is far better for you. The prognosis for a full recovery is far better if you where using Nitrox in preference to air. There was a huge row about this when the medical team stood on up and said this at a BSAC conference, because the BSAC, at the time, still band the use of Nitrox for recreational diving.

 

[Celt]

The computer does not calculate NDL using any assumed amount of offgassing on ascent. How could it? What value of ascent do we use? 20 fpm? 30 fpm? 60 fpm? You are correct in that two divers, one using air and one using EAN36, if dove to the same NDL would have an equal N2 loading. On ascent to the surface the nitrox diver would have less residual N2 in their tissues because of "wash out" due to the higher O2. So, these are two different issues. NDL is defined as the time remaining at the current depth before mandatory deco stops are needed. Or, to put it another way it is the time remaining where a diver may ascend directly to the surface. There is no ascent implied in the definition. In my spreadsheet the algorithm I use does not use ascents. I posted the algorithm in another thread.

Will the computer not tell both divers to slow if there ascending to quickly, in other words control the assent so they are both offgasing enough

 

[boulderjohn]

I would like to jump in as someone who was once a career English teacher and talk about the fact that there has been much argument about of definitions of key terms in this thread.

In an American football game yesterday, the announcer said that a defensive player had tied an all-time record with his interception return. I will bet that no one in the listening audience shouted out, "No! He didn't set a record! A record is a vinyl disc that will reproduce a sound!" That is because he overwhelming majority of people understand that when we use words, the definitions depend upon the context. A word correctly used in one context would be incorrect in even a slightly different context.

Here are two examples:

Recreational: This word has many different meanings in many different contexts. If you do a google search for the phrase "recreational diving," you will find many of sources explaining what that means. Most will show that it can mean different things in different contexts. I certainly did not do a statistical analysis of the results when I did the search, but my wild guess is that more than 90% of the time, people who use the phrase "recreational diving" are contrasting with a common definition for technical diving and are referring to diving above a 40 meter depth with no decompression obligations and no hard overhead environments. Some people will use the term differently in different contexts, but that does not mean everyone else is wrong.

Safe: The word "safe" is used within the context of the activity. If you get technical about it, nothing is 100% safe. Whatever the activity, there is a range of behaviors that is considered to be within the safe zone. If you are standing at a street corner and a light signal tells you it is safe to cross the street, you can probably cross the street safely 99.9% of the time. On the other hand, when I was in Paris last year, I started to cross a street at just such a time and almost got hit by a car making a right turn at the highest possible speed, a violation of the law. A Parisian explained to me that in Paris, that law is routinely disobeyed, so in the context of Paris, one has to add an extra step in the process of judging safety. The same is true for diving. What constitutes a safe dive depends a generally accepted range of behaviors with the context of the dive.

 

[tursiops]

The tables don’t make any assumptions about ascent, safety stop, etc. in figuring out the NDL

The SSI tables are based on the US Navy tables, which are calculated based on an assumed ascent rate. If you go too fast, you don't spend enough time outgassing on the way up, and decrease your ambient pressure too fast for safe outgassing. If you go too slow, you effectively spend more time at depth than the tables are designed for, so you should use a reduced NDL.

 

[boulderjohn]

Air dive is to NDL which is 20 minutes. EAN32 dive is to NDL which is 30 minutes. So both divers now need to ascend. The tables don’t make any assumptions about ascent, safety stop, etc. in figuring out the NDL. Neither do the computers.

After writing an article on deep stops in decompression diving, I decided to write one on deep stops in NDL diving. I gave up after my research showed me

• There is not nearly enough research upon which to draw any conclusions whatsoever.
• The huge difference between decompression diving and no decompression diving is that in NDL diving, there does not seem to be any real difference in the ascent rate, provided you are in between a mad sprint to the surface and such a slow ascent that you eventually go into decompression. If you slow your ascent too much on a decompression dive, you will need to do extra time in shallower decompression stops. If you slow your ascent on an NDL dive but stay within NDL limits, you can still go directly to the surface, especially if you do a recommended safety stop.
• There is a gray area between NDL dives and decompression dives, a fuzzy region where you as an individual might be better off doing a brief decompression stop. The people trying to explain that were apparently stymied in finding a concise way to to say "OK, you are getting close to a required decompression stop here, so in view of individual physiological differences, we really think it would be a good idea to do one in this range of dives." They settled on the phrase "required safety stop."

 

[tursiops]

In simplistic terms, you can either:

• dive Nitrox as if it where air and get a safety benefit

OR

• dive Nitrox as Nitrox and gain extra in water time. And get no safety benefit [1]

This is NOT an either/or proposition. You need not choose solely between diving 21% and (say) 32%. If you dive your 32% as if it were (say) 28%, you would get BOTH increased time and MORE safety. You just have to mind your MOD for the actual 32% you are using.

 

[Gareth J]

So that’s what I’m saying if a gas is dived to its max NDL there’s no safety benefit no matter what O2 %: people on here are saying the nitrogen loading is less using nitrox when leaving the water even if it’s dived to its NDL.

That was certainly not what I said. If it was interpreted as such I apologise.

The whole point, way back early in the thread. It was noted that Nitrox is of particular benefit where repeat diving is being carried out.
This was because less Nitrogen was in the breathing gas, allowing a greater NDL dive time compared with air. This becomes more noticeable as the number of dives increase.
If you have the luxury of breathing optimum mix for each dive this becomes significantly pronounced over a weeks diving.
Recreational facilities may only provide standard gases, which generally means Nx32 or Nx36.
I have certainly done my 'shallow' second dive on a rich / optimum mix to flush Nitrogen so that the following morning I can maximise my dive time. [A typical behaviour in Scapa.] Yes, if I avoided the second dive altogether, I would have had more time available than if I did the second dive on an optimum Nitrox mix. But I am on holiday, with limited time and opportunity, so I am maximising my dives.
Which is back where we started.



[*] This is somewhat academic these days for me. Most of my dives are on CCR, so I am always on optimum mix

 

[Gareth J]

This is NOT an either/or proposition. You need not choose solely between diving 21% and (say) 32%. If you dive your 32% as if it were (say) 28%, you would get BOTH increased time and MORE safety. You just have to mind your MOD for the actual 32% you are using.

You are absolutely correct. I was taking the two extremes for simplicity.

In one of my slightly earlier posts I made your point. i.e. only diving to within 5 minutes of the NDL or a similar adjustment.

A similar option would be to choose to do a 10 minute stop at the end of each dive when diving within the NST.

Or as we used to do, breath 50% for 5 minutes at 6m at the end of every dive. Without adjusting the schedule for the 50%.

 

[Angelo Farina]

@mac64
To my view, diving to the edge of the NDL is aggressive diving.

Which is exactly my point...