Oxygen narcotic?

[VE7DAC]

I normally fly my breather at a 1.0 setpoint..

Fair enough, I've heard of people running setpoints ranging from 0.8 to 1.6.

I guess my real question is, if oxygen is narcotic, why is there no discussion about it when talking about accelerated deco? Or is it not a significant factor when shallow? And if it isn't a significant factor when shallow, why should it be an issue at depth?

 

[Jeremy Williams]

All gasses have a narcotic effect at atmospheric pressure. It turns out that Nitrogen and Oxygen are pretty similarly narcotic. Our nervous systems are adapted to air so that it feels completely normal at 1 ATM. Depth obviously increases the partial pressure of each gas. The narcotic factor of each gas is then multiplied by the partial pressure. At 2 ATM the small narcotic effect becomes twice as large but it’s still relatively small. As the partial pressure increases so does the narcotic effect.

 

[Storker]

Generally, narcosis is seen as an issue at depths from 30-40m and downwards. If we for simplicity assume that N2 and O2 are roughly equally narcotic, that means that the total narcotic gas pressure is at least 4 ATA before we start worrying about narcosis.

Now, still using the ballpark assumption that N2 an O2 are equally narcotic, what is the total narcotic gas pressure of a) 50% at 21m? b) 100% at 6m? c) An RB loop with 1.3 bar pPO2 and 10/70 dil at, say, 50m?

 

[taimen]

Gas blender toolkit android app currently has an illustrative calculator for rebreather diluent END with different setpoints.
Not a complex calculation to do, but still that app is illustrative.

 

[Storker]

Not a complex calculation to do, but still that app is illustrative.

I totally agree that it isn't a complex calculation to do. And because of that, I also think that anyone thinking about dabbling in mixed gases ought to do those calcs by hand (or by calculator app, or by spreadsheet) to get a feel for the numbers. Because I also believe that doing the actual calculations helps developing a feeling that just punching stuff into an app where the math takes place behind a curtain doesn't.

Just MNSHO, mind you.

 

[taimen]

Luckily that app has all the parameters adjustable, making it illustrative for someone who wants to figure out what is the difference between O2 considered narcotic or not. All divers are not as comfortable with math as you @Storker (and me as well)
I actually asked the developer of that app to include this feature into the already excellent app to demonstrate these effects for some not so scholarly divers. Honestly, this simple calculation is harder than you imagine for many people, who can be great divers. Struggling with calculus stops learning for many and teaching calculus on a diving course is not the point.

 

[ofg-1]

I am not sure about narcotic, but if you combine it with hydrogen to make hydrogen hydroxide it is definitely a inhalation hazard.

 

[Storker]

All divers are not as comfortable with math as you @Storker (and me as well)

I agree. But still, shouldn't someone planning to use mixed gas (beyond simple rec nitrox: "use 32%, don't go below 33m") be able to calculate at least the first two examples I gave?

I'll concede that the last example I gave may be a bit complicated for those who aren't mathematically inclined, but I believe that calculating ATA for a given depth (which the two first examples really are) was covered in my OWD curriculum.

Spoiler

a) 21m = 3.1 ATA
b) 6m = 1.6 ATA

Both numbers are well below the 4-5 ATA which are generally considered somewhat narcotic

 

[cerich]

Sorry to wake this thread up again, but how does the idea of oxygen narcosis get along with rebreather use?

If you accept 1.4-1.6 PPO2 as a safe CNS limit, but also think it causes narcosis at that level, shouldn't rebreather divers be constantly narced?

What about deco stops on 100%? If oxygen is narcotic, I'd expect to feel those effects at a 6M stop.

Or am I missing something here?

using a setpoint of 1.4 or 1.6 is really mostly reserved for deco by many ccr divers. You will find 1-1.2 more common with longer duration dives or multi day dives planned to the lower end.

 

[cerich]

I normally fly my breather at a 1.0 setpoint..

I see people saying that all the time, yet I have NEVER seen anybody arrive at a dive site flying their CCR like a jetpack. In fact, I can't find the darn fly button on either of mine and I have a pilots license and kinda want to. I've seen them come in trucks, vans, cars, even a motobike once, but never being flown. Plus, why would you be on the loop while flying below 10,000 agl anyhow?