Using gas mixtures to eliminate surface intervals?

[Diver0001]

Just as a theoretical possibility....

Suppose you made two dives, the first with Nitrox and the second with Heliox (or visa versa).

Would you need to account for the surface interval between dives in the planning for the second dive?

My thinking is no. I'd like to hear some expert opinion.

R..

 

[limeyx]

Just as a theoretical possibility....

Suppose you made two dives, the first with Nitrox and the second with Heliox (or visa versa).

Would you need to account for the surface interval between dives in the planning for the second dive?

My thinking is no. I'd like to hear some expert opinion.

R..

so you think the helium means you dont need to decompress? I dont think that's the case. You still need to offgas the helium, it's not "magic"

 

[PerroneFord]

"Helium is your friend."

Your ascent profile and gas selection controls your surface interval. But it's always planned. In fact, some of the deco programs force you to enter your surface interval as a dive to 0ft.

 

[Diver0001]

so you think the helium means you dont need to decompress? I dont think that's the case. You still need to offgas the helium, it's not "magic"

No, I think if you make the first dive with nitrogen and then second dive with helium (heliox - ie zero nitrogen) that you don't need to account for the residual nitrogen in your system.

At least in theory it makes some kind of sense.

R..

 

[rjack321]

You will eliminate the residual gas (N2 or He) really really fast on the botttom portion of the second dive since it won't be in your current mix.

 

[ianr33]

I think you guys are missing the point. If the first dive was done on heliox,then during a second dive done on nitrox the helium will be constantly offgassing (because there is no He in the breathing mix)

It might get interesting though when you start to ascend and the N2 offgasses as well as whatever He is left. The second dive on nitrox would probably have to be long enough to completely desaturate the helium from the first dive??

this was in reply to posts #2 and #3. You guys type too fast

 

[jimclarke]

Purely looking at disolved gas tensions you are right.

I think there is a snake under the grass. Microbubbles formed by one gas will have a an added gradient to enlarge them from the second gas.

This would make me very nervous.....further there is as far as I know no imperical data.

Perhaps breathing oxygen on the surface would be a fruitful line of enquiry?

Either way thanks for the stimulating idea.

 

[Diver0001]

Purely looking at disolved gas tensions you are right.

I think there is a snake under the grass. Microbubbles formed by one gas will have a an added gradient to enlarge them from the second gas.

This would make me very nervous.....further there is as far as I know no imperical data.

Either way thanks for the stimulating idea.

Exactly the reason I wanted some expert opinion. This thought had occurred to me too.

R..

 

[ianr33]

Purely looking at disolved gas tensions you are right.

I think there is a snake under the grass. Microbubbles formed by one gas will have a an added gradient to enlarge them from the second gas.

This would make me very nervous.....further there is as far as I know no imperical data.

Either way thanks for the stimulating idea.

But if you stay at a constant depth there will be no microbubbles formed (or at least no more than exist when not diving)

I think Diver0001 should volunteer to test this.

 

[rjack321]

I think there is a snake under the grass. Microbubbles formed by one gas will have a an added gradient to enlarge them from the second gas.
.

Not true, the gases are independent. The bubbles - if any- of N2 or HE (depending on dive order) will be driven to zilch very very fast. This is the whole principle behind accelerated deco on O2 or 50%. This dive is just ascending then switching gases. So throughout the 2nd dive you off-gas whatever you on-gased from dive 1.

And the deeper you go on dive 2, the faster you offgas the residuals from dive1. Its like being at negative ATAs from the perspective of that residual gas.