Nitrox course. What's the point?

[dmaziuk]

Actually, @Storker brings up a good point, and one that hadn't occurred to me before. Maybe I'm missing something, or don't know enough about dive science to realize the answer.

Why DO dive computers let you adjust for salt water or fresh water?

For depth gauge display/log. Adjusting to salt/fresh/middle-whatever-that's-called does not change deco calculations, only the depth display.

 

[dmaziuk]

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%.

Sorry, it was just like the dead divers with weights on: a study of 320 "undeserved" DCS cases, i.e. the subjects were within the "safe" limits a priori. Less than 5% of them were over GF .9 (at any point?) in the dive indeed. But they were all diving "safe" to begin with.

(Interestingly, >14% were below GF .7, so we can "scientifically conclude" that GF90 is "safer" than GF70.)

 

[Blackcrusader]

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.

Shearwater Perdix do not have an audio alarm which is one of the things I like about it. I've been with divers when their alarm sounds and they look and point to me. They don't even know the sounds of their own dive computer. Dive computers should have test sounds so divers know what they sound like.

 

[Blackcrusader]

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

Even in Asia a few dollars more Dody it's cheap.

 

[Blackcrusader]

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.

Being limited on NDL than gas is when you do deeper dives. If you did a max 20m and stayed at 20m for the dive on nitrox 32% most divers on an AL80 will run out of air before they get to NDL. It's easier to get to NDL when you are deeper. Even though the MOD on Nitrox is 34m I prefer to limit myself to around 30m maybe 31m as it's too easy for people to see something and descend. So pushing MOD and NDL not the best practice one might say. There are some pigmy sea horses at one location I dive at but they are generally at 32m depth. Nitrox is good for getting extra time to NDL to take photos.

 

[Centrals]

Even in Asia a few dollars more Dody it's cheap.

May be not for you!
I won't pay for nitrox unless the dive justified it!
Multi-level all the way!

 

[Blackcrusader]

May be not for you! I won't pay for nitrox unless the dive justified it! Multi-level all the way!

Multi level what? Dody wrote he is not interested in deco dives for now. He might change his mind later or or even take a course like the TDI Advanced nitrox and Decompression just to learn from it.

 

[Gareth J]

You’re computer calculates the whole dive not just the bottom time

Well, I don't know what tables/computers that you dive but I learned diving a table that said 60 minutes was NDL for air at 60 feet. My current computer says 55 minutes.

Modern computers calculate in real time. TTS, (time to surface,) is an estimated figure, based on current (real time) data about the dive, (and diving history) an assumed ascent rate, (potentially an assumption on what decompression gas will be breathed during the ascent).
If you deviate from the assumed elements used to calculate the ascent, , the computer will recalculate the TTS.
NOTE - this is why, when you have a decompression stop to do, and you choose to decompress deeper than the ceiling stated by the computer, e.g. decompress at 6m rather than the 3m, your decompression time will be greater than stated. Modifying your TTS. In the UK, we often choose to decompress at 6m, rather than the computers preferred decompression depth of 3m. Surface conditions often preclude shallow stops. Most UK divers allow for this in their dive planning. If I am on a no-stop dive, I will do safety stops at 6, 4.5 and 3m, the computer will not acknowledge the safety stop at 6m, but will once at 4.5m or shallower.

It should also be remembered that there are differing theories on how to calculate decompression requirements resulting in different no stop time for the same dive, dependent on table (theory used). This variation is increased in the dive computer market, where additional elements may be added, conservatism, padding, adjustment for number of repetitive dives, poor dive practice, etc.
A manufacture may have multiple models across its range e.g. Suunto, which uses RGBM, RGBM 2, Fused RGBM and Tech RGBM. Some of the Tech computers allow you to change models dependent on your preference, e.g. Bulhmann or VPM.

I quoted NST figures for a dive using differing mixes earlier in the thread, I qualified the statement by stating WHICH model was being used. Differing models, differing times!

 

[David Novo]

Ask @Wookie about that.

What do you mean?

P.S.: Despite what I said, I always dive Nitrox when available.

 

[Celt]

Really? Your computer knows how long you are going to take to ascend? Mine doesn't know that. If I am out of time at 100' I may go to the surface or I may just go up a ways. I'm impressed that yours knows.

I never dive on the fly, I plan all my dives. I always know what I intend to do. Force of habit.