Nitrox course. What's the point?

[doctormike]

I kinda agree, but to me "required" means "mandatory". And this is an official publication from the biggest certifying agency in the world. Which I used to make a rhetorical point about deco being a hard black line drawn through a fuzzy gray area.

Right. There is a lot of fuzziness about decompression stress. There shouldn't be about the definitions of words, otherwise there is no point in having a discussion.

Assuming the standard definition of "safety stop", saying something is a mandatory safety stop is ludicrous. The fact that the biggest certifying agency in the world said it is irrelevant. We can all cite examples of very powerful individuals and organizations, with far more global influence than PADI, saying things that don't make sense on examination.

So it's a safety stop, or it's a mandatory stop.

 

[Gareth J]

My main point is it is a safety stop, not a hard stop.

For new divers. You are doing NDL diving, which means if you have an issue, return to the surface. You DO NOT NEED to stop at 3 minute. Being out of the water, or at least on the surface is safer than being under the water with a problem!

Yes it is DESIRABLE to do the safety stop.

 

[doctormike]

My main point is it is a safety stop, not a hard stop.

For new divers. You are doing NDL diving, which means if you have an issue, return to the surface. You DO NOT NEED to stop at 3 minute. Being out of the water, or at least on the surface is safer than being under the water with a problem!

Yes it is DESIRABLE to do the safety stop.

SUPER important point. And we often forget - as we bicker about bailout and isobaric counterdiffusion in the new divers forum - that a lot of people read these threads who are new divers!

If you feel the need to get out of the water NOW, get out of the water NOW. Even if skipping a safety stop contributes to your overall decompression stress. There are plenty of stories of tragedies that happen near the surface, with panic spirals.

 

[ginti]

What you could say Dody is that doing dives without planning are not safe. Let's take the vacation diver.

As someone already stated, diving is in general very safe (that is, very low risk calculated as a ratio between accidents and dives). And most dives are made by vacation divers, so they are in theory very safe according to that definition of risk.

So what exactly do you mean with safe?

 

[Storker]

You are doing NDL diving, which means if you have an issue, return to the surface. You DO NOT NEED to stop at 3 minute. Being out of the water, or at least on the surface is safer than being under the water with a problem!

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. You might spend your next 5-6 hours being thoroughly bored in a very narrow room with steel walls, and that would suck a lot (so don't go there). But being dead would suck a lot more.

 

[Belzelbub]

@mac64

You answered your question yourself .

I don’t believe the examples you provided relate to the part that Mac is struggling with.

Instead take two dives to the same depth. One done on air, one done on EAN32. For example, let’s assume the dives were done to 98’. My SSI table has depths great are pretty close for both. Air is 100’, EAN32 is 98’ on my table.

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.

Assuming both follow the same ascent rate, and do a safety stop at 15' for 3 minutes, the EAN diver may end up with lower tissue saturation as they would have been breathing less N2 during the decompression stage of the dive. Is it a consequential amount? Probably not, but if there wasn't a difference, there would be no reason for decompression divers to switch gas during their deco stops.

Main thing is that both tables and computers calculate NDL as the time left at a particular depth. They don't include the ascent, because they don't know what that would be, and in most cases (barring real slow ascents) the on gassing is done at that point and off gassing is beginning.

In reality, in the dives above, most computers would actually show more time in the NDL box because they would give credit to the time spent above the max depth. Tables don't. The computer takes a snapshot at a certain frequency. They don't calculate continuously, but probably more like every 5 or 10 seconds. Some computers allow you to change the frequency, others don't. A diver manually calculating at that frequency would be kind of pointless as they wouldn't get to join the dive.

Oh, and to address another statement about divers who don't pre-plan dives being unsafe. I guess it would depend on what is meant by pre-planning. Most of my dives are around 50' or so. I dive EAN 32 generally. At that depth I have plenty of NDL, so my dives are gas limited. I don't plan each dive. Now, if diving to a less typical depth, I will take a quick look at the plan function on my computer to get an idea about how much time I have available, and during the dive, I will pay closer attention to the NDL.

 

[Celt]

@mac64

You answered your question yourself .

If two divers enter the water as a buddy pair. One on Air (Nx21), on on Nitrox 32 (Nx32). Both dive with the computers set for air.

Dive Option 1
One diver is breathing 79% Nitrogen, one diver is breathing 68% Nitrogen.
When they leave the bottom following the air profile, the diver using Nitrox has less nitrogen in their system (tissues), than the diver on air. Hence the diver using Nitrox has a safety buffer.

Dive Option 2
One diver is breathing 79% Nitrogen, one diver is breathing 68% Nitrogen.
The diver using air dives with the computer set to air, does a square profile and follows the NDL dive time, surfacing
at the limit of the NDL.
The second diver (Nx32), does the same square profile, with his computer set to Nx32. He files the NDL dive time, surfacing at the limit of the NDL.
The computer (Nitrox table) gives the second diver a longer dive. When he leaves the water, he has the same nitrogen saturation as the air diver had when he left the water at the end of his dive.

The two options produced to different risk factors.
Option 1 gave the Nitrox diver a 'safer dive' (with regards to DCI), than the air diver. Because although haveing the same dive as the air diver, his had less nitrogen in his system at the end of the dive.
Option 2 gave the Nitrox diver the same 'risk' of DCI as the air diver, i.e. he dived to the edge of the NDL, but he had a longer dive.

The air diver could reduce his risk of DCI, if he choose to ascend with 10minutes of NDL left on every dive.
The Nitrox diver could follow the same practice, choosing to ascend with 10 minutes of NDL left on every dive, he would have the same risk as the air diver, but longer dives.


In the above example we are assuming a perfect profile to the same depth for each diver. (i.e. how a table is calculated.)

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

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.

 

[Gareth J]

Yes @doctormike & @Storker

"It's better to be on the boat wishing you where in the water, than in the water wishing you where on the boat!"

 

[Celt]

The computer calculates the NDL as the amount of time you can remain at the current depth without incurring deco obligations. At that point, it has no idea what is going to happen next. If you go up a bit, the computer will recalculate and display a new number.

If using a dive computer that displays surf GF, both dives to NDL will show the same at NDL = 0. Assuming of course that the same GFs were used. The EAN diver will now offgas more N2 due to the richer mix.

What I’m saying is at the surface they will both have offgased the same amount of nitrogen when the dive is finished.

 

[EFX]

If you dive them both to the NDL, then the safety levels are comparable, though I would give a slight nod to the EAN dive as you’d be breathing less N2 during the ascent and Safety Stop.

I can’t get my head around this, if a computer calculates a dive to NDL. Surely it calculates the same amount of offgasing needed on the assent no matter what gas is been used. Therefore there is no difference in the nitrogen loading at the end of the dive. What I missing. Are you saying that 2 divers using the same computer set to the same conservancy . One dives 21% O2 to NDL the other dives 32% O2 to NDL the computer sends one of them to the surface with a greater nitrogen loading?

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.