It finally happened - my CCR tried to kill me
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One small point with regard to dwell time and RMV. The curve of respiration is a sine wave - slow initial velocity, fast mid-portion, and a trail-off at the end.
Think of breathing your max, say 40 full chest-fulls pet minute. Your can't sustain that, I agree. But let's look at the numbers. If you're a huge guy, 2.5 liters per breath x 40 = 100 lpm, which equals 3.5 CFM. You're above the testing limit, but as @tbone1004 points out, that's not sustainable, and it's therefore not a stress to the scrubber as tested.
But let's look at the dynamic of it. Let's presuppose hard work, but only half of that max, i.e., 50 lpm (1.75 CFM). Easily below scrubber max. But what is the max flow? The 50 lpm is both in and out, so at a minimum, the 50 liters is going out during only half the minute, which makes the minimum expiratory flow 100 lpm. And due to the acceleration and deceleration phases of each breath, max flow may only be occurring during 60% of each cycle. In other words, if 85% of each breath is packed into the middle 60% of each cycle, the max flow is now not 50 lpm, not 100 lpm, but 141 lpm or nearly 5 CFM.
Now do the same calculations at 132 feet, or 5 ata. Five times as many molecules are passing thru the scrubber, irrespective of WOB.
You can begin to see how you might actually get CO2 breakthru at depth with a workload, irrespective of the added CO2 stress of increased gas density, and potentiality lesser volume movement.
Now during CE testing, the artificial lung is supposed to mimic all this, but I don't know that for a fact. So it is not inconceivable that despite a scrubber capability of handling "3CFM for 120 minutes with lots of added CO2", that high gas flow in mid breath coupled with decreased granule efficiency from thick gas at depth might allow some CO2 breakthru.
Think of breathing your max, say 40 full chest-fulls pet minute. Your can't sustain that, I agree. But let's look at the numbers. If you're a huge guy, 2.5 liters per breath x 40 = 100 lpm, which equals 3.5 CFM. You're above the testing limit, but as @tbone1004 points out, that's not sustainable, and it's therefore not a stress to the scrubber as tested.
But let's look at the dynamic of it. Let's presuppose hard work, but only half of that max, i.e., 50 lpm (1.75 CFM). Easily below scrubber max. But what is the max flow? The 50 lpm is both in and out, so at a minimum, the 50 liters is going out during only half the minute, which makes the minimum expiratory flow 100 lpm. And due to the acceleration and deceleration phases of each breath, max flow may only be occurring during 60% of each cycle. In other words, if 85% of each breath is packed into the middle 60% of each cycle, the max flow is now not 50 lpm, not 100 lpm, but 141 lpm or nearly 5 CFM.
Now do the same calculations at 132 feet, or 5 ata. Five times as many molecules are passing thru the scrubber, irrespective of WOB.
You can begin to see how you might actually get CO2 breakthru at depth with a workload, irrespective of the added CO2 stress of increased gas density, and potentiality lesser volume movement.
Now during CE testing, the artificial lung is supposed to mimic all this, but I don't know that for a fact. So it is not inconceivable that despite a scrubber capability of handling "3CFM for 120 minutes with lots of added CO2", that high gas flow in mid breath coupled with decreased granule efficiency from thick gas at depth might allow some CO2 breakthru.
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@rsingler
standard NEDU settings
40lpm/1.5cfm=20 breaths/min at 2 liters/breath
75lpm/3cfm=30 breaths/min at 2.5liters/breath
Of note to think about, depending on the configuration of the unit, mainly if there are counterlungs on the inhale and exhale side, and if they are "balanced", then the scrubber is experience more consistent flow rates. Most rebreathers with dual counterlungs have them in balance *TOS, OTS, BMCL, etc* where there is dwell time on both the exhale and inhale cycles. On rebreathers with single counterlungs like the SF2 and KISS Sidekick, they only have an exhale counterlung, so there is no real "dwell" time it's just pulled through the scrubber on inhale. Either way this is all dealt with during the testing and is part of why some rebreathers have better efficiencies than others.
standard NEDU settings
40lpm/1.5cfm=20 breaths/min at 2 liters/breath
75lpm/3cfm=30 breaths/min at 2.5liters/breath
Of note to think about, depending on the configuration of the unit, mainly if there are counterlungs on the inhale and exhale side, and if they are "balanced", then the scrubber is experience more consistent flow rates. Most rebreathers with dual counterlungs have them in balance *TOS, OTS, BMCL, etc* where there is dwell time on both the exhale and inhale cycles. On rebreathers with single counterlungs like the SF2 and KISS Sidekick, they only have an exhale counterlung, so there is no real "dwell" time it's just pulled through the scrubber on inhale. Either way this is all dealt with during the testing and is part of why some rebreathers have better efficiencies than others.
OP
Stupid question, are these performance parameters to be corrected for depth e.g. at 10 meters, do we double these performace parameters?
OP
No correction needed. The CE results are scrubber performance at depth. @tbone1004 can correct me if I'm mistaken. And the volumes in question won't change at depth - only the number of molecules per unit volume due to ambient compression. That's one of the reasons scrubbers are challenged at depth: the constant number of CO2 molecules produced by the body are now hidden among five times as many breathing gas molecules in the circuit, making it harder for the scrubber granules to be exposed to CO2 to grab them.
I thought these CE figures were for a specific depth, (possibly 40M or 90M, I could be wrong). Again pardon my ignorance, why would the number of CO2 molecules not change with depth if the volume of the inspired gas we breathe in does i.e. if I am inspiring 20 lpm at 1 bar then at 10 bar I am inspiring 200lpm (20lpm x 10bar) therefore would the O2, N2 and Co2 molecules not increase in line with the gas density?
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