Switching diluents on ascent

Dirkadiver

I'm not a Rebreather diver yet, but dive with a few who are & am always asking questions as to the how's & why's of it all. I'm mostly a lurker on this board soaking up as much info as I can as I look to head down the RB path next year, but after a dive we did a couple weeks ago, I felt the urge to post this question.

My specific question is this: we did a dive to 250' for about 15min. 3 of us were on OC & 1 was on a Rebreather. The RB diver was diving 10/50 as diluent. At our 70' stop, I noticed bubbles coming up from below me & I knew the only person below me was the RB diver, so I quickly started dropping back down & was frantically trying to get his attention to see what was going on. He kept signaling ok & was fumbling around with one of his offboard tanks & his CL. Once everything settled out, and I was convinced everything was ok, we finished our deco. On the boat, I asked what had happened and he said that he switched from his onboad diluent(10/50) to his offboard air(drysuit bottle). On this specific dive, does this make sense? I've run it on V-Planner, which I'm not proficient at yet w/ CC profiles, but based on what I ran, it only made a 2 minute difference in total run time (please correct me if I'm wrong on this) & displayed an IBCD warning. How many of you would have gone through the trouble/risk of doing this for the seemingly nil benefit(again, on this specific dive)?

My 2nd question is what kind of dive profile, if any, would this be a common/sensible practice?

Remember, I'm asking this never having dived a Rebreather before, and as I mentioned, I'm far from proficient w/ planning CC dives using V-planner(this was my first go at it).

Ferg3333

It considered good practice to run the same diluent throughout the dive on CCR.

You are off gassing helium into the loop which could cause problems.

If you need to use multiple diluents then you would be aware of the associated problems etc

By keeping the same diluent and pushing your ppo2 up at the final stops (CNS clocks) permitting you can reduce the decompression obligation.

Just my 2p worth

Dirkadiver

To maintain your PO2, do you not have to flush/exhaust gas from the loop & replace it w/ O2 on ascent? Would this not rid the loop of a lot of the HE?

aainslie

It won't change the N2/He ratio.

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Andrew Ainslie

Dirkadiver

I realize that. That relationship will be fixed througout the dive. That's physics:) Ferg just mentioned that you're offgassing He, which could be a problem, but didn't elaborate.

Care to share your thoughts on my original post?

rossh

Using V-Planner, put in your 10/50 bottom dil, and Air swap in ascent. Take a look at the ppInert graphs. See how the N2 inspired increases dramatically? You shouldn't do that, particularly on bigger dives. The best deco will come with a continuous reduction of each inert gas through the ascent.

Regards

divetheworld

Google "Isobaric Counter Diffusion".

By the time your at 6m, you'll be on either 80% or 100% O2. The distance/time between the switch point and 6m will only be a handful of mins. Not worth the extra grief or hazards for a few mins of deco.

Brent

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teomannaskali

I am only speculating but I think the chap did not want to have a hypoxic dil plugged in to his adv . So he swapped to air, so in case of emergency he had something breathable (never need to use it if you are ascending, and maby he did not do a loop flush, just let out some bubbles because of the expanding gas during ascent).

So the reason had nothing to do with decompression, just safety (increased risk due to high N2 but decreased risk of breathing low O2, which can get you quicker)

So while he ascends he is still on his bottom mix that he is constantly adding oxygen, and if something goes wrong, he adds air in to his loop, saves the moment and then he can re calculate his decompression.

Teoman

jkaterenchuk

I do not have a lot of experience with switching except to practise but it seems to be alot of hassle and extra risks to manage for saving a few minutes of deco.

Ross:

Can you explain how you ICB alert works in V Planner?

Can it be modified by the user? If so why?

Can you point us to any literature or reports in which your alert is based?

John

lizardland

I've tried swapping dils on the way up and it wipes me out post dive. I feel tired and a bit niggly, staying on helium dil all the way up I feel totally different, no major fatigue. For the diving I do generally (max 2-3hrs) I just don't think it's worth the hassle.

Big problem is where you end up spending a lot of time without ascending, i.e. the last stop. At 6m after a deep and/or long dive it can be a struggle maintaining a high setpoint due to inerts being offgassed into the loop, especially in the first 20min or so.

Cheers,

Stuart