Calculating CO2

[Meivi]

So ive been thinking. There are a lot of different forums and ppl talking about getting the most out of our scrubbers.

All different rebreathers got a different amount of scrubber capacity and all of the have their "max time" for the scrubber and so on.

Taken from the manual on my Prism 2:

240 min @(60fsw, 40F, 1,3 l co2/min, 0,5% CO2 SEV)

And now i wonder, how much is 1,3 l co2/min... How do I determine how much CO2 im acutally breathing out through the loop... For this I have been searching for a good answer, now is since we always easily can calculate how much O2 l/min we are using, isnt there a way to make some calculating over how much CO2 we've been exhaling?

 

[northernone]

Parliminary reply based on my reading.

Depending on metabolism (and diet primarily) it's our respiratory quotient which is 1l o2 to produce .7-1.0l of co2... This ratio can vary greatly.

Regards,
Cameron

 

[tbone1004]

pretty simple. You know the weight of sorb that is in the scrubber and based on that you can extrapolate how much CO2 it can hold. You then have to "fudge" it based on breakthru since the activated area isn't flat and linear. That comes back to how much CO2 you have from up there.

So, as @northernone said above, you typically assume a 1:1 ratio of O2 to CO2 to be safe and that's your duration. Put a bar pressure gauge on your O2 bottle and know how many bar you can used based on liters of CO2 capacity and you have a close enough number

 

[Meivi]

Thanks alot,
and If i may ask @northernone, by diet what do you mean?

well I'm counting about 1,5l/min O2, just becuse its kinda difficult to reed the exact numbers on an spg... but i might be counting a bit high on that, i dont know...

 

[KevinNM]

In theory you can get someone to measure your metabolic efficiency. I have never done this, so here's an explanation: Metabolic Efficiency Testing: Explanation and motivations

 

[northernone]

Thanks alot,
and If i may ask @northernone, by diet what do you mean?

well I'm counting about 1,5l/min O2, just becuse its kinda difficult to reed the exact numbers on an spg... but i might be counting a bit high on that, i dont know...

Learned this reading something about the space shuttle's scrubber and nutrition. Depending what you eat the body metabolizes o2 into co2 at a different ratio. Hard science and can be personally tested with the right equipment.

Google says:
The Respiratory Quotient value indicates which macronutrients are being metabolized, as different energy pathways are used for fats, carbohydrates, and proteins. A value of 0.7 indicates that lipids are being metabolized, 0.8 for proteins, and 1.0 for carbohydrates. The approximate respiratory quotient of a mixed diet is 0.8

I'm knee deep in dark waters but that's the little I know about it.

It's worth noting estimating scrubber duration using math is risky.... I've had a few intentional breakthroughs testing homebuilds and even expecting it, the experience is not pleasant. A Co2 hit can pretty quickly become lethal.

Safety margins are built into manufacturer recommendations. A few builds even have published test results....

Regards,
Cameron

 

[Meivi]

Yea ofcourse its riskt, and i wouldnt change the way im doint it now for the little I know about it,
But that with diet is really interesting, and there seems like ive found another "bad" thinkg about carbs (im a no carbs kinda man)


This has been good to get some answers about this, I've been trying find this out for some time now.
Thanks all!

 

[tbone1004]

use 1:1, added conservatism and all that, but more importantly for real world diving, it's easy math that you can do in your head. "I have a 300l co2 capacity in my scrubber, I've used 250l out of my O2 bottle, hrrrmmm time to go up?"