Nitrox course. What's the point?

[boulderjohn]

I have quoted the references for the last post so people do not have to seek them out.

Exact oxygen % in Nitrox are nothing like as important and most of the nitrox course teaching leads candidates to believe. The values used are designed to give a very good margin of safety in recreational diving. HBOT treatment takes place using pure oxygen at up to 3 bar, in theory (as taught in nitrox class) that would be well over twice the pressure needed to kill someone, yet it does not.

If you OxTox in the chamber, which happens, you don’t drown. There is a BIG difference between high PPO2 in a chamber vs underwater.

The scenarios are quite different. OxTox doesn't kill you, anywhere. In the chamber the dive you carefully and bring you up if required. Spasm in the water and you'll drown. The margins are different because the consequences are different.

In reply to posts 217 & 218. What you say is reasonably accurate, but what I posted is true in regard to the context in which it was posted. Posters were implying that a small variation in % around the 32 % mark was critical which it is not. Ox Tox does not only cause the convulsions that usually result in drowning, there may be no convulsions but longer term damage can occur to other organs.

The reference is to the significantly higher levels of PPO2 encountered in a chamber than are allowed by standards, and the two people responded by pointing out that toxing in a chamber does not lead to drowning. That is true, but there is another factor involved.

For reasons that are not well understood, people will tox in the water at considerably lower PPO2s than will occur in a chamber. IIRC, people can generally tolerate as much as twice the PPO2 level in a chamber than they can in the water.

On the other hand, I agree that the tolerances in the standards for PPO2 in scuba are very generous, and divers generally don't have to worry about a percent difference in their PPO2 measurement.

 

[Neilwood]

That’s not true, a 10 or 15 sec difference in assent to the first stop isn’t going to make a blind bit of difference to the total decompression schedule (if it did everyone doing deep air dives would be bent) and you’d have to fall asleep to miss half a minute of a stop time. I would kick myself if I missed half a minute. I can time stops to 5 seconds. 30 feet/min. assent used to be 60feet/min.

I was making a a point that by slow ascents etc that times and deco schedules can quickly be made useless by failing to stick to the full criteria behind the deco schedule and did not expect that someone would pick me up on a minor point - with hindsight I should have said by minutes instead of by seconds.

You might not miss a stop time by half a minute but I bet there are a lot of divers who do and if that is on tables the remainder of the schedule becomes increasingly useless as the gap between plan and action becomes larger. Depending on depths & stops etc, an extra minute or so ascending might add more than a minute to the "correct" schedule but that wouldn't be reflected in the dive plan which the divers are following.

Divers are taught to plan the dive and dive the plan - the plan is useless if not followed in all respects. Ascent rates are almost as important as stop times (which is something that I suspect a lot of people do not understand) especially when considering large deco schedules.

 

[Celt]

I was making a a point that by slow ascents etc that times and deco schedules can quickly be made useless by failing to stick to the full criteria behind the deco schedule and did not expect that someone would pick me up on a minor point - with hindsight I should have said by minutes instead of by seconds.

You might not miss a stop time by half a minute but I bet there are a lot of divers who do and if that is on tables the remainder of the schedule becomes increasingly useless as the gap between plan and action becomes larger. Depending on depths & stops etc, an extra minute or so ascending might add more than a minute to the "correct" schedule but that wouldn't be reflected in the dive plan which the divers are following.

Divers are taught to plan the dive and dive the plan - the plan is useless if not followed in all respects. Ascent rates are almost as important as stop times (which is something that I suspect a lot of people do not understand) especially when considering large deco schedules.

And the same goes for a computer if you don’t follow it. It’s as easy to make a 10m/min. assent with a watch and depth gauge as a computer with practice. For me its the same as walking a 10 metre line in a min. Zero the bezel and watch the second hand. Watch and depth gauge on one arm keep buoyancy slightly negative. There’s something wrong with divers training and practice if they can’t maintain a steady assent. The real problem with maintaining deco stops and timing them is thinking you can do them without a station.

 

[Angelo Farina]

Depending on depths & stops etc, an extra minute or so ascending might add more than a minute to the "correct" schedule but that wouldn't be reflected in the dive plan which the divers are following.

Divers are taught to plan the dive and dive the plan - the plan is useless if not followed in all respects. Ascent rates are almost as important as stop times (which is something that I suspect a lot of people do not understand) especially when considering large deco schedules.

When I was trained to deco dives during my first course, we were given the US Navy tables, but we were also instructed to not use them as the US Navy manual was specifying.
In particular the ascent rate had to be slower than 10 m/min, instead of the 18 m/min specified in the manual.
And the additional time spent in such a slow ascent had to be included in the dive time, instead of ending the dive time at bottom, before ascending.
With these modifications, any extra slowing of ascent has no consequences...
Example: the classic small-deco dive of 30 minutes at 30m, requiring a mandatory deco stop of 3 min at 3m.
If this dive is conducted following the US Navy manual, the diver stays at bottom for 30 minutes, then he ascends quickly at a speed of 18 m/min to the deco stop at 3 meters, does the 3-minutes stop and surfaces.
According to the method I was taught, instead, the diver stays at bottom for something as 24 or perhaps 26 minutes, then he ascends slowly, and arrives at 3 meters when the watch is showing 30 minutes.
At that point there is the mandatory stop of 3 minutes, followed by the additional safety stop of other 3 minutes, and finally another minute for coming up from 3 meters to the surface.
So there is no constraint in ascending at less than 10 m/min, and any extra time spent while ascending must be included in the bottom time, so it makes the deco to be certainly over-estimated and hence safer...

 

[Searcaigh]

@Dody the OP thanks for starting this thread, my coffee break was longer than planned.

Thankfully we are not using one of the original marine depth measurements these days e.g. fathoms, imagine the calculations for 1 Atmosphere 5.5 fathoms, but let's not go there.

Nitrox is worth it for many divers, especially when doing multiple dives per day over multiple days.

Personally when I dive locally my first mix is 34% for the first dive to 28-30m and my second dive the mix is 40% to allow me to have a reasonable second dive at a depth of 20-24m .... you'll learn that stuff when doing a Nitrox course by an instructor who knows his/her stuff.

Unfortunately all my local dives are square profile, no luxury of a multilevel dive in the Gulf of Oman unless you're interested in jellyfish, salp, or plankton.

I won't comment on the drysuit other than some people get colder quicker than others, especially when doing photography and not moving around much, at 23C for an hour I love my drysuit, plus it keeps me warmer during the SI and on the boat ride back to the marina when people in wetsuits shiver.

Interesting. So, it is possible to safely do 4 dives a day on air.?

I did 29 dives over a week in the Maldives on air back in 1998 and all dives no less than 50 mins

so if Dody wants to do a dive to 40m he would do that on air not nitrox.

I'd do it on EAN28, but hey that's just me

The Perdix allows up to 5 gases.

Only in Tech Mode as far as I am aware

 

[boulderjohn]

And the same goes for a computer if you don’t follow it.

Not following a computer is the same thing as not having a computer. I am amazed that I have to point that out.

If you think you are following a watch and depth gauge on a decompression ascent and make a mistake without realizing it early in the ascent, your mistake will be magnified as you continue the ascent. As you look at your watch and depth gauge for guidance, it will have no indication that you are now off schedule. Follow it as carefully as you can and you will still be wrong, and you will never know it.

If you think you are following a computer on a decompression ascent and make a mistake without realizing it early in the ascent, the computer will adjust and give you a different ascent profile. As you look at your computer for guidance, it will give you a new schedule that accounts for your earlier mistake.

I will admit you are right about your main point--if you don't look at your computer, it won't help you. As the musical group Faces told us so many years ago, "A Nod's As Good As a Wink... to a Blind Horse."

 

[Celt]

If you think you are following a watch and depth gauge on a decompression ascent and make a mistake without realizing it early in the ascent, your mistake will be magnified as you continue the ascent. As you look at your watch and depth gauge for guidance, it will have no indication that you are now off schedule. Follow it as carefully as you can and you will still be wrong, and you will never know it.

how much of a “mistake” are you taking about. How much of a difference is there between max conservancy and min conservancy on a computer. I most certainly don’t make that kind of a mistake. The depth gauge and watch don’t disappear of your wrist if you take your eyes of them, the information is still there.

 

[boulderjohn]

I most certainly don’t make that kind of a mistake.

How do you know?

My friends who got bent would have sworn they did not make any mistakes if they had not had a computer profile of the dive to show them what mistakes they mare.

Now, I realize there is a difference between people who are perfect and mere mortals.

 

[dmaziuk]

The depth gauge and watch don’t disappear of your wrist if you take your eyes of them, the information is still there.

Whereas computers vanish leaving behind a shimmering outline of the word GAMEOVER

 

[boulderjohn]

he depth gauge and watch don’t disappear of your wrist if you take your eyes of them, the information is still there.

But with none of their history.