Question Isobaric counter diffusion in CC

[AeroSpartacus]

SP on 0.5, I manage pO2 manually in the last 15-20m to manage minimum loop. From what I have seen, this is common practice with eCCR. Do you do differently?

I was taught to drop the SP down (to ~0.7) while ascending so that the solenoid isn't firing a bunch as the pressure drops, but would switch back to 1.2+ (after generally doing an O2 flush check sensors and also get back to a high PO2) once we reached the deco stop at ~20 feet. (To add, I'm only MOD1 so I haven't really had any required stops deeper than 20 feet yet, or longer than ~15 minutes).

 

[mr_v]

i dont. that sounds like a waste of gas. for what reason?

You establish a new baseline and flush out all unwanted gases. E.g., after switching to a low He mix, you remove He residuals from the loop. The same thing you do on deco when you periodically flush with O2, as you off-gas.

 

[LFMarm]

You establish a new baseline and flush out all unwanted gases. E.g., after switching to a low He mix, you remove He residuals from the loop. The same thing you do on deco when you periodically flush with O2, as you off-gas

Only difference between dil flushes with gas switch and O2 flushes is that you need to update the diluent on the computer otherwise it will assume the wrong fraction of He in the loop. For O2 flushes, the sensors take care of it.

 

[flymolo]

The same thing you do on deco when you periodically flush with O2, as you off-gas.

yup, this is why i think it's unnecessary. not really concerned with whatever minimal deco i'd maybe be saving

 

[Whitrzac]

SP on 0.5, I manage pO2 manually in the last 15-20m to manage minimum loop. From what I have seen, this is common practice with eCCR. Do you do differently?

That is not what your graph is showing or what you've been talking about doing.

So what are you actually doing in the water? What commentt are we suppose to believe.

 

[kensuf]

I think it depends on the dive profile.

Some general thinking points, if it's a <2-3 hour planned run, I wouldn't bother to perform a gas switch since the deco time savings would be incredibly negligible (oh wow, I saved 5 minutes, yippie!).

If it's deeper than 330' (100m), stairstepping down may be appropriate. Dropping to something with 30-45% helium in the 180-200' (55-60m) range is reasonable.

While I personally have done more than my fair share of switches to 32% at 120' (36m), I've seen some people get completely whacked with narcosis from it, so I'd recommend keeping helium in the mix to offset that.

Also keep in mind that every gas switch you make adds an opportunity to screw up. Sometimes keeping it simple really is the best solution.

 

[LFMarm]

Same dive profiles, now showing partial pressures instead of fractions.

Ascent with flushes

Ascent on bottom dil

 

[Snusmumrik]

What is the advantage of changing diluent during a dive?

Not being experienced Tx diver, me always take into account possibility of bailing out.
So, dil same as bailout mix lowers PPN2 and PPHe changes.

 

[justinthedeeps]

Good to exercise the switch drills when you can put it into the plan
Seems ok to skip when it's not practical

The effect on the computer of switching CC dil to a lower He fraction is evidently significant enough to lower runtimes by up to 10+%, probably because of the "helium penalty." Easy to like that benefit. But does it actually affect our true decompression or DCI risk? I think we can agree that nitrogen is a ghetto gas below ~20 metres, so not wrong to skip a big hit of that just to make the computer happy.

Gas in loop?
The amount to which off-gassing back into the loop affects deco is probably overstated. Much less than total loop volume, and we are constantly venting the loop as it expands on ascent, limiting buildup or concentration. He gas solubility in bodily fluids is limited, and lower that for nitrogen, albeit real. Maths could be done here

It is not wrong to stay on a high setpoint (including manually) on an ascent--this is flushing out unwanted gases every time you re-establish minimum loop volume (frequently). "Oxygen is cheap" they say--at least you're not chugging down a high O2 mix on OC.

 

[LFMarm]

I just checked the speed up in deco from doing dil switches for the 100m example dive. Here are the total dive times:

• Keeping bottom gas as dil for the whole dive (SP 1.3): 115 min
• Loop flush with oxygen at 6m (SP 1.3 until that point): 111 min
• Switching through all gases (11/74, 21/35, 40/0, 100/0): 100 min

Given the increased complexity and higher potential for ICD, on such a dive I would likely go for the second option and sacrifice 11 min of longer deco compared to the optimal gas switches. For dives with significantly longer deco, I may opt for the option of switching to all available gases and get a 10% acceleration on deco.

Thanks everyone for sharing their perspective.