Nitrox Question

[Nutter1]

I was talking with a freind of mine who is involved in the NAUI organization, he proceeded to tell me that they will be going public with a recommendation to max % on nitrox to 32%. He said the reason for this being outside influences such as water temp, body composition, thermoclines, alcohol, stress etc. can change the oxygen % as the air enters your body. Thus potentially causing oxygen poisoning at less depth. Has anyone else heard anything about this, if not maybe I will contact NAUI. Any feedback is welcome.

[Thunderball]

Many factors play a role in tissue-gas dynamics (temperature, BMI, stamina, exertion, depth, partial pressure, etc.). Oxygen toxicity is just one facet of the equation. In fact, those variables play a role in the development of DCS and narcosis too (nitrogen-dependent phenomena). Navy and NOAA divers have been safely using enriched air for years. Why the sudden change with NAUI? I am unaware of any reports that the incidence of oxygen toxity (siezure, death) has increased, even with the advent of recreational nitrox use. What data does NAUI have to base this change in philospy on? I was taught, that unless you are repetitively diving your limits on nitrox, it is your nitrogen alarm that will max out before your oxygen alarm does.

I think the place to ask these questions would be DAN.

[Nutter1]

Thanks

[300psi]

DAN has an article related to this at:

http://www.diversalertnetwork.org/medical/articles/article.asp?articleid=35

It is a little long but the article seems to push for using a PO2 of 1.4 for divers who exert or dive cold or warm water. No mention about 32% limit.

[Double Down]

During my nitrox training, my insutructor really pushed for a max of 1.4 to be safe. It was explained 1.6 would be "marginally safe, and 1.4 would be the ticket. Because of the exertion spearfishing entails, I choose to stay below 1.4.

Here's my question...
as Thunderball pointed out, with all the varying factors at work, how accurate is it to go by the tables and assume a certain EANx percentage will equal a certain PO2 the same for all divers? During my training, it was made to sound like the PO2 would always be a constant for all divers, but that doesn't seem to make sense to me anymore. Any input?

James

[PatMyGreen]

How often do you guys use anything more than 32%? (other than for pony bottles near the surface) I can't often go more than that anyway due to the fact that I am consistently diving 85-105'.

[Double Down]

Good point, I always seem to use 32% myself, except in the Keys. There are some times down there I use 36%.

[kitefisherman]

Pat: I generally use mixes around 36% (1.4 ata @ 95 FSW, 1.6 ata @ 114 FSW) when I am diving in 90-100 feet here. I am very familiar with the area, and the sand on the outside of the reef is no deeper than 100 feet so I couldn't get deeper if I tried. Also, I generally hunt 15 to 20 feet above the bottom to get a better vantage point and to avoid spooking fish, so the time that I spend below 90 feet is usually minimal. I am always well within my 24 hour O2 exposure limits. The higher mix allows me to minimize my deco stops on subsequent dives. It is not unusual for me to do 3 or 4 90-100 foot dives in a day with each dive averaging about 55 minutes.

[Nutter1]

I also normally use 36% but rarely ever exceed 100' and if I do it is only to pick up a fish and only up like 105' typically only 2 dives in a day. If what I have heard is true I may just consider limiting myself to 32%

[PatMyGreen]

I like to stay about 15' off the bottom too but you never know when you'll be elbow to shoulder in a hole trying to grab shovelnose to round out the stringer. I am also not comfortable pushing too close to 1.6 over and again. I know I can't get any deeper on my reefs but you never know, you know.?

[PatMyGreen]

I just read the whole post, you aren't divin on AL80'S are you Kite. I don't own any tamks of my own so thats what I'm stuck with for now and I often erroniously assume that everyone else dive's similarly. Its never seemed neccesary to up the mix because we are down for such a short period of time that after an hour s interval we can go again and again with no deco on just 32%.
Makes since that your longer dives would make a higher blend a requirement. Also what are you on that is big enough to stay down on for an hour?

[kitefisherman]

Pat: You are correct. I'm diving steel tanks (HP 120, LP 121s, 112s & 98s). All these tanks are cranked to about 3800 PSI so I have more than 150 CFT of gas with the LPs. The reef here runs North and South for miles (with some gaps). I may cover half a mile or more on a single dive especially if I have a current to help me along. We don't have the fish density that they have in the MGs or SC, so I need to find the fish. If I dove 32%, I would have significant deco on every dive - especially the latter ones. With 36%, I usually have no deco on the first dive and no more than 10 minutes of deco on my last dive. I always carry a pony bottle in case I have a failure with my main tank or regulator so that I can get up and do any deco. I'd rather avoid or minimize my deco - and especially if I am carrying my fish with me because of the sharks that we have here. Usually, I lift bag my fish up as I shoot them if that is compatible with how the boat is being run. I get no thrill from diving deep or going into deco - I just want to get where the fish are - and get back safely.

[PatMyGreen]

Must be nice to have somebody driving the boat so you can come up anywhere and be picked up. I don't have enough friends I trust to not run me over or keep the boat floating (you really have to meet these guys) so we have to anchor and stay in one area.

[PatMyGreen]

One day when my friends and I evolve into higher primates and can work our thumbs I'd like to imitate what you have desribed. Thanks for the explanation and sorry for highjacking the thread nutter!

[Thunderball]

As for accounting for all the variables that play a role in tissue-gas dynamics and safety, I would say this: buy a really good computer. Less chance of making a mistake. And a good compute will take into account water temperature in addition to pp02, depth and time. If you have an gas-integreated computer (that keeps track of your gas consumption and therefore your exertion), all the better. UWATEC make some great recreational computers that will even allow you to change the level of microbuble formation, depending on how much repetitive diving you are doing (change the risk). Using the tables will obviously not account for anything except time, depth, ppo2. Finally, computers that break the body up into compartments are probably even safer (each tissue compartment has its own dynamics and averaging the entire body to one model is probably less precise). Make sense?

[kitefisherman]

Pat: One more thing that you might not realize from my post and because you aren't diving here is that we dive with a dive flag float and a reel here. Unless you are diving a wreck, you are almost always drift diving. You have to tow a flag because it's the law and because of the density of boats diving and fishing the reefs within a mile of shore where we are diving. It doesn't guarantee that you won't get run over, but usually it won't be by your boat. :D I know it sounds like a pain, but you get used to it.

[Letrappes]

Thanks for the encouraging words Pat. I'll remember that next time we're out.

[Double Down]

Thunderball

Thanks for the info. I have a Suunto Vyper air integrated which I am very comfortable with, however I wasn't sure if it would take into account such things as you stated like temp and exertion. Anyway, it give me something to think about for deeper dives.

[Speargun]

DD
If your Suunto is AI it should be a Cobra or Vytec (multigas & wireless). I'm pretty sure that your computer takes all of that into account. Check Suunto's website for more info.
Even if it doesn't account for temp, exertion, etc, the Suunto computers are about the most, if not the most, conservative computers on the market today; especially when it comes to multiple dives. I have done multiple dives with my Vyper and Nitek3 and ended up with 9 minutes of deco on the Vyper vs none on the Nitek. The good thing about the Suunto's is that they have a "no DCS hit" record for anyone following the computer. So if you like safe and conservative, Suunto is the way to go. If you want to push the limits a little more, buy another computer.

[Double Down]

Speargun,

Actually it is a Cobra. Not sure why I put Vyper, guess it is a snake thing. Anyway, thanks for the info. Good to have another's input. I will be checking there website shortly.

James

[WreckDiver]

Temp YES, Exertion NO

[Speargun]

I really like Suunto's computers due to the ease of use and download software. But, I hate the fact that they are so conservative. They knock 5 or 6 feet off of the M.O.D. because they figure that if you were standing on the bottom, holding the computer at eye level, your feet would be below the M.O.D. for a given mixture.
There isn't a noticable difference on your first dive as compared to another computer, but each additional dive will result in steeper penalties to bottom time and/or additional deco time.

[ichthyophobia]

Here's one more thing to consider. I dive my computer set at 1.6 PO2 because when I select my mix for my dive, I make sure that the max depth of the mix is at least 10 feet deeper than the bottom of my dive. This effectively gives me a safety margin on my PO2 since I don't ever reach the max depth for the mix.

Remember, PO2 is a factor of O2 mix, max depth and time spent at that max depth. Don't hit the max depth and don't hit the max time, and you effectively reduce your O2 exposure.

As an example, 38%O2 at 1.4PO2 goes to 88ft max. for 150 minutes. At 1.6PO2, it goes to 105ft max. for 45 minutes. So, you dive where max depth is 100 ft or less, and you spend most of your time hovering at 90ft looking for fish. Effectively, your actual PO2 exposure would be around 1.45PO2. And most of us won't get the 45 minutes of full bottom time, depending upon the tanks anyway, so there's some safety margin in tank size, too.

If you are diving 32%O2 at 100ft and you lay on the bottom for your entire 60 minute dive (if you have enough air) your PO2 is only 1.3. If you hover at 80-90ft, your PO2 is less!

If you aren't comfortable with the numbers, then stay conservative. A good computer certainly increases your comfort level!

I will dive 40% up to 90ft deep since this gives me max bottom time. I'll run out of air before I run out of bottom time using HP100's.

I've read about a few diver deaths lately, and I can't seem to recall any of them due to oxygen toxicity, except for maybe tech divers sucking off the wrong bottle.

Bottom line is that you really have to understand what all the numbers mean.

[fishnfool13]

I think its time to get rid of my Suunto . I never realized that they were that conservative . I just thought my dive buddies were that wreckless .

[Nutter1]

I appreciate all the info. This is the exact reason I joined the forum.

Thanks
Nutter1

[Denny]

As new divers become less and less fit, certification agencies are going to become more conservative in their recommendations.
1.4 atm? Maybe for extreme diving, but even then, that sounds a little TOO conservative.
36% at 110 fsw has never been a problem for me or anyone I know. I always dive the max mix that I can, using 40% whenever possible. If you are going to pay for the mix, I figure that you should get the most bang for your buck.
Conservative computers SUCK. I recently used a liberal computer to dive the elbow and was able to complete four dives at 180', with my max deco at 8 minutes. Doing the deco on 80% helps clean out the nitrogen quickly. Stopping halfway up on ascent for one minute is a something I learned recently that seems to be a very sound practice.

[kitefisherman]

Denny: I agree with you and especially with regard to O2. DCS is rather common compared to the rather rare cases of O2 toxicity especially considering how popular nitrox has become.

I generally use my Oceanic for routine familiar, one gas repetitive dives to 130 FSW or less. The Oceanic is rather liberal and uses the Navy dive tables. When I do a deco dive (especially a series of them and especially if they are at extreme depths), I use my Suunto Vytec both to take advantage of the ability to scrub with a deco mix as well as to add an extra measure of conservatism. A prime example of that would be last weekend’s rig trip when we were doing 4 bounces to 200 FSW a day for two days. I have absolutely no problem doing an extra few minutes of deco when I am fifty miles and 3 hours out of port in a rather remote part of the world. Regardless of how liberal or conservative your computer is, remember that your computer does not “let” you do anything. Your body and its reaction to changes in pressure determine whether you get bent, and it may not react the same way every time. I understand that with about 75% of the cases of DCS that the doctors cannot determine why the diver got bent even after reviewing the data in the diver’s computer. If you push you body far enough and enough times, you will probably eventually get bent. Something to think about on your safety or deco stop . . .

That said, I’m out the door to get in a couple of tanks before I start work. :)

[Thunderball]

From a physician's point of view, it all boils down to this. Risk. What can you do to minimize it and what increases it. Some of those factors we have control over (temp=exposure suit, exertion=fitness, water skills, time.depth=don't push the limits). With experience comes a skill set that allows you to manage these variables.

Computers make it much easier to manage some of these things, but remember, the real reason that computer were developed was to MAXIMIZE your RNT. Before they were invented, your dive profile was determined by your max depth, even if you only spent a fraction of your bottom time at that depth. Computers compute the "area under the curve" and constantly update your RNT (and % O2). As conservative as the most conservative computer is, it is not nearly as conservative as the tables.

I like my two UWATEC computers because they are sort of moderate in their gas politics. They do not, however, make a model that allows multi-gas (3) management or allow you to change the FiO2 underwater. Suunto Vytec is a great instrument for that.

I am long winded, but on last thing. If the incidence of O2 toxicity is so low, then only huge changes in the way our computers currently manage the algorithm are going to reduce it even more. Changing the limit from 1.5 (PADI) to 1.4 (NAUI) is probably not going to reduce the risk or inicdience of accidents by very much. I'd like to see a radomize control trial looking at exactly that, but such a study will never happen.

One thing that NAUI has proposed, and DAN has published some data about, is that any dive deeper than 60 feet should include a 1 minute safety stop at 1/2 your maximum depth in addition to usual 3 minute stop. I think that, more than anything, will decrease your risk of microbubbles. As for O2 toxicity, does a safety stop really play a role in O2 degassing? Hmmmm.

[Denny]

Shoot 'em up, Kite! Wish I could get out for a couple of tanks before work, but then, I'd be working the afternoon shift.:D

[Double Down]

Several things have been mentioned about being able to change the F02 of a Suunto while under water. I did not realize this. Am I understanding this right?

[kitefisherman]

Double Down: The Suunto Vytec is a 3 gas computer that allows you to switch between preselected gases on the computer while underwater. When you switch to a deco gas (e.g., 80%) on the computer, your deco or no deco time is adjusted accordingly. Your gas choices (e.g., 32%, 50% and 80%) and partial pressure tolerances (e.g., 1.4, 1.5 or 1.6) are set at the surface and cannot be changed underwater.

A single gas nitrox computer (e.g., Cobra) doesn't allow you to do this. So if you scrub with high O2 your computer will not give you credit although your body still will. :D

Denny: 1 tank each: Limits of hogs & a mutton! Nothing of any great size.

[Double Down]

Kite,

Thank you for clearing that up. That was the best explaination I've heard. I understand now. I have the Cobra which was the basis for my confusion. I thought maybe I was missing something the comuter could do.

Thanks again...

[SANDIBOTTOM]

I believe I'm right about this..
Navy I'm told dives around 2.0
Commercial dive table i looked at and have a copy of is 1.8
We usually dive at 1.6 which is very safe.


every week we dive and are mixed at 1.6
our tanks ( Hogfish 8 and I) are always at 40%...This we have been doing for about 7 - 9 yrs...I feel yhis is very safe!!!!!I'm no spring chicken.........Our meter usualy indicates high o2 at the end of the day..This is 5 - 6 hp 120's,,,,,
but every one needs to do what they feel comfortable diving with....

[Hogfish8]

40%er all the way Baby!

[dlock]

I agree with the 40% mix. I dive just about every weekend. We dive three tanks, if there are no fish around we'll cut that dive short and go somewhere else to finish the tank. The deepest we've been going lately is 70'. I want to get the most out of my $7.50 EANx charge:D

[Doc]

1.6 ATA is still pretty conservative; unless your predisposed (read: epilepsy) this should not cause you to sieze.

In certan medical application of HBOT (hyperbaric o2 therapy) patients are run at 2.5 ata or higher for a full hour with no or very few attributable seizures; slight pulmonary congestion does occur on ocassion, but this is self limiting and usually reported as mild dyspnea. I have done this profile myself in a single place chamber and had no problems.

I have never heard of any case of a diver seizing at 1.6 ata; no matter how hard he was working. Has any body else? Almost all nitrox related deaths I hear about are breathing the deco gas at depth... that is the danger.

I personally choose to dive at 1.6 and dont worry a bit.

Getting bent is not black and white. Bubbles form on every dive; I like to limit them. But then again seizures at depth are fatal, the bends rarely are.