Suunto Cobra & Nitrox

I have a newly purchased Cobra with only 20 dives on it. I am wondering about diving on standard air and using my pony for my safety stop. My pony is currently filled with 36% Nitrox. I plan on breathing the pony on my safety stop. While I am at 15' making my safety stop, the computer will register that I am stopped but will also register that I haven't used any of my main gas. I understand that I will not be able to register the 36% nitrox into my computer, even though it wouldn't matter because the main gas would not deplete. Has anyone else run into a similar situation? How did your computer handle the situation?

On the computer the remain air calculation may be off. But I never pay much attention to it. Further, the dive software may not be in correct for your SAC as I am not sure how that calculation is done over time. I.e. if no gas is used during the time period then it is added into the running average or not. You could easily test this by doing a dive to 33' for ten minutes and just sit on the bottom. Then do another but breath off of your back gas for five then your pony five. See what the sac rates are.


Also on a side note breathing 36% for your safety stop will not add much of a margin over just air as the partial pressure difference is not significant enough to do alot of good.

PO2 21% = 0.31
PO2 36% = 0.52

To reduce the nitrogen efficently you want the window to be greater than 1. As such, what you want to do is get on the Nitrox at the highest PO2 you can. Which means at depth or a higher concentration.

First off, the Cobra only stores starting and ending PSI.

Second, the Cobra won't change its calculations for your time spent on the EAN36, because you can't change gasses during the dive -- for that, you'd need the gas-switching capabilities of the Vytec, D6 or D9.

I've used EAN50 on a deco stop using the Corbra and it just ticked down the stop as if I were diving on air or EAN32. It doesn't really know that you are not breathing from the main tank. It's ill-suited for gas switching and I plan to replace it.

I've done this with my Cobra. The SAC will be off (but so what?) and the deco calcualtions will be off, in your favor, slightly.

Go ahead and use your pony bottle, and don't worry about the computer.

Turtle Diver:

I've done this with my Cobra. The SAC will be off (but so what?) and the deco calcualtions will be off, in your favor, slightly.

Go ahead and use your pony bottle, and don't worry about the computer.

Click to expand...

"in your favor" -- you mean on the side of safety, right?

Yes ... in your favor because your computer is set for air but you are breathing gas at a higher PO2 which will allow for slightly better off gassing but as I posted previously not that much better.

Given the hassle of replenishing the Pony bottle, I can't see the point in doing this at all. The pp02 at 15ft on EAN36 is so low, it's really not worth switching over for a few minutes.