How Far Do You Push Your Scrubber?

grahamsp · 6th January 2006, 23:57

Quote: (Originally Posted by Sutty)

I'm very doubtful whether the 4.5% (exhaled CO2/consumed O2) figure is reliable in a diving scenario. This is about right for a normal person breathing normally on the surface, however experienced divers have been shown to have a modified response to CO2, and therefore may have higher expired CO2% but with a lower air comsumption/min. Even non-divers will put up their CO2 with increased WOB.

Can you give me any references to follow up? I'd be interested to know more.

Quote: (Originally Posted by Sutty)

If this figure is your basis for determining your scubber duration it may well be seriously flawed.

Happily, no. My scrubber limit is based on an implied CO2 production rate of 1.16 lpm.

Graham

grahamsp · 8th January 2006, 10:28

Further to my response to Sutty asking for references on CO2 retainers, I found a good one. Go to http://www.njscuba.com/njscuba/misc_co2_retention.html. Seems to cover the subject of CO2! Talking of which, I qualified on CDBA in 1977 (the Royal Navy's CCR/SCR). This set, a direct descendent of the RN's WW2 rebreathers and primitive by today's standards, had a single hose into/out of the counterlung so CO2 build up was real risk. The RN's solution to the problem was not twin hoses but regular and deep breathing, even to the point of bottoming your lungs on the exhale; I remember this being heavily stressed during our initial training. After while, it became second nature and, even with the luxury of the Evo's twin hoses and one-way gas flow, I still have the habit nearly 30 years later. Whether or not you are a CO2 retainer, I'd recommend it as good practice.

Graham

Sutty · 8th January 2006, 16:14

Quote: (Originally Posted by grahamsp)

Can you give me any references to follow up? I'd be interested to know more.
Happily, no. My scrubber limit is based on an implied CO2 production rate of 1.16 lpm.

Graham

"Diving and subaquatic medicine" by Edmonds et al (4th edition), p226-227 is a good start. There are more references at the end of the chapter but I haven't followed them up.
One interesting quote in this context is:

"significant exercise (O2 uptake >60 percent of maximal) under water and using scuba, produces an elevation of PACO2 and is some times marked (PACO2>60mmHg)"

60mmHg works out as a surface-equivalent of 7.9%

My point was that if you were assuming your CO2 production was 4.5% multiplied by your open-circuit respiratory minute volume, you may well underestimate it significantly. If you then used an inaccurate figure for CO2 production to estimate scrubber life you could come unstuck!

e.g. if actual exhaled CO2 was 6% then at your 15l/min it would be 0.9l/min.

Neil

leadking · 8th January 2006, 17:23

Earlier in the posts someone brought up the old rule of thirds, which I like. I have run my scrubber over recommended limits during decompression but I would never intentionally start a dive with the intent of grossly exceeding the limits. I do deepwater penetration and would like to think if something went wrong and I over extended my bottom time, I want to know that all the "cushion" available to me. My life is not worth a few bucks worth of chemical. My safety motto is "Just because your getting away with it does not mean it's safe"

grahamsp · 9th January 2006, 10:18

Quote: (Originally Posted by Sutty)

"Diving and subaquatic medicine" by Edmonds et al (4th edition), p226-227 is a good start.

Thanks, I'll follow this up.

Graham

Karlos · 9th January 2006, 11:36

Quote: (Originally Posted by grahamsp)

....The RN's solution to the problem was not twin hoses but regular and deep breathing, even to the point of bottoming your lungs on the exhale

Graham

Graham, wasn't only the English Navy they were teaching this and I can remember it still being taught in the 80's...

Question: Does it belong to the training agencies syllabus now a days.... IANTD, TDI, SSI etc...

View Poll Results: How Far Do You Push Your Inspo/Evo Scrubber
I Won't - I Stick to APD's Limits	34	39.53%
I'll Go 25% Over	25	29.07%
I'll Go 50% Over	12	13.95%
I'll Go 75% Over	6	6.98%
I'll Go 100% Over	9	10.47%
Voters: 86. You may not vote on this poll

6th January 2006, 23:57	#51 (permalink)
grahamsp New Member Current Rebreather/s: Evolution Other Rebreather/s: Join Date: Jun 2005 Location: Sydney, Australia Posts: 117	Re: How Far Do You Push Your Scrubber? Quote: (Originally Posted by Sutty) I'm very doubtful whether the 4.5% (exhaled CO2/consumed O2) figure is reliable in a diving scenario. This is about right for a normal person breathing normally on the surface, however experienced divers have been shown to have a modified response to CO2, and therefore may have higher expired CO2% but with a lower air comsumption/min. Even non-divers will put up their CO2 with increased WOB. Can you give me any references to follow up? I'd be interested to know more. Quote: (Originally Posted by Sutty) If this figure is your basis for determining your scubber duration it may well be seriously flawed. Happily, no. My scrubber limit is based on an implied CO2 production rate of 1.16 lpm. Graham
(Offline)

8th January 2006, 10:28	#52 (permalink)
grahamsp New Member Current Rebreather/s: Evolution Other Rebreather/s: Join Date: Jun 2005 Location: Sydney, Australia Posts: 117	Re: How Far Do You Push Your Scrubber? Further to my response to Sutty asking for references on CO2 retainers, I found a good one. Go to http://www.njscuba.com/njscuba/misc_co2_retention.html. Seems to cover the subject of CO2! Talking of which, I qualified on CDBA in 1977 (the Royal Navy's CCR/SCR). This set, a direct descendent of the RN's WW2 rebreathers and primitive by today's standards, had a single hose into/out of the counterlung so CO2 build up was real risk. The RN's solution to the problem was not twin hoses but regular and deep breathing, even to the point of bottoming your lungs on the exhale; I remember this being heavily stressed during our initial training. After while, it became second nature and, even with the luxury of the Evo's twin hoses and one-way gas flow, I still have the habit nearly 30 years later. Whether or not you are a CO2 retainer, I'd recommend it as good practice. Graham
(Offline)

8th January 2006, 16:14	#53 (permalink)
Sutty Classic Kiss diver Current Rebreather/s: Classic Kiss Other Rebreather/s: Join Date: Jun 2005 Location: Glossop, Derbyshire, UK Posts: 815	Re: How Far Do You Push Your Scrubber? Quote: (Originally Posted by grahamsp) Can you give me any references to follow up? I'd be interested to know more. Happily, no. My scrubber limit is based on an implied CO2 production rate of 1.16 lpm. Graham "Diving and subaquatic medicine" by Edmonds et al (4th edition), p226-227 is a good start. There are more references at the end of the chapter but I haven't followed them up. One interesting quote in this context is: "significant exercise (O2 uptake >60 percent of maximal) under water and using scuba, produces an elevation of PACO2 and is some times marked (PACO2>60mmHg)" 60mmHg works out as a surface-equivalent of 7.9% My point was that if you were assuming your CO2 production was 4.5% multiplied by your open-circuit respiratory minute volume, you may well underestimate it significantly. If you then used an inaccurate figure for CO2 production to estimate scrubber life you could come unstuck! e.g. if actual exhaled CO2 was 6% then at your 15l/min it would be 0.9l/min. Neil Last edited by Sutty : 8th January 2006 at 16:15. Reason: Spelling
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8th January 2006, 17:23	#54 (permalink)
leadking New Member Current Rebreather/s: Inspiration Classic Other Rebreather/s: Join Date: Dec 2005 Location: Minneapolis Posts: 57	Re: How Far Do You Push Your Scrubber? Earlier in the posts someone brought up the old rule of thirds, which I like. I have run my scrubber over recommended limits during decompression but I would never intentionally start a dive with the intent of grossly exceeding the limits. I do deepwater penetration and would like to think if something went wrong and I over extended my bottom time, I want to know that all the "cushion" available to me. My life is not worth a few bucks worth of chemical. My safety motto is "Just because your getting away with it does not mean it's safe"
(Offline)

9th January 2006, 10:18	#55 (permalink)
grahamsp New Member Current Rebreather/s: Evolution Other Rebreather/s: Join Date: Jun 2005 Location: Sydney, Australia Posts: 117	Re: How Far Do You Push Your Scrubber? Quote: (Originally Posted by Sutty) "Diving and subaquatic medicine" by Edmonds et al (4th edition), p226-227 is a good start. Thanks, I'll follow this up. Graham
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9th January 2006, 11:36	#56 (permalink)
Karlos New Member Current Rebreather/s: Inspiration Classic Evolution Other Rebreather/s: Join Date: Jun 2005 Location: Sydney, Australia Posts: 55	Re: How Far Do You Push Your Scrubber? Quote: (Originally Posted by grahamsp) ....The RN's solution to the problem was not twin hoses but regular and deep breathing, even to the point of bottoming your lungs on the exhale Graham Graham, wasn't only the English Navy they were teaching this and I can remember it still being taught in the 80's... Question: Does it belong to the training agencies syllabus now a days.... IANTD, TDI, SSI etc...
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