Nitrox course. What's the point?

[Angelo Farina]

Right. But there is a clear line between a dive that requires staged decompression and one that does not, whether you call it tech or European style rec.

So we all need to be careful in implying that "significant modifications to the original plan [involves] no other consequences than some longer deco stops."

You made that statement in the new divers forum. In my opinion, that is a very dangerous implication.

I understand the problem, and I apologize if I did not express the concept more clearly, risking of confusing new divers.
The whole point is: planning a dive with deco stops is safer than planning a dive within NDL.
Most "underserved" DCS occur to divers planning dives within NDL, and exceeding the NDL limit. If the dive was planned with deco, it had been less dangerous...
If your certification agency calls this "tech", I have no problem: I recommend to all new divers to extend their training up to the point where they can plan and perform deco dives safely. That for me is the minimum level of training for making diving truly safe.

Which bring us back to the original topic of this thread: the Nitrox course.
For moderate depths, say 36 or 38 meters max, Nitrox gives a good advantage over air, allowing a larger NDL. But it is wrong to think that using Nitrox, as the dive now is within NDL, then it is safer. Actually, I consider it less safe, as you are not anymore taking the precautions and equipment required for a dive with deco.
In fact, if I get Nitrox for a dive at moderate depth, I use it happily, but I keep my plan as if I was diving air (or, using the computer, I do NOT switch it to Nitrox mode). I will make my planned deco stops, albeit probably unneeded, and exit water safer and happy...

 

[EFX]

Yes, you can also use your DPA formula, but that requires adjusting the DPA based on what kind of water you are diving in to get your MOD, which will give you a MOD in feet or meters (for that specific salinity). And then using the SAME SALINITY FACTOR to convert your DC's measured ATA to a depth in feet or meters, so that you can compare the two numbers and see if an alert sounds. That's what I mean by putting it in and taking it out.

OR, you can just take the ATA value that your DC measures directly, multiply it by the FiO2 that you know from analyzing your gas, and if that number is more than 1.4, you ascend. And that simple process does not require a salinity factor.

Finally.

It's needed for the USER, since divers like to see how many feet deep they were. I have no idea what the source code is, but I understand the math enough to know that if they work by using that salinity factor to convert from ATA to feet, then they have to use the SAME factor on the other side of the equation when you are figuring out what your PO2 is at that depth. Which means that you are putting it in and taking it out.

Seriously, PPO2 = FiO2 x ATA. PPO2 is what we care about for ox tox, ATA is what the dive computer measures. Not sure why you think that that calculation needs to be adjusted for salinity.

OK. Go and reread my posts. I am NOT promoting one method over another. In fact, I agreed with you. However, *IF* you want to use a pre-determined MOD you have to use a salinity factor. I think you like to argue. Have you met my wife?

 

[Gareth J]

The conversion from pressure to depth is always needed for the user. How do you know that dive computer's use only the pressure and not calculated depth for MOD. Do you have access to a dive computer's proprietary source code? If so, give me a link.

I think I said the conversion was always required because divers thing in terms of depth, either meters or feet dependent. All modern computer can have the units displayed changed.

I am not sure if any computers on the market allow you to adjust for salinity (water density). Certainly my different Suunto's don't, neither does the OSTC or the APD CCR.
The CCR computer is certainly particularly concerned with accurate pressure measurement in preference for depth measurement. It certainly works with pressure, mbar measurement is part of the startup and calibration procedure. The earlier version didn't even display depth. Depth and time where only introduced on the second generation control systems. Similarly, the VR3 worked with pressure, again, part of the start up and calibration procedure for those units setup for CCR.

I do know the water density changes for my different dive destinations. Local fresh water quarry, pure spring water (Iceland), UK south coast, UK sea lochs, and UK freshwater lochs, mediterranean, Red Sea.

I don't really care about the water density, because the computer is working off pressure. I don't care if I am in real terms 30cm deeper of shallower than the depth shown.

 

[doctormike]

I understand the problem, and I apologize if I did not express the concept more clearly, risking of confusing new divers.
The whole point is: planning a dive with deco stops is safer than planning a dive within NDL.
Most "underserved" DCS occur to divers planning dives within NDL, and exceeding the NDL limit. If the dive was planned with deco, it had been less dangerous...

Sorry to beat a dead horse, but apparently that's what I do.

Yes, PLANNING a dive with deco stops is safer than planning a dive within NDL and then having no idea what you are going to do if you mistakenly get a deco obligation.

But it seemed to me that this sentence of yours says precisely the opposite of that:

"significant modifications to the original plan [involve] no other consequences than some longer deco stops."

So if you made a plan to do an NDL dive, and you had enough back gas for that, but then you significantly modified that plan, how do you know that you would have enough back gas for your "longer deco stops". Or is this just a semantic thing, and now your modified plan is your new plan, because you did the modification before the dive and made sure you had enough gas. Maybe I'm misunderstanding what you are saying.

 

[ginti]

The whole point is: planning a dive with deco stops is safer than planning a dive within NDL.
Most "underserved" DCS occur to divers planning dives within NDL, and exceeding the NDL limit. If the dive was planned with deco, it had been less dangerous...
If your certification agency calls this "tech", I have no problem: I recommend to all new divers to extend their training up to the point where they can plan and perform deco dives safely. That for me is the minimum level of training for making diving truly safe.

Are you talking about just adding a stop at 6m for 5 or more minutes even if the dive was within deco limits?

Nowadays agencies call that safety stop, and as far as I remember they teach it very often and they also recommend to do it any time you can...

 

[Storker]

I'm not sure what you mean by deco stops if you aren't trained as a tech diver. What method are you referring to?

Not all agencies have a hard border between no-stop/"rec" and staged deco/"tech". My CMAS 3* cert certifies me for "light" backgas deco, as would several BSAC certs. We're still recreational divers. If I were to present as a technical diver, I'd want to be trained for accelerated deco and preferably also trimix.

 

[dmaziuk]

I am not sure if any computers on the market allow you to adjust for salinity (water density). Certainly my different Suunto's don't, neither does the OSTC or the APD CCR.

Shearwaters let you choose between fresh, salt, and EN13319

 

[doctormike]

Not all agencies have a hard border between no-stop/"rec" and staged deco/"tech". My CMAS 3* cert certifies me for "light" backgas deco, as would several BSAC certs. We're still recreational divers. If I were to present as a technical diver, I'd want to be trained for accelerated deco and preferably also trimix.

Gotta put the Tech / Rec argument on the bingo card. If you don't get paid to dive, you are a rec diver, right?

 

[Blackcrusader]

You know, someday they will invent a fork, a thing to help you eat. I'll bet it will be a whole lot safer and easier than spearing everything with your knife.

Mostly I use chopsticks but knife and fork does come in handy at times

 

[Gareth J]

Shearwaters let you choose between fresh, salt, and EN13319

To be honest I struggle to see the advantage. The sensible thing would be to choose a 'average salinity', and not bother with any further adjustment.
The Salinity changes between different oceans are
Baltic Sea 10g/kg
Black Sea 13-23g/kg
Oceans (general) 34-36g/kg
Mediterranean 38g/kg
Red Sea 36 - 41g/kg

I can't find a figure to show the actual effects on depth (density).

As we have discussed, it doesn't really matter, calculations are based on pressure not depth.

I suspect the variation in accuracy of the depth sensors is greater than the effects of the salinity of the water. Different computers regularly give different depths, even the same model.