Nitrox course. What's the point?

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If you don't set your DC correctly for water type it will give you a MOD alarm at the wrong depth. The MOD calculation is dependent on the depth of 1 atm (gauge) of water. Here is the formula for calculating MOD:

MOD = ((ppO2 / fO2) - 1) x dpa ...........;where dpa (depth per atmosphere) is 33 ft for salt and 34 ft for fresh water.

For a ppO2 of 1.4 atm, a fO2 of 36%, the MOD for salt and fresh water is 95 and 98 ft respectively. If you set your DC for fresh water instead of salt your DC will alarm at 98+ ft where you actually have exceeded your ppO2 limit of 1.4 atm. Setting your computer correctly for the actual dive you're going to do is important!

I don't think that this is correct.

MOD alarms are entered into the computer as PPO2 (i.e. 1.4), not as a depth. You aren't calculating a MOD, you are selecting a MOD.

You also enter into the computer what type of water you are diving in, but what that does is change the internal constant that the computer uses to convert a measured ambient pressure into a depth in feet or meters.

The computer uses the measured ambient pressure and the FiO2 of the breathing mix to calculate the PPO2, and if it's over the alarm value that you selected, you get an alarm. The constant that is used to generate your depth display has nothing to do with that.

So if you dive in salt water with your DC set for fresh water, you will get an inaccurate depth readout, but the only way you would know that would be to somehow compare it to a linear measure like a marked line. It would not change the safety of the dive. Your computer would alarm when your PPO2 reached 1.4 (or whatever you had set it to), and that calculated value is not going to be dependent on what you selected as the salinity constant.

PPO2= ATA x FiO2, whether you are diving in water, floating in air, or submerged in mercury.
 
Right, that's exactly what I think. From an educational point of view, it doesn't make sense to try to get people to think about diving to "atmospheres".

I guess the basic issue is that the salt/fresh setting on your DC is a fudge factor that will make - for example - your MOD alerts accurate in terms of actual depth under water, even though we have no way of detecting our linear depth under water, so that's really irrelevant. All we can do is detect ambient pressure. If we set the setting wrong, the linear depth will be wrong, but that wouldn't change the MOD or N2 loading calculations.
A thing a lot of people forget with regards to the "fresh, EN13319 and Salt is that they are arbitrary figures to try to give a "best guess" actual depth for display purposes. They relate to densities which are 1000kg/m3, 1020 kg/m3 and 1030 kg/m3 respectively.

A few dives I have done show the relative futility of worrying about these settings:
1) The Red Sea ranges from 1035-1041kg/m3 (as an enclosed sea with high temperatures it is more susceptible to evaporation) so the Salt setting is "theoretically" too low.
2) Sea lochs in Scotland (open to the sea at one end and freshwater at the other) - a number of these are subject to tides and as such the salinity can vary between fresh and salt. This can be sometimes be noticed in even as short a stretch of water as 30-50m during a dive as you can see the halocline where the two bodies of water meet. So on a single dive you can have all three settings of you DC being valid at separate stages of the dive but invalid on others.

With both of these situations, there is no way you can set a DC to the correct "depth" range unless you have a hydrometer on hand.

None of this affects the important calculations such as MOD/N2/NDL as they are calculated by ambient pressure as you state.
 
The answers I write are really for Dody as he is the one asking.
You could make this clearer by addressing the post to him.
 
You could make this clearer by addressing the post to him.

Yes sorry about that but as he is the author of the post I assume he is reading it. I'm sure those who are nitrox certified and also instructors and technical divers won't be learning anything from my post. :)

I love to see all the equations that are shown from a relative simple post like should I dive using nitrox.
Dr. Dody is good at equations. Phd in Physics
 
My previous post mentioned the advantages of using nitrox to extend No-Stop time ( or reduce the decompression penalty for dives including decompression).
This has also been mentioned numerous times.
Again, we have all mentioned the particular advantages where multiple dives, and multiple days diving in using Nitrox.

For those who are risk adverse.
Even where the dives are not limited by the limits of the NDL time, i.e. where available gas is the limiting factor. Using Nitrox increases the 'safety' buffer, if Nitrox is the breathing gas but the NDL for air is applied. Significantly less Nitrogen will be absorbed by the diver using Nitrox than the diver using Air for an identical dive.

It should be remembered
For dives using an air table (computer setting), it is safer using Nitrox than air.
For dives using Nitrox using the optimum Nitrox table (computer setting), the risk is the same when diving to the NDL limits. [1]


[1] DDRC have said, it is far safer to get a bend on Nitrox than Air. The prognosis for a full recovery for a diver who was bent on Nitrox is better than for the diver bent on Air. I believe, in part this is due to the effect of the additional oxygen saturation in the tissue surrounding or down stream of the bubble, increasing the time before tissue is damaged due to oxygen starvation. Also the additional oxygen increases the Nitrogen gradient, increasing the potential for reducing any 'bubble' faster. (In much the same way as oxygen is the first aid treatment for a bend and should be given at the earliest opportunity, the earlier Oxygen is administered, the better the prognosis.)
 
My previous post mentioned the advantages of using nitrox to extend No-Stop time ( or reduce the decompression penalty for dives including decompression).
This has also been mentioned numerous times.
Again, we have all mentioned the particular advantages where multiple dives, and multiple days diving in using Nitrox.

For those who are risk adverse.
Even where the dives are not limited by the limits of the NDL time, i.e. where available gas is the limiting factor. Using Nitrox increases the 'safety' buffer, if Nitrox is the breathing gas but the NDL for air is applied. Significantly less Nitrogen will be absorbed by the diver using Nitrox than the diver using Air for an identical dive.

It should be remembered
For dives using an air table (computer setting), it is safer using Nitrox than air.
For dives using Nitrox using the optimum Nitrox table (computer setting), the risk is the same when diving to the NDL limits. [1]


[1] DDRC have said, it is far safer to get a bend on Nitrox than Air. The prognosis for a full recovery for a diver who was bent on Nitrox is better than for the diver bent on Air. I believe, in part this is due to the effect of the additional oxygen saturation in the tissue surrounding or down stream of the bubble, increasing the time before tissue is damaged due to oxygen starvation. Also the additional oxygen increases the Nitrogen gradient, increasing the potential for reducing any 'bubble' faster. (In much the same way as oxygen is the first aid treatment for a bend and should be given at the earliest opportunity, the earlier Oxygen is administered, the better the prognosis.)

How many bent divers are there in recreational diving who are diving within NDL limits? I know of one DCS case in the Philippines but the diver was diving to catch food and kept doing back to back deep dives and ascending too quickly he eventually passed away as no chamber where he was. He was already feeling ill before the final dive but kept on diving. This was late last year.
 
@peter_guy told me a story how he presented a trimix card and was turned down for nitrox. He then asked for trimix, but without helium. lol
You may be misremembering. I believe that was Bob Bailey.

How many bent divers are there in recreational diving who are diving within NDL limits?
A recent European study found the overwhelming majority of divers with DCS were within NDLs. Off the top of my head, I believe it was over 90%.

Don't make a faulty conclusion from that, though. I am sure over 95% of the people who die in Germany are Germans; that does not mean it is more dangerous to be a German in Germany than a visitor from abroad. Almost all recreational divers stay within NDL, so it is logical that most of the DCS cases would be within NDLs as well. The study suggested (needs further work) that algorithms do not give enough time for the mid level compartments to reach safe levels.

Contrast that statistic to cave diving. Nearly all people who dive in caves these days are certified cave divers, yet nearly half the people who die on cave dives are not certified cave divers. That clearly indicates a huge difference in safety between being cave certified and not cave certified. Yet, some people look at that statistic and say it shows that the odds of dying in a cave are the same for certified and non-certified divers, which would only be true if the two groups were entering caves in equal numbers.
 
Even if you dive at altitude, for recreational dives you only need to know that the MODs for altitude are actually deeper than at sea level, so you can safely use the sea level MODs.
BTW, a lot of people do NOT know this, and they in fact believe the opposite to be true. Not knowing how to calculate MOD at any altitude, if you ask them how to adjust for altitude, they will likely pull out the equivalent altitude table used to compute NDLs at altitude, but that has no relationship to MOD, and it will lead to a shallower MOD.
 
How many bent divers are there in recreational diving who are diving within NDL limits? I know of one DCS case in the Philippines but the diver was diving to catch food and kept doing back to back deep dives and ascending too quickly he eventually passed away as no chamber where he was. He was already feeling ill before the final dive but kept on diving. This was late last year.

It's important not to oversimplify decompression stress as being a binary thing, saying that you are either within limits or you aren't. There is a lot that goes into the risk of DCS. Multiple ascents, even if your magic bracelet says you are OK, are a risk factor. Some algorithms account for that.

That certainly was an issue with my "within NDL" hit. Here's my writeup. It's 10 years old, so I may have not understood things quite as well back then, but still interesting.
 
I don't think that this is correct.

MOD alarms are entered into the computer as PPO2 (i.e. 1.4), not as a depth. You aren't calculating a MOD, you are selecting a MOD.

You also enter into the computer what type of water you are diving in, but what that does is change the internal constant that the computer uses to convert a measured ambient pressure into a depth in feet or meters.

The computer uses the measured ambient pressure and the FiO2 of the breathing mix to calculate the PPO2, and if it's over the alarm value that you selected, you get an alarm. The constant that is used to generate your depth display has nothing to do with that.

So if you dive in salt water with your DC set for fresh water, you will get an inaccurate depth readout, but the only way you would know that would be to somehow compare it to a linear measure like a marked line. It would not change the safety of the dive. Your computer would alarm when your PPO2 reached 1.4 (or whatever you had set it to), and that calculated value is not going to be dependent on what you selected as the salinity constant.

PPO2= ATA x FiO2, whether you are diving in water, floating in air, or submerged in mercury.

I agree. See post #139.
 
https://www.shearwater.com/products/teric/

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