Inerts Offgassing and Sharing Deco Gases

[GLOC]

Another thing that came up this weekend.

If you are diving a 21/35 mix to 48m for 30mins and have 35mins of deco to do. You end up with 18mins of deco at 6m and then an ascent. With 8 mins of deco left to do, your buddy signals that he is running low on deco gas (50%).

Do you?

A. Give him your Deco Reg and switch to backgas and increase the stops by 1.5x or 2x, and then when he has finished his time, you take the deco reg back, he does * amount of time extra on his backgas, you finish your extension on deco gas and ascend.

or

B. As you only have 12 or 16 mins at 6m, you continue to breathe your deco gas and he switches to BG, and remain there until your normal stops would be completed and ascend

B is based on the assumption that the nitrogen makes up 46% of the BG you are breathing and the He will both ongas and offgas more quickly and as at 6m your deco method is primarily based on pressure gradient, rather than gas gradient, it won't make much difference for the timescales involved.

Again, please try not make this an agency thing, just interested in people's thoughts about the deco theory.

Regards

[deepbull]

send up a yellow bag and get some more gas sent down?


go ccr do less deco!


ada

[GLOC]

Originally Posted by deepbull

send up a yellow bag and get some more gas sent down?


go ccr do less deco!


ada

For that amount of time, by the time the Skipper was over, you would have run out!!

[nigelH]

Ignoring the fact that on CCR he can have any of my bailout gas...

Switch to my next richest mix, give him the gas and punch the a gas switch into the computer.
He might have to wait for me to finish but I bet he won't whinge.

If I was switching back onto a high helium mix I might pull the 'hold my weights in my hand' trick in case my ears go silly.

Actually some friends just buddy breathed the deco mix. A lung full of 50% isn't going to go down appreciably while your buddy has a slurp.

[David]

now did that 7 seem as good as an 80 at that point in time LOL

[japandiver]

I think that you have to go back to basic principles to decide the right answer:

a) Everyone must breath
b) Don't (unnecessarily) endanger yourself

Hence my solution would initially depend on how much back-gas is available - if OOG has adequate back-gas they should go onto back-gas and a yellow blob should go up. If a drop-tank appears then all is well, if not, when the non-OOG has completed their deco I would suggest that they could hand-off their deco cylinder and go onto back-gas whilst the OOG uses the dregs.

If OOG doesn't have adequate back-gas then I think that you would donate back-gas at 6m whilst continuing your deco and when complete hand-off deco gas.

It's not a great situation to be in and demonstrates why some groups use yellow blob=deco gas and why some of us have invested in gas-switching computers.

There's also a whole other debate possible here on doing mixed-gas dives with only one stage. Personally I almost always use a 32% stage and a hot mix / O2 stage when He is involved - cheaper and safer.

Hugh

[Juz]

Originally Posted by GLOC

With 8 mins of deco left to do, your buddy signals that he is running low on deco gas (50%).

What's 'running low'.

6m for 8 mins at 15l/min is only 192 ltrs
27b in a 7, 28b in an AL80

If by 'running low' you mean will run out, I have them breath their tim until they do actually run out, have them then go to back gas until I have finished and then hand off my tin.

In reality missing 4-5 mins of stops on that dive is likely to have no effect as most deco programs build in more conservatism than that.

Juz

[GLOC]

Just to clarify, OOG diver has loads of backgas.

A point you raise JD, can gas switching computers deal with multiple gas switches back and forth?

Originally Posted by Juz

What's 'running low'.

6m for 8 mins at 15l/min is only 192 ltrs
27b in a 7, 28b in an AL80

If by 'running low' you mean will run out, I have them breath their tim until they do actually run out, have them then go to back gas until I have finished and then hand off my tin.

In reality missing 4-5 mins of stops on that dive is likely to have no effect as most deco programs build in more conservatism than that.

Juz

10bar or so.

Agreed about the last point Juz.

[Juz]

Oh yeah, I'd slap the fool too for putting me in that position!

Juz

[Richie]

Originally Posted by GLOC

Just to clarify, OOG diver has loads of backgas.

A point you raise JD, can gas switching computers deal with multiple gas switches back and forth?

10bar or so.

Agreed about the last point Juz.

VR£ can apparently. I have not tested that in anger though.