Remove co2 while filling diluent?

[Gowanus Diver]

Disclaimer: Not a rebreather diver, never planning on going tech. Just a curious nerd.

Does anyone try to remove atmospheric CO2 when filling your diluent tank? Most dil mixes, as I understand it, are just filtered air @ 21%, or if Trimix air + helium. However air is already ~0.05% CO2. You’re carrying that down just to have the scrubber absorb it.

I guess that really doesn’t matter relative to the metabolic respiration, after all you’re going to convert the ~full O2 tank to CO2, while the dil will mostly circulate and only <=0.05% was CO2 to begin with…

 

[bubblemonkey2]

I have wondered this too for compressed gases in general, but really for anything that happens to be in the compressor intake (CO2, CO, argon, etc) CO is obviously lethal, and argon (highly narcotic?) can be a problem in some oxygen concentrating systems. Probably the finest setups have CO2, CO, argon meters to monitor this in the input and output gases.

The short answer I think is ordinary ambient CO2 doesn't matter at the depths where air is considered appropriate as a diluent (i.e. 40 meters and shallower). It probably is fine even at the open circuit MOD around 60 meters, but air is not a good diluent there for other reasons.

Additionally, as soon as you stop adding diluent to the loop, the portion of 'ambient' CO2 quickly vanishes because you are recycling the same diluent through the scrubber without adding any more ambient CO2 than what was in the initial loop fill.

I'd be curious whether the ppCO2 in air used as an open circuit breathing gas at or below 60 meters has been a factor for people who've been desperate or brave enough to engage in such foolishness.

Assuming 500ppm CO2 (0.0005 as a fraction)
At 40 metres (~5 atm), with air the ppCO2 is 0.0025 which would 'feel like'(?) like 2500ppm on the surface
At 90 meters (~10 atm), with air the ppCO2 is 0.005 which would 'feel like' 5000ppm on the surface

5000ppm is listed as an '8-hour exposure limit.' Anything near or above that might affect a diver?

On a 90 meter dive one should be on a diluent and/or bailout with ~65% helium in it, and a squirt of pure oxygen, so the ambient ppCO2 would be reduced to about one third.

 

[tbone1004]

Disclaimer: Not a rebreather diver, never planning on going tech. Just a curious nerd.

Does anyone try to remove atmospheric CO2 when filling your diluent tank? Most dil mixes, as I understand it, are just filtered air @ 21%, or if Trimix air + helium. However air is already ~0.05% CO2. You’re carrying that down just to have the scrubber absorb it.

I guess that really doesn’t matter relative to the metabolic respiration, after all you’re going to convert the ~full O2 tank to CO2, while the dil will mostly circulate and only <=0.05% was CO2 to begin with…

the ppCO2 in the gas that you start with is negligible. In compressed breathing gas it is often slightly higher than ambient because any CO from the compressor is converted to CO2. The scrubber is capable of scrubbing roughly 120liters of CO2 per kg and our scrubbers are usually around 3kg, so 360liters of CO2 can be scrubbed. A CCR has call it 10L of air volume at the surface including what's in your lungs and .0005 that's .005 liters it has to remove which out of 360l capacity is not deemed significant.

Also of note that many divers start our dive with a loop full of O2 to best facilitate the 1.6ppO2 check at 20ft though to get pure O2 in the loop the ambient gas has to be flushed out so it's getting scrubbed regardless.

 

[nadwidny]

Disclaimer: Not a rebreather diver, never planning on going tech. Just a curious nerd.

Does anyone try to remove atmospheric CO2 when filling your diluent tank? Most dil mixes, as I understand it, are just filtered air @ 21%, or if Trimix air + helium. However air is already ~0.05% CO2. You’re carrying that down just to have the scrubber absorb it.

I guess that really doesn’t matter relative to the metabolic respiration, after all you’re going to convert the ~full O2 tank to CO2, while the dil will mostly circulate and only <=0.05% was CO2 to begin with…

It's negligible.