O2 consumption Question- CCR divers ???

[turnburglar]

Hey Divers,

I am doing preliminary research into high altitude life support systems and have a few questions that where not covered in open water.

If a rebreather was loaded with 100% O2 and the wearer where under 30% atmospheric pressure (~5psi) how long would that bottle last? Is there some formula for figuring out bottle capacity = hours of metabolic consumption?


Any help is greatly appreciated.

 

[tbone1004]

Atmospheric pressure does not impact your bodies consumption of O2. Your body doesn't necessarily have a "fixed" rate of consumption, but consumption is typically 1-1.5 liters of O2 per minute. This will depend on body type, size, etc. Large males will use more than small females. Will also depend on exertion levels. If you're sitting on the couch watching tv in a warm environment it will be much lower than if you are hiking up a mountain.

 

[turnburglar]

Hey tbone thanks for the reply.

So at a consumption rate of 1 liter/min how long would one of these 3 liter bottles filled to (3400 psi) last?

Faber FX-23 (3L) Cylinder for CCR, White | Dive Gear Express®

Obvisouly the scrubber will be next on the list:

How much scrubber would be needed for 8-12 hour of scrubbing? And can scrubbers work in cold temperatures? (-50f to +40f)?

 

[victorzamora]

So, a 3L tank holds 3L at one atmosphere. Full (3442psi) is 232bar, which means that the 3L tanks hold 696 liters (3L*232bar) when full. Filling oxygen to that high of pressure makes me a little nervous, but even 3000psi (200bar) is 600L. That's 10 hours.

Cold temperatures make scrubbers less effective, but a fairly conservative rule of thumb is one pound of scrubber per hour.

This all begs the question, though: What kind of high altitude life support are you talking about?

 

[The Chairman]

I use just a tad under 0.5 Bar/minute on my 3L. typically my O2 is filled to 200 bar. I tend to burn more when shallow.

 

[Gareth J]

One of the advantages of being at altitude, is the scrubber won't suffer the efficiency issues of being at depth, that diving scrubbers do.
One of the issues that diving scrubbers have to contend with is as the diver gets deeper, they amount of gas in the breathing loop increases, but the O2 consumed remains the same, and therefore the CO2 produced remains the same. However the CO2 becomes proportionally less and less within the loop as the depth increases and the breathing loop pressure increases. This makes it harder for the scrubber to remove the CO2. So the gas travels further through the scrubber before all the CO2 is removed.
This is why the life of the scrubber is given as time at a specific depth, and the time increases as you get shallower.

A more poetic explanation is here

The Drogon Network [Inspiration]


Gareth

 

[turnburglar]

Gareth thanks for the great reply. It makes sense that the scrubbers become less effective when there's more pressure. So if I have a constant pressure of .3 ATM the scrubbers should last awhile compared to diving?

 

[Gareth J]

Gareth thanks for the great reply. It makes sense that the scrubbers become less effective when there's more pressure. So if I have a constant pressure of .3 ATM the scrubbers should last awhile compared to diving?

Yes, it's more efficient, the target contaminant makes up a larger proportion of the gas passing through the scrubber, so the sofnalime has more opportunity to react with it.
However, you still have the issue of cold interfering with the scrubber efficiency. Once the chemical reaction starts, it is exothermic, so produces heat and the issue is less of a concern. In diving we are breath the scrubber, this achieves two things. It confirms the scrubber is working, (if you don't black out ), it gets the chemical reaction initiated prior to cooling once you enter the water.

Gareth

 

[tbone1004]

you also only get so much scrubbing capability. The scrubbers can be matched to the O2 bottles. I.e. the scrubber is theoretically capable of scrubber say 600l of CO2, so if you have a 3l tank and fill it to 200 bar, then the o2 and scrubber should run out around the same time which prevents people from staying on the loop too long and going into a hypercapnia scenario which is bad

 

[turnburglar]

Hey guys new questions about scrubbers. For this thought experiment let's assume it doesn't make sense to use the common sofnalime. Whatever scrubber material is chosen needs to be reusable. Doing some reading: submarines and the space station are using silver oxide canisters for CO2 scrubbing. From what I understand the silver oxide turns into silver carbonate after absorbing Co2 in 25% humidity. They can then take the used canisters and bake them at 400F and the silver carbonate reduces back to silver oxide making the canisters almost indefinitely reusable. The trouble with silver oxide- really expensive and doesn't hold that much CO2. I watched an MIT lecture it was mentioned that silver oxide only holds 15% of its weight in Co2 where LiOH (non re useable) can almost retain 100% of its weight in CO2.


How much CO2 can be made in the typical 8 hour work day?

Silver oxide reference:

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