Deco penalties

[Rec Diver]

Dude.....you have things way mixed up. The presence of helium in a trimix is the reason why it is less narcotic. This is why END calculations only use the Helium concentration and do not include the O2 concentration.

Dude, the reason you are using helium is to replace the nitrogen you have removed from the mix. Air is a mixture of 21% oxygen and 79% nitrogen. A trimix of 35/25 uses a mixture of 35% oxygen, 25% helium, and 40% nitrogen. It is the reduction of nitrogen that reuces narcosis not the increase of oxygen. Since you have reduced the nitrogen level in the mix, and can not add anymore oxygen, you must make up the difference with some kind of breathable gas. And for now it is helium.

Counting on Nitrox to reduce narcosis is not a good idea IMHO

I would suggest that you read the link that Miketsp gave.

Oxygen is just as narcodic as nitrogen. I've felt sudden narcosis from switching from Trimix to an EAN deco. This is also one reason (of several) why I put some He in my deep deco mix. 21/35 for my 190 MOD bottle, and 35/25 in my 120 MOD bottle.

If I am understanding correctly, when you switched from one of your trimix mixtures with a low quantity of nitrogen to an EAN mixture with a higher quantitiy of nitrtogen you felt the effects of narcosis. This exactly what should happen. The higher percenatge of nitrogen the higher the effects of narcosis. Again I would suggest that you read Miketsp's link.

 

[Rec Diver]

Oh, you quoted very recent research

Hmmmm....what to do?

 

[dumpsterDiver]

Well, on our second dive off the boat in Monterey, my computer thought I should spend 20 minutes at 10 feet.

Of course, I'd forgotten to set it for 32%.

I'd been watching my depth meter and timer, and I knew where I really stood. That was the dive where I decided that carrying the computer didn't make any sense. If I was going to ignore what it said anyway, why have it beeping at me? And it was furious when I took it out of the water, and refused to talk to me for two days.

As you gain more experience in computer diving, you will learn to do computer Deco-diving when problems like this arise.. This involves tieing the computer off to a line at 15 feet and allowing it plenty of time to deco while you eat cookies and tell lies on the boat.

 

[MonkSeal]

Hmmmm....what to do?

To do more research:
http://dir-diver.com/en/knowledge/nitrox_1.html
http://dir-diver.com/en/knowledge/nitrox_2.html

A simple Google of "oxygen narcotic" will give enough results.

 

[Rec Diver]

To do more research:
http://dir-diver.com/en/knowledge/nitrox_1.html
http://dir-diver.com/en/knowledge/nitrox_2.html

Oh no, not George Irvine! Is he even a doctor? This is the problem with DIR...brain washing. I thought that you might be getting your information from another sourcce rather than form experience and knowledge.

But as we all know if your not Doing It Right, well then you must be doing it wrong. I wonder who was the savior of the diving world before George Irvine?

 

[Dan Gibson]

So if I were to plan my deco schedule and it calls for 15 minutes on O2 at 20' after diving on air, and then I loose the use of O2 at 20', I can just double my 20' stop to 30 minutes?

You sure about that ratio?

Deco stops with 100% oxygen can cut your deco time in half as compared to deco stops on air. This is very, very efficient.

 

[Rec Diver]

So if I were to plan my deco schedule and it calls for 15 minutes on O2 at 20' after diving on air, and then I loose the use of O2 at 20', I can just double my 20' stop to 30 minutes?

You sure about that ratio?

No. you would have to check your tables for an accurate deco times and stops. I was only making a quick analogy. However, you bring up a good point don't you? If you loose your deco gas you could always revert to your back gas for your decompression obligation. As long as you conserved the required amount for reduntancy.

 

[JeffG]

Oh no, not George Irvine! Is he even a doctor? This is the problem with DIR...brain washing. I thought that you might be getting your information from another sourcce rather than form experience and knowledge.

But as we all know if your not Doing It Right, well then you must be doing it wrong. I wonder who was the savior of the diving world before George Irvine, Jacques Cousteau?

I guess if it was George that wrote that your point might be stronger. But as it is....

 

[limeyx]

Dude, the reason you are using helium is to replace the nitrogen you have removed from the mix. Air is a mixture of 21% oxygen and 79% nitrogen. A trimix of 35/25 uses a mixture of 35% oxygen, 25% helium, and 40% nitrogen. It is the reduction of nitrogen that reuces narcosis not the increase of oxygen. Since you have reduced the nitrogen level in the mix, and can not add anymore oxygen, you must make up the difference with some kind of breathable gas. And for now it is helium.
...

I think it's fair to say that there is as yet no definitive answer to whether or not O2 is narcotic.

There is one camp that says not, and another camp that says "Maybe, maybe not, but let's split the difference and assume O2 is about as narcotic as N2"

One other benefit of adding He to replace N2 is that it's a smaller molecule and easier to breathe, so you tend to build up less CO2 than if you had N2 (and weighs less so carrying your tanks doesn't hurt so much

Also, diving deep with elevated PPO2 will increase your CNS% for the dive, and may (depending on what research/anecdotal evidence you believe) make deco less effective as you've already exposed your lungs/circulatory system to elevated levels of O2.

I tend toward the conservative side and always assume O2 is about as narcotic as N2, try to dive with an average PPO2 of 1.2/1.6 and use generous amounts of helium.

I have never noticed a difference diving nitrox to 100 feet compared with air, but then I haven't really made an attempt at a scientific/blind test.

 

[Dan Gibson]

I guess I should have put some emiticons on my post since you actualy thought I was seriously asking a question. I know what to do and would not need to check my tables as I know the ratio is not 2:1.

No. you would have to check your tables for an accurate deco times and stops. I was only making a quick analogy. However, you bring up a good point don't you? If you loose your deco gas you could always revert to your back gas for your decompression obligation. As long as you conserved the required amount for reduntancy.