Nitrox on boat with air refill

[IVC]

The NOAA CNS oxygen exposure table is simple and easy to memorize. I recommend that you go ahead and memorize these oxygen exposure limits (along with the associated couple of rules).

This seems to be the consensus and I will definitely do it. Extra knowledge can only help.

---------- Post added November 6th, 2015 at 06:29 PM ----------

Why would you even need a calculator? Surely you can get close enough in your head.

That's the best answer yet (thanks,) but it wasn't possible.

The problem is that by the time I was out of the water (I wasn't exactly one of the last ones out; quite the opposite ,) rushing to put my equpiment back in the rack and looking around to enjoy the day for a moment, they were already refilling my tank, so the calculation option was off the table. I was monitoring my air until the three minute stop, then didn't bother during the surface swim, which I realized was a mistake since I couldn't write it down in my log book for future reference.

Anyway, my primary concern was just how at least two experienced divers on the boat completely ignored the *concept* of CNS O2 exposure, not that it was in any way pushing the limit. If either one said what several posters here said, e.g., "don't worry, you'll never reach your CNS limit with depth and time you're doing," I'd be completely fine - it would show they knew about it and considered it safe vs. pretending it didn't exist.

---------- Post added November 6th, 2015 at 06:41 PM ----------

No offense, but as a relatively new diver, it sounds like you're way overthinking the issue and losing sight of diving for fun (maybe that comes from having a physics background)! If you do the math, to exceed the daily recommended O2 CNS limit, you would be doing several long, deep dives.

"Overthinking" is perspective-relative. To you and several more experienced divers it might be "overthinking," but not because there is a simple way to say "it's not an issue."Instead, you went through all the calculations and you've done your homework so you already *know* it's not an issue. On the other hand, for *me*, this is that first time where I *learn* that it's not an issue and that I can "relax and enjoy the dive." I wouldn't want to be the person who looks at a potentially dangerous concept and says "nah, won't happen to me." I'd much rather be the person who recognizes a potential problem, evaluates it and then *determines* it's not an issue.

The more I read responses in this thread, the more I realize that the basic Nitrox training (PADI) could be improved by spending a bit more time on actual calculations and a bit less time on color-matching tanks and fins.

 

[Bob DBF]

Having notes/tables is something I'm realizing more and more is a must. Dive computers are nice and handy for coral reefs, but once you start deviating from the standards, you suddenly find yourself having to work out solutions instead of following an established protocol. The only way to make this "working out solutions on the fly" safe is to have both the understanding of the problem and the data with which to calculate. I'm beginning to really appreciate the way tech divers think.

I've ordered Nitrox tables. They provide cumulative CNS O2 exposure based on time-depth so I can quickly see if I'm anywhere near the limits.

First, you might want to talk to your Nitrox instructor and have him go over the course again and give you the tables that you should have had to begin with. The idea of a Nitrox class is to learn how to use and understand Nitrox, and to know how to deal with various issues, one of which you brought to us today.

As far as buying the O2 analyzer, it seems like overkill unless you are blending your own gas in your garage. I've always found one available to me when I've picked up tanks from a facility that blends Nitrox, as well as on boats that deliver Nitrox. I only only use Nitrox on multi day multi dive trips so it is not a real issue for me.

Tech divers think the same way as any experienced diver does, plan the dive to minimize the risk to you and your buddy so you can make it back to the pub for a beer at the end of the day.



Bob

 

[IVC]

The level of O2 that requires "oxygen clean" configuration seems to be a contentious issue. I don't have a dog in this race, but from what I found it looks as if the correct PPO2 level for needing to be oxygen clean for anything *outside* scuba community is in the 23%-25% range.

From one of the manufacturer's FAQ: Why does Luxfer require cleaning for oxygen concentrations above 23.5%. They have a pretty good overview of how different agencies look at what requires "oxygen service." Even OSHA uses 29 CFR 1910.134 which states that anything over 23.5% needs to be clean. However, since the primary concern with high PPO2 is during filling, I leave to the shop to decide what standard they want to use.

---------- Post added November 6th, 2015 at 06:55 PM ----------

First, you might want to talk to your Nitrox instructor and have him go over the course again and give you the tables that you should have had to begin with.

It was PADI e-learning. I begin to see a pattern emerge here...

 

[rx7diver]

Why would you even need a calculator? Surely you can get close enough in your head.

Doing EAD calculations and oxygen clock calculations (especially when backing things out to calculate a max NDL EAN repetitive dive) are a bit too tricky for me to do in my head.

EDIT. A calculator isn't necessary. But, I would prefer to have a pencil and slate—at the very least.

 

[stuartv]

"Overthinking" is perspective-relative. To you and several more experienced divers it might be "overthinking," but not because there is a simple way to say "it's not an issue."Instead, you went through all the calculations and you've done your homework so you already *know* it's not an issue. On the other hand, for *me*, this is that first time where I *learn* that it's not an issue and that I can "relax and enjoy the dive." I wouldn't want to be the person who looks at a potentially dangerous concept and says "nah, won't happen to me." I'd much rather be the person who recognizes a potential problem, evaluates it and then *determines* it's not an issue..

+1000!

Good call! In my opinion, anyway. I've been chastised here on SB many times in the last year for "overthinking" things. But, I agree with you. I don't want to be the guy that dies because I listened to some of the people here and decided I did not need to fully think through whatever action I was contemplating.

First, you might want to talk to your Nitrox instructor and have him go over the course again and give you the tables that you should have had to begin with.

I don't know which Nitrox class IVC took, but when I took SDI Computer Nitrox, the tables and how to use them were not part of the curriculum. Unless, by "should have had to begin with," you're really talking your personal disagreement with what current course standards are. Regardless, going back to the instructor may not help as I'd guess IVC understood what the instructor taught and the instructor taught the standard curriculum. So, going over it again probably wouldn't make any difference.

 

[dumpsterDiver]

A minor issue but.. the advice to just buy an analyzer and follow it is not all that robust either.. Your analyzer should be used to CONFIRM your mix. You should have a good idea of what the mix is before you measure it and then use the analyzer to verify it. You should be able to do the fractions and percents in your head.. more or less.

It sounds like the OP has a healthy dose of skepticism about what people tell him. A good trait in my book.

 

[Brandon]

"Overthinking" is perspective-relative. To you and several more experienced divers it might be "overthinking," but not because there is a simple way to say "it's not an issue."Instead, you went through all the calculations and you've done your homework so you already *know* it's not an issue. On the other hand, for *me*, this is that first time where I *learn* that it's not an issue and that I can "relax and enjoy the dive." I wouldn't want to be the person who looks at a potentially dangerous concept and says "nah, won't happen to me." I'd much rather be the person who recognizes a potential problem, evaluates it and then *determines* it's not an issue.

That's why I was reluctant to just simply state "it's not an issue" without at least giving you a basic rundown of the types of calculations and though processes I went through to determine it wasn't an issue for me.

Intuition requires experience - both to be familiar with the concepts involved, but also to know when the conditions have changed such that the base assumptions no longer apply. You can't instill that on another diver just by telling them; they need to gain the experience and knowledge themselves. We do a disservice to our fellow divers when we suggest they follow any number of shortcuts, rules or guidelines that we've learned work for us through our personal experiences, but that they may not understand well enough to utilize safely.

In your case, it's just a matter of taking the dive-planning skills you've already learned, and applying them to a series of hypothetical dives. Change the depth/bottom time, change the gas blends, and change the surface intervals; and look at how that affects your no-deco time. Those exercises are a way of gaining experience and knowledge, and will likely provide numerous discoveries that will aid you in making more intuitive decisions next time around.

 

[geoff w]

... Extra knowledge can only help.

...

The more I read responses in this thread, the more I realize that the basic Nitrox training (PADI) could be improved by spending a bit more time on actual calculations and a bit less time on color-matching tanks and fins.

Take a look at https://decodoppler.wordpress.com/2009/02/10/daily-limits-for-cns-oxygen-toxicity/ for a good explanation on the subject

When I took the PADI Nitrox specialty in 2005, we did tables, calculations, etc, and maybe touched on how to set a dive computer to nitrox mode. From what I've seen, the current specialty is a bit more streamlined and geared toward getting divers to just dive nitrox. From my experience, if you're a typical recreational diver, the current PADI nitrox specialty is fine, since the two things you're going to run into first as far as limits are maximum operating depth (MOD) and no decompression limits (NDL).

 

[gcarter]

Why would you even need a calculator? Surely you can get close enough in your head.

For example, if you were diving EAN32, and you had 1000 psi left, and topped off with air to 3000 psi, then the pct02 would be 32*1/3 + 21*2/3 which is close enough to EAN25. Which you might as well treat as air anyway for deco planning.

If you had more than 1000 psi left, you should have stayed down longer...

This is what I have done when face with air on top of nitrox. Easy peasey.

---------- Post added November 7th, 2015 at 01:55 AM ----------

The more I read responses in this thread, the more I realize that the basic Nitrox training (PADI) could be improved by spending a bit more time on actual calculations and a bit less time on color-matching tanks and fins.

If that is what you discussed in your training, the problem is with the instructor, not the course.

 

[Centrals]

If that is what you discussed in your training, the problem is with the instructor, not the course.

He did it through e-learning!
When I did the course back in 1997, I have to do TWO dives + numerous calculations on Best mix, MOD, PPO2 etc etc. And the instructor was sitting right next to me ready to give a me a hard time or help!!