Nitrox course. What's the point?

[Storker]

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.

Back when I only dived air, my min gas time was very close to my NDL time at most depths between 20-ish and 30-ish meters. These days, when I have basically standardized on EAN32, my NDL is roughly 1.5 times my min gas time.

Not only that it makes life underwater a lot easier (monitor my gas, don't care about NDL), I get shorter SIs and/or increased bottom time on my 2nd and possibly 3rd dive of the day. Win-win. I love EAN32. And that's not considering the fact that I'm one of those who believe that I feel less fatigued after 2-3 long/deep (by NDL standards) dives on nitrox than I do after 2-3 long/deep dives on air.

 

[Degenerate]

It is partially a joke. I have never seen a dry suit just pictures, movies and people are like the Michelin bibemdum wearing it. Very different with the freedom of movement that one has with a wetsuit.

I have considerably more freedom of movement in my drysuit than I do in a wetsuit.

 

[doctormike]

I agree with what you wrote above in a followup post earlier. Here is my original post that you quoted:



Just to be clear the first sentence in my original quote above is misleading. The final paragraph is correct. There are two methods for alarming exceeding MOD on the DC. One method is to use the pressure reading, calculate the ppO2 and compare it with the limit entered by the user. When the measured value exceeds the user limit generate a MOD alarm. If you enter the wrong water type, for Nx36, for example fresh instead of salt the DC display will read 98+ ft instead of 95+ ft.

The second method (from my original post) uses a calculated MOD depth once the user enters in fO2% and the ppO2 limit. Using Nx36 and a ppO2 of 1.4 atm the calculated MOD is 95 ft in salt water or 98 ft in fresh water. The diver, however, selects fresh water instead of salt water. The diver starts the dive in salt water. At the actual depth of 95 ft the MOD alarm will sound. Why? The DC "thinks" the depth is 98+ ft and this is what is displayed to the diver. So, it compares the measured depth with the calculated MOD which is greater and generates the alarm.

Whichever method is used the result is the same; the MOD alarm will be generated at the correct water depth but what is displayed to the diver depends on the water type setting. OK, time to move on.

I dunno, I find this stuff interesting! The discussion has helped me to understand things better. But let me ask you about the second method.

I don't know if some dive computers have an internal algorithm like you describe - one that converts the ambient pressure to a depth based on a specified salinity, and then compares that result to an MOD calculated from the desired maximum PPO2, known mix and salinity. But even if that were to be the case, salinity would still be irrelevant in terms of giving you an unsafe PPO2, since you are just mathematically putting the salinity constant (right or wrong) in, and then taking it back out again, right?

And yes, I agree, in the example you give, the diver would hit 95 feet (actual depth), the PPO2 would hit 1.4 and the alarm will sound, no matter what you set the salinity to. The only outcome of that salinity setting error would be that the DC display would erroneously read 98 feet, even though the diver was actually at 95 feet.

 

[Dody]

Its not an absolute question, there is no absolute answer. Water temperature is a key ingredient, and your tolerance to cold. A thick wetsuit takes an eternity to get on and greatly restrict movement, but there are some exotic ones that have less impact. I'd wear my drysuit over a 7mm any day of the week, I also think its more comfortable than a 3mm with extra insulators. A drysuit is far more comfortable to wear for surface intervals, and much better to put back on after that first dive when everyone on the boat is whimpering about how cold and miserable their wetsuit is.

Skills are different, risks are different, ballast is also different. I'd be hesitant to suggest one is safer than the other because its very dependent on the specific risk. I'm not going to detail the specifics because its a limitless scenario, and you're not going to understand some of the concepts as a newbie to diving. Also not all scenarios would be relevant to you, and as a recreational sport diver a wetsuit meets your needs superbly. You'll come around by yourself, or you never will, but they're expensive so you won't change your mind because of lobbying on SB.

I hate the cold . And I get on my 5.5 mm wetsuit in less than 10 seconds. Taking it off ( especially the feet) is the real hassle . My comment about drysuit was not the point. It was just a piece in a stupid argument. All I want is to learn and get advices from more experienced divers without being disrespected. Your post is why I am on SB.

 

[Dody]

Dody... Nitrox 32% means I get longer times to NDL and is better over repeated dives. However dive times still depend on your sac rate and how long you stay at depth. If I was using nitrox 32% staying at 30m depth for 24 minutes I do not exceed NDL and can ascend and finish the dive. If on air at 30m can stay for 15 minutes but that is then 0 NDL so for safety lets make it 12 minutes at 30m on air. Nitrox gives me longer time to stay deep. It does not change the sac rate. So on multiple dives where you want to go deep to 30m then you get longer times to NDL. Even when I dive with my friends are also good on gas consumption we do not push staying deep on our dives. Sometimes we use nitrox but leave our computers on 21%. Some DM guides will do this as well.

Thanks.

 

[Gareth J]

It is important to understand that NO decompression MODEL guarantees that you will not get bent.

There are many variables, some of which are not fully understood. Because we don't really understand the biology or physiology of what is happening we have made assumptions. Then tested these assumptions (see Haldane and his goats).

If you want to be aggressive, you dive to the edge of the NDL.

To be conservative you either dive well inside the NDL, add stops on the end of the No-stop dive (safety stops), or remove the gas that is the issue, Nitrogen.

Understanding that workload, temperature, fatigue, dehydration, medication, mental exhaustion, stress, etc all change the decompression obligations is fundamental to safe diving.

 

[tursiops]

I dunno, I find this stuff interesting! The discussion has helped me to understand things better. But let me ask you about the second method.

I don't know if some dive computers have an internal algorithm like you describe - one that converts the ambient pressure to a depth based on a specified salinity, and then compares that result to an MOD calculated from the desired maximum PPO2, known mix and salinity. But even if that were to be the case, salinity would still be irrelevant in terms of giving you an unsafe PPO2, since you are just mathematically putting the salinity constant (right or wrong) in, and then taking it back out again, right?

And yes, I agree, in the example you give, the diver would hit 95 feet (actual depth), the PPO2 would hit 1.4 and the alarm will sound, no matter what you set the salinity to. The only outcome of that salinity setting error would be that the DC display would erroneously read 98 feet, even though the diver was actually at 95 feet.

I am unaware of ANY dive computers that calculate MOD based on some table of MOD depths. They could, but they don't. Using a table would be incredibly stupid, because the tables themselves depend on pressure, salinity, and actually on temperature if you are seriously anal.

The whole point of MOD (as others have said) is the PPO2 based on diver-input FO2 and computer-measured ambient pressure. Depth is irrelevant. it is not useful to the computer, just to the person wearing the computer.

 

[Dody]

Recreational dive times are always limited by either NDL or Gas. It is a good thing to point out. I would hope that even a green diver would "get" that but it is always a good idea to be explicit. If you are finding that your dives are always being limited by available breathing gas instead of NDL than Nitrox will be of little value to you.

As an example. The vast majority of my dives are in a HP100 in the 40-90' range and I have a respectable SAC (.40-.45 cfm on average). If I were to dive air my dives would ALWAYS be turned by NDL. By using nitrox given the diving I do my dive times are increased significantly. However, these facts don't apply to everyone. It is important to understand the parameters under which you are diving to assess how valuable Nitrox will be to you.

I understand. I believe that my dives are more limited by NDL than by gas so Nitrox when it is available could be of value. But really, my post was about the value of taking the Nitrox course if you already know the theory. Now, I know why I need to take it. I need the card to dive nitrox.

 

[Dody]

I get it. I'm agreeing with you.

One more benefit to consider. Even if your dive is limited by your air consumption Nitrox will give you an added safety factor. Less nitrogen loading is always good even if your dive is turned by your air consumption. Less nitrogen in your blood equals added safety factor assuming all other diving guidelines (MOD, ascent rate, safety stop) are followed. This is another reason I always dive Nitrox. Not the primary reason, but just another reason.

In the US, where obviously, Nitrox is available everywhere, is it more expensive to dive Nitrox than air?

 

[doctormike]

I am unaware of ANY dive computers that calculate MOD based on some table of MOD depths. They could, but they don't. Using a table would be incredibly stupid, because the tables themselves depend on pressure, salinity, and actually on temperature if you are seriously anal.

The whole point of MOD (as others have said) is the PPO2 based on diver-input FO2 and computer-measured ambient pressure. Depth is irrelevant. it is not useful to the computer, just to the person wearing the computer.

Right, that's what I was getting at. Even if you did want to calculate a MOD including salinity as a variable, you would just take it out when converting between ambient pressure and depth.