Sorb surprises

[doctormike]

But surely your ridiculously high tax rates should translate into impeccably maintained roads throughout the province!!!!!

As for the incident that @doctormike posted, I'd say CO2 retention. A hard packed axial is pretty hard to overbreathe. Coupled with cold water, high work load, I think it's not a scrubber issue. The fact that going SCR alleviated the issue speaks to me that he changed his breathing pattern and tidal volume pretty significantly in order to maintain SCR, and probably in doing so, was able to evacuate any retained CO2.

Yeah, I would be surprised if the take home was that a four hour scrubber is no good after 90 minutes. That test data should mean something, even though it's limited to very specific conditions. Workload means something, sure, but I never heard anyone claim that scrubber function declines in a linear fashion over it's rated lifespan.

 

[tomfcrist]

As for the incident that @doctormike posted, I'd say CO2 retention. A hard packed axial is pretty hard to overbreathe. Coupled with cold water, high work load, I think it's not a scrubber issue.

Where did he say anything about cold water? “He” being “I” in this case. FWIW I don’t think the scrubber being partially used was the root cause of the incident. I think it was a combination of a few things. I thought I spelled that out well in the RBW post.

 

[tomfcrist]

Yeah, I would be surprised if the take home was that a four hour scrubber is no good after 90 minutes

The takehome for me was that knowing I was going to do a 235’ dive in what is usually a high current area on a scrubber that has been used for over a third of its rated life is kinda like putting on a slightly used condom before banging a bar wench....It might work in a pinch...but it might not...and herpes is forever.

 

[JohnnyC]

Where did he say anything about cold water? “He” being “I” in this case. FWIW I don’t think the scrubber being partially used was the root cause of the incident. I think it was a combination of a few things. I thought I spelled that out well in the RBW post.

Sorry, I must have got that from Tom's post instead of your post on RBW.

Anyway, I don't know that I agree with your assessment, but I'm glad you're still kicking! Kudos for sorting the problem and getting out of the water in one piece, not to mention bringing the other dude with you.

 

[doctormike]

The takehome for me was that knowing I was going to do a 235’ dive in what is usually a high current area on a scrubber that has been used for over a third of its rated life is kinda like putting on a slightly used condom before banging a bar wench....It might work in a pinch...but it might not...and herpes is forever.

Well that's the important question, isn't it?

Is a scrubber that has been used for 1/3 of it's rated life less effective at removing CO2 under a given set of conditions than a fresh scrubber, or does it maintain approximately equal efficacy over it's life span and then fail at some point after the reaction front has moved through the entire stack? I'm a rebreather newbie, but I had always been taught the latter.

 

[tomfcrist]

Well that's the important question, isn't it?

Is a scrubber that has been used for 1/3 of it's rated life less effective at removing CO2 under a given set of conditions than a fresh scrubber, or does it maintain approximately equal efficacy over it's life span and then fail at some point after the reaction front has moved through the entire stack? I'm a rebreather newbie, but I had always been taught the latter.

The former makes logical sense to me, if considering low dwell time in the scrubber due to elevated breathing rate and increased production of CO2.

No doubt there is a retention peice to this puzzle as well. But I’ve worked that hard or even harder before and had no issue.

I don’t pretend to have all the answers here by any stretch of the imagination.

 

[doctormike]

The former makes logical sense to me, if considering low dwell time in the scrubber due to elevated breathing rate and increased production of CO2.

No doubt there is a retention peice to this puzzle as well. But I’ve worked that hard or even harder before and had no issue.

I don’t pretend to have all the answers here by any stretch of the imagination.

And once again - in a different forum - thanks for the writeup and kudos on the rescue! You have a different screen name here, didn't recognize the one on RBW (I'm not on there that much).

 

[tbone1004]

About over breathing a unit with insufficient dwell time for the co2 in the loop.... there is dozens of personal accounts of divers overbreathing a scubber. It's easy to demonstrate by a few minutes finning against a fixed obstacle to simulate a hard workload.

The out of shape diver with a poor VO2 max have an advantage in this regard. For me, it's not difficult to overbreath, depending on exertion, gas density and fitness level. Of the 4 entry level courses materials I've read, all warn of this common risk. The few manufacturers who post scrubber data also show this.

Am I misunderstanding something?

the CE testing requires at least the equivalent of a 1.4cfm sac rate and maintains this until scrubber breakthrough. You have to be in VERY good shape to maintain this for any length of time and most will pass out. They do this with a 20bpm breathing rate which is pretty quick and will get most people light headed in less than 2 minutes.
Those that have passed the EN14143 test have passed with a 75lpm or 2.65cfm breathing rate *most of the "big boys" in CCR have passed that for reference*. You have to be in REALLY good shape to maintain that without passing out for any length of time. Read Navy SEAL's can't do it for more than a couple minutes.

Regarding those with poor VO2 max, and anyone who thinks they can overbreathe a scrubber that was packed properly, I'm willing to bet it is more their inability to expire the CO2 from their body than the CO2 actually getting through the scrubber.

 

[stuartv]

@tomfcrist I read the thread over in that other forum. My account there is still awaiting approval, so I can't post there.

The question "over there" was asked about how you managed the sorb between day 2 and 3 and I didn't see an answer? Did the sorb just stay in the CCR overnight? Sealed up the whole time? Or what?

 

[rjack321]

Well that's the important question, isn't it?

Is a scrubber that has been used for 1/3 of it's rated life less effective at removing CO2 under a given set of conditions than a fresh scrubber, or does it maintain approximately equal efficacy over it's life span and then fail at some point after the reaction front has moved through the entire stack? I'm a rebreather newbie, but I had always been taught the latter.

A new scrubber, a 1/3rd used scrubber and a 2/3rds used scrubber are all functionally equal. The efficiency does not decline linearly at all. It's even (and very close to 100%) until the reaction front starts to poke through and then declines precipitously. Once again posting one of John Clarke's excellent articles
How Long Will Your Rebreather Scrubber Canister Last?