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Thread: Calculating scrubber

  1. #1
    Captain Meivi is an unknown quantity at this point Meivi's Avatar
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    Calculating scrubber

    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?

  2. #2
    RBW Member DaleHawerchuck is an unknown quantity at this point DaleHawerchuck's Avatar
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    Re: Calculating scrubber

    Manufacturers are rating scrubber capacity at high CO2 production rate to account for different other factors but mainly to be on the safe side, as in normal conditions nobody would produce that much CO2 during the whole dive.

    Usual O2 consumption rates (CO2 is basically at 1:1 ratio)
    - 0.5L / min at rest
    - 1.0L / min during moderate work
    - 2.0L / min during heavy work

    This being said, I strongly recommend sticking to these conservative numbers. Overbreathing scrubber to get the "most out of it" and to save a few bucks is breaking the 1st rule of CCR: THOU SHALL NOT BE CHEAP...

    Just keep this margin in case you have a real emergency and to account for all the variables affecting your scrubber life.Your life is worth more than a 30$ scrubber refill...


    Sent from my SGH-I337M using Tapatalk

  3. #3
    Captain Meivi is an unknown quantity at this point Meivi's Avatar
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    Re: Calculating scrubber

    Quote Originally Posted by DaleHawerchuck  View Original Post
    Manufacturers are rating scrubber capacity at high CO2 production rate to account for different other factors but mainly to be on the safe side, as in normal conditions nobody would produce that much CO2 during the whole dive.

    Usual O2 consumption rates (CO2 is basically at 1:1 ratio)
    - 0.5L / min at rest
    - 1.0L / min during moderate work
    - 2.0L / min during heavy work

    This being said, I strongly recommend sticking to these conservative numbers. Overbreathing scrubber to get the "most out of it" and to save a few bucks is breaking the 1st rule of CCR: THOU SHALL NOT BE CHEAP...

    Just keep this margin in case you have a real emergency and to account for all the variables affecting your scrubber life.Your life is worth more than a 30$ scrubber refill...


    Sent from my SGH-I337M using Tapatalk
    Yea i know all this about conservativity, and I will still be conservative, but its been buggning me not to have nothing more to look at then "Scrubber duration" in my prism 2 manual!

    Thanks!

  4. #4
    Photographer & Journalist John Liddiard has a spectacular aura about John Liddiard has a spectacular aura about John Liddiard has a spectacular aura about John Liddiard has a spectacular aura about John Liddiard has a spectacular aura about John Liddiard's Avatar
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    rEvo II Mini Hybrid

    Re: Calculating scrubber

    Read the recent Shearwater blog post
    https://www.shearwater.com/monthly-b...rs-rebreather/

  5. #5
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    Re: Calculating scrubber

    This seems to be a pet bitch of mine.

    I was fortunate enough to be involved in the development of a lower cost respiratory calorimeter back in 2003/4. These machines basically measure tidal volume as well as PPO2 and PPCO2 exhaled. Used for a few different applications including calculation of dietary requirements for coma patients, in our case the target was monitoring of resting metabolic rate to track the effectiveness of different weight loss strategies on individuals.

    Anyway. One of the key metrics this thing pumped out was the respiratory quotient (RQ). A decade ago or more so I can't remember which was on top or bottom of the dividing line but it's the ratio of O2 consumed to CO2 produced. Every time I see scrubber duration linked to O2 consumption I cringe because I've seen how variable that RQ can be. From 0.6 to about 1.3, not only from person to person but for an individual. A nominal RQ is about 0.8, not 1:1. But I've run a sample on myself at 0.7, drunk a can of coke and watched it climb to 1.25 over ten minutes and stay there for an hour before crashing back. Ever heard about aerobic vs anaerobic exercise? I could slowly ramp up from a walk at 0.8 and, at a certain heart rate, watch it smash up to 1.2. Get the rate down from r a bit and it would settle back.

    My point is that 1:1 is a bloody dangerous way to calculate it. Even before you consider things like channelling, wall creep, dwell time etc.

    Use manufacturer recommendations for a COTS breather or, for a home build, use some very conservative calls and preferably a temp stick too...

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    Re: Calculating scrubber

    Quote Originally Posted by Meivi  View Original Post
    Yea i know all this about conservativity, and I will still be conservative, but its been buggning me not to have nothing more to look at then "Scrubber duration" in my prism 2 manual!

    Thanks!
    Donn the prism and get a rEvo rms and you will have way more to look at than scrubber duration in the manual.
    You will have remaining cycle time, remaining scrubber time, temperature ;)

  7. #7
    So many CCR So little etc Mark Chase has a reputation beyond repute Mark Chase has a reputation beyond repute Mark Chase has a reputation beyond repute Mark Chase has a reputation beyond repute Mark Chase has a reputation beyond repute Mark Chase has a reputation beyond repute Mark Chase has a reputation beyond repute Mark Chase has a reputation beyond repute Mark Chase has a reputation beyond repute Mark Chase has a reputation beyond repute Mark Chase has a reputation beyond repute Mark Chase's Avatar
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    Re: Calculating scrubber

    Its a simple calculation

    C02 = 85% of V02

    Vo2 = 4% of RMV

    So my avg RMV is 15lpm so that's X 4% = 0.6 C02

    So in theory 1.6 divided by 0.6 X test duration

    That's 3 X (1.6 / 0.6) = 7.99 hours

    The margin for error then comes in with depth and water temperature.

    Personaly, I run 6 hours max for dive planning

    Scrubber failure due to bad packing can happen instantly or well within 3 hours, so I don't consider it an issue that affects overall dive planning. I pack tight ALL the time to give consistency and I know that's good for 6 hours because I have done it many times.
    Saying stick to 3 hours is great but CCR was reely the tool for deep tec stuff and 3 hours isn't enough


    ATB

  8. #8
    Captain Meivi is an unknown quantity at this point Meivi's Avatar
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    Re: Calculating scrubber

    Quote Originally Posted by Mark Chase  View Original Post
    Its a simple calculation

    C02 = 85% of V02

    Vo2 = 4% of RMV

    So my avg RMV is 15lpm so that's X 4% = 0.6 C02

    So in theory 1.6 divided by 0.6 X test duration

    That's 3 X (1.6 / 0.6) = 7.99 hours

    The margin for error then comes in with depth and water temperature.

    Personaly, I run 6 hours max for dive planning

    ATB

    Correct if I'm wrong.

    My RMV is about 16

    4% of 16 = 0,64

    That's 0,64 litres of oxygen per minute.

    And my Co2 should then be 85% of 0,64 = 0,54litrs co2 per minute.


    But from where did you get that 1,6?

  9. #9
    RBW Member Concerned Diver is an unknown quantity at this point Concerned Diver's Avatar
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    Re: Calculating scrubber

    I have a prism 2. I have done numerous 5 hour dives never been an issue. I don't really want to stay under much longer than that just because I'm ready to get out by then. I consider my breathing under a moderate workload.

  10. #10
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    Re: Calculating scrubber

    Quote Originally Posted by Meivi  View Original Post
    But from where did you get that 1,6?


    1,6 l/min CO2 is the test condition for the CE test of the scrubbers on which the manufacturers base their recommendation from usually (with exceptions, so always look at the fine print)

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