Why does depth matter for scrubber lifetime

Hanssing · 28th October 2007, 08:36

Quote: (Originally Posted by DanDunfee)

.... Till then I believe that the 14 % per ata of depth is a reasonably good approximation of the depth effect on a scrubber with a given charge weight (given a previously validated scrubber duration, at analogous dive and work conditions, but another depth).
......

Given that theres now been published Ansti-results for different sports Rebreather, I for one would like to see some simple tests performed (just on the major different scrubberdesigns ie. radial, axial and EAC) that meassure:
a) duration @ 40msw, 4dgrC, 1,6lpm CO2 (or 50-60m) - divephase
b) duration @ 6msw, 4dgrC, 1,2lpm CO2 (or 10m) - decophase
And test really interesting:
c) 50% time of a) at samecondition. Then total time to breakthrough with the rest of the test being performed at conditions b).

This would correlate to sportsdives, where the Rebreather is used throughout the whole dive (ie. no bell or surface supplied Deco). The difference in time between b) and c) would also tell us how much of the scrubber comes back to life when ascending.

I dont care on wich unit as long as it is axial

Sorry if test as above has been conducted allready, and I'm just not aware.

Regards
Nicolai

Mdemon · 28th October 2007, 09:10

Whilst genuinely fascinated by design theory, ANSTI tests etc, it doesn't really change our in-water experience.

The bottom line is, at depth you need to give the demons more time to catch the CO2. So keep the workload down, breath normally, have bailout and use it quickly if the demons drop the ball.

York · 28th October 2007, 09:21

Quote: (Originally Posted by Shawn)

So then if I understood correctly that break through is the largest determining factor when determining the scrubber life using calcium hydroxide then would (on deep dives) lythium hydroxide be a better alternative? (sorry about the spelling)

And if alot of this problem is negated by the use of radial scrubbers, how come Rebreather's don't come with a radial scrubber instead of the axial scrubber?

Thanks

(wow...I just went back and re-read some of the posts and ..I'm a little out of my league) But still very good stuff to know.

Abbo touched the effect of the radial scrubber on CO2-removal performance. Allow me to formulate an additional view on the functionality:
For the axial scrubbers used in recreational rebreather diving, breakthrough is expected at around 50% ish of total capacity. This means that in the axial configuration, the remaining 50% absorbent is insufficient to reduce PCO2 to the required limit, although CO2 is still being removed from the breathing gas stream. Radial scrubbers by their design have a different ratio of surface area to bed depth, resulting in a greatly reduced flow speed. You should be able to achieve similar results by using a very wide but short axial scrubber, with inherent drawbacks. In this radial configuration, slower gas flow results in more time that a gas molecule spends in the vacinity of a scrubbing pellet, resulting in a sharper defined, shorter reaction zone. Overall breakthrough occurs later, as more than 50% of total scrubber capacity is utilised - I have seen estimates of 70% or more. The big drawback, as pointed out by Abbo and Tom, is that you now end up very close to the edge of the performance with no buffer left - staying longer on the scrubber, or heavy breathing at the end of the dive puts you over the edge completely, whereas an axial scrubber is more forgiving (still lots of capacity available).

I agree with Hanssing that scrubber performance should be tested differently, and I personally would like to see not only the point of breakthrough, but also the slope at that point, meaning what happens if you exceed it a little bit. I don't think that a "binary" scrubber (works nicely/doesn't work at all) is desireable.

As a side note, it is technically possible to achieve scrubber efficiency of more than 90% even in the size range of our recreational rebreathers - however at a huge cost so that throwing away your axial absorbent with only 50% used suddenly doesn't seem such a waste after all.

Joerg

York · 28th October 2007, 09:50

Quote: (Originally Posted by DanDunfee)

================

Re the red-highlighted statement above ....
Au Contrare !! There are substantial bodies of both lab and manned testing data showing irrefutibly, in principle, that scrubber breakthrough times decrease as a function of depth, at least down to 100 m. I know of no credible scientific test data to the contrary. The question, as above is simply how great is the effect.

Dan,
I believe that in principle we are arguing the same point. I certainly do appreciate your view - great discussion!
I am aware of at least one IMO credible study that shows otherwise, and the study also shows the influence of breathing on the overall performance. I left all my research papers in Florida, so I have to refer to my digital notes here and hope I pick the right reference:
MacGregor, C.D., Fraser, M.G., The Effect Of Pressure On The Efficiency Of Carbon Dioxide Absorbents, The Winter Annual Meeting of the American Society of Mechanical Engineers, Phoenix, Arizona, November 14-19, 1982
As far as I remember, the data required transformation to show the pressure-dependance for one specific flow rate. I hope I will be able to verify in about a months time - sorry about that

Joerg

DanDunfee · 28th October 2007, 16:10

Quote: (Originally Posted by Tom Rose)

Great discussion material at DEMA....Tom

===============

Yep........BTW Tom, thats a great new avitar pic with You and Superboy..nuther Rebreather diver in the making ??? :-)

devilgas · 28th October 2007, 19:10

Quote: (Originally Posted by DanDunfee)

================
Joerg

I certainly agree that there are many, many chemical, thermal, and physical factors and functions which go into determining scrubber breakthrough times for a given set of conditions. It's a complex problem... some of the individual functional component effects are additive, and some offsetting, to a point of being rather confounding. Extensive computer modelling has helped understand some of the major individual component effects, However, efforts to model the whole integrated function, from the grass roots up have been, at best, marginally successful. None that I know of have yielded a simple set of parametric factors which are usable, even by divers who are especially skilled in the arts, for making on-site, pre-dive estimates of probable scrubber duration under prevailing dive conditions.

This many-faceted scrubber duration challenge is, in many ways, analogous to the complex thermo-chemical and physical function factors in a solid-propellant rocket motor. In dealing with these types of complex problems, especially those which defy comprehensive detailed understanding and modelling of each functional contributor,.. One often can, by reverting to simplified parametric correlation of the total observable effect, deduce simplified but valid functional factors which arch-over the complex interactivity and yield reasonable computational factors which are useable by skilled, working-level users (on site divers). Thats a principal goal in the studies I spoke of in my previous post, but this remains to be documented and validated to the Rebreather diver community. Till then I believe that the 14 % per ata of depth is a reasonable approximation of the depth-dependent effect on a scrubber of a given charge weight (given a previously validated scrubber duration, at analogous dive and work conditions, but another depth).

Re the red-highlighted statement above ....
Au Contrare !! There are substantial bodies of both lab and manned testing data showing 'irrefutibly', in principle, that scrubber breakthrough times decrease as a function of depth, at least down to 100 m. I know of no credible scientific test data to the contrary. The question, as above, is simply how great is the effect, and can reasonably reliable computational factors be deduced to support on-site, pre-dive estimates. Anything would be welcome to fill the vacuum that exists now.
============================

Lithium Hydroxide has seen significant use in rebreather diving equipment, including SCUBA-type rebreathers.... just not significantlly in the recreational or civilian technical diving arenas.

Get um, Danno....

DanDunfee · 28th October 2007, 21:17

Quote: (Originally Posted by devilgas)

Get um, Danno....

==============
Bill

Good to see ya up on line...You goin to DEMA ?? I'll be there nearly all the time. My cel is 703 220 4336. Lets split a beer.

Shawn · 30th October 2007, 00:48

All good information...the Lithium Hydroxide question came from the Extend air link on the ISC web page under options. So I started reading everything I could about Extend air.

Back to the radial scrubber thing for a minute....

I have seen a lot of talk about a radial scrubber especially the CIS lunar. If break through is easier (??) with a radial scrubber then why all of the hub bub? or is it more that the Cis Lunar had a water resistant scrubber canister?

I get to these posts kinda of late so sorry about all of the slow responeses.

thanks

York · 30th October 2007, 07:19

Quote: (Originally Posted by Shawn)

All good information...the Lithium Hydroxide question came from the Extend air link on the ISC web page under options. So I started reading everything I could about Extend air.

Back to the radial scrubber thing for a minute....

I have seen a lot of talk about a radial scrubber especially the CIS lunar. If break through is easier (??) with a radial scrubber then why all of the hub bub? or is it more that the Cis Lunar had a water resistant scrubber canister?

I get to these posts kinda of late so sorry about all of the slow responeses.

thanks

as usual, there is no free lunch

the radial scrubbers offer you longer duration for normal breathing, but less safety once the scrubber is closer too its end. That still leaves you with 1.5 to 2-times longer performance.
The water-resistance is independent of the scrubber's performance. E.g. you can buy a radial scrubber for the Meg but without the hydrophobic membrane Golem Radial Scrubber for the Meg
Overall, IMHO, the radial scrubber is the way to go
J

Abbo · 30th October 2007, 08:23

Quote: (Originally Posted by Shawn)

...the Lithium Hydroxide question came from the Extend air link on the ISC web page under options. So I started reading everything I could about Extend air.

To be clear, the ISC link takes you to Extendair, a company that makes products for all kinds of uses. Extendair uses lithium hydroxide for subs and mines.
ExtendAir
If ISC comes up with a way of obviating the risk of caustic cocktails and thereby making lithium hydroxide viable in its rebreathers, you are likely to get crushed flat in the rush to buy them.

28th October 2007, 08:36	#21 (permalink)
Hanssing flap-flop ..... flap-flop Current Rebreather/s: rEvo Other Rebreather/s: rEvo Join Date: Nov 2005 Location: Denmark Posts: 405	Re: Why does depth matter for scrubber lifetime Quote: (Originally Posted by DanDunfee) .... Till then I believe that the 14 % per ata of depth is a reasonably good approximation of the depth effect on a scrubber with a given charge weight (given a previously validated scrubber duration, at analogous dive and work conditions, but another depth). ...... Given that theres now been published Ansti-results for different sports Rebreather, I for one would like to see some simple tests performed (just on the major different scrubberdesigns ie. radial, axial and EAC) that meassure: a) duration @ 40msw, 4dgrC, 1,6lpm CO2 (or 50-60m) - divephase b) duration @ 6msw, 4dgrC, 1,2lpm CO2 (or 10m) - decophase And test really interesting: c) 50% time of a) at samecondition. Then total time to breakthrough with the rest of the test being performed at conditions b). This would correlate to sportsdives, where the Rebreather is used throughout the whole dive (ie. no bell or surface supplied Deco). The difference in time between b) and c) would also tell us how much of the scrubber comes back to life when ascending. I dont care on wich unit as long as it is axial Sorry if test as above has been conducted allready, and I'm just not aware. Regards Nicolai __________________ Woohooo - I can change my rEvo! Its going to be bitchin' tricked out piece of gear
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28th October 2007, 09:10	#22 (permalink)
Mdemon SiegeEngine II Current Rebreather/s: Inspiration Classic Home Build Other Rebreather/s: Inspiration Classic Home Build Join Date: Feb 2007 Location: SWUK Posts: 1,931	Re: Why does depth matter for scrubber lifetime Whilst genuinely fascinated by design theory, ANSTI tests etc, it doesn't really change our in-water experience. The bottom line is, at depth you need to give the demons more time to catch the CO2. So keep the workload down, breath normally, have bailout and use it quickly if the demons drop the ball. __________________ www.southwestmafia.com "small minds talk about people, Average Minds Talk About Events, GREAT MINDS TALK ABOUT IDEAS!" The WRONG Attitude will get you killed. "Once the agenda-monkeys and perfect-worlders have moved on, perhaps we can do some diving?"
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28th October 2007, 09:21	#23 (permalink)
York probubbly not Current Rebreather/s: Megalodon Other Rebreather/s: Join Date: Mar 2005 Location: Bristol, UK Posts: 118	Re: Why does depth matter for scrubber lifetime Quote: (Originally Posted by Shawn) So then if I understood correctly that break through is the largest determining factor when determining the scrubber life using calcium hydroxide then would (on deep dives) lythium hydroxide be a better alternative? (sorry about the spelling) And if alot of this problem is negated by the use of radial scrubbers, how come Rebreather's don't come with a radial scrubber instead of the axial scrubber? Thanks (wow...I just went back and re-read some of the posts and ..I'm a little out of my league) But still very good stuff to know. Abbo touched the effect of the radial scrubber on CO2-removal performance. Allow me to formulate an additional view on the functionality: For the axial scrubbers used in recreational rebreather diving, breakthrough is expected at around 50% ish of total capacity. This means that in the axial configuration, the remaining 50% absorbent is insufficient to reduce PCO2 to the required limit, although CO2 is still being removed from the breathing gas stream. Radial scrubbers by their design have a different ratio of surface area to bed depth, resulting in a greatly reduced flow speed. You should be able to achieve similar results by using a very wide but short axial scrubber, with inherent drawbacks. In this radial configuration, slower gas flow results in more time that a gas molecule spends in the vacinity of a scrubbing pellet, resulting in a sharper defined, shorter reaction zone. Overall breakthrough occurs later, as more than 50% of total scrubber capacity is utilised - I have seen estimates of 70% or more. The big drawback, as pointed out by Abbo and Tom, is that you now end up very close to the edge of the performance with no buffer left - staying longer on the scrubber, or heavy breathing at the end of the dive puts you over the edge completely, whereas an axial scrubber is more forgiving (still lots of capacity available). I agree with Hanssing that scrubber performance should be tested differently, and I personally would like to see not only the point of breakthrough, but also the slope at that point, meaning what happens if you exceed it a little bit. I don't think that a "binary" scrubber (works nicely/doesn't work at all) is desireable. As a side note, it is technically possible to achieve scrubber efficiency of more than 90% even in the size range of our recreational rebreathers - however at a huge cost so that throwing away your axial absorbent with only 50% used suddenly doesn't seem such a waste after all. Joerg Last edited by York : 28th October 2007 at 10:27. Reason: spelling - flow, not slow
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28th October 2007, 09:50	#24 (permalink)
York probubbly not Current Rebreather/s: Megalodon Other Rebreather/s: Join Date: Mar 2005 Location: Bristol, UK Posts: 118	Re: Why does depth matter for scrubber lifetime Quote: (Originally Posted by DanDunfee) ================ Re the red-highlighted statement above .... Au Contrare !! There are substantial bodies of both lab and manned testing data showing irrefutibly, in principle, that scrubber breakthrough times decrease as a function of depth, at least down to 100 m. I know of no credible scientific test data to the contrary. The question, as above is simply how great is the effect. Dan, I believe that in principle we are arguing the same point. I certainly do appreciate your view - great discussion! I am aware of at least one IMO credible study that shows otherwise, and the study also shows the influence of breathing on the overall performance. I left all my research papers in Florida, so I have to refer to my digital notes here and hope I pick the right reference: MacGregor, C.D., Fraser, M.G., The Effect Of Pressure On The Efficiency Of Carbon Dioxide Absorbents, The Winter Annual Meeting of the American Society of Mechanical Engineers, Phoenix, Arizona, November 14-19, 1982 As far as I remember, the data required transformation to show the pressure-dependance for one specific flow rate. I hope I will be able to verify in about a months time - sorry about that Joerg
(Offline)

28th October 2007, 16:10	#25 (permalink)
DanDunfee Custom Title Allowed! Current Rebreather/s: Other CCR Other SCR Home Build Other Rebreather/s: Not Bought Yet Other CCR Other SCR Home Build Join Date: Sep 2005 Location: Virginia, USA Posts: 204	Re: Why does depth matter for scrubber lifetime Quote: (Originally Posted by Tom Rose) Great discussion material at DEMA....Tom =============== Yep........BTW Tom, thats a great new avitar pic with You and Superboy..nuther Rebreather diver in the making ??? :-) __________________ There are 'Bold' divers and old divers, but NO old, ' Bold ' divers
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28th October 2007, 19:10	#26 (permalink)
devilgas CCR Instructor Current Rebreather/s: Inspiration Classic Optima Other Rebreather/s: Inspiration Vision Evolution Join Date: Apr 2005 Location: FL, USA Posts: 191	Re: Why does depth matter for scrubber lifetime Quote: (Originally Posted by DanDunfee) ================ Joerg I certainly agree that there are many, many chemical, thermal, and physical factors and functions which go into determining scrubber breakthrough times for a given set of conditions. It's a complex problem... some of the individual functional component effects are additive, and some offsetting, to a point of being rather confounding. Extensive computer modelling has helped understand some of the major individual component effects, However, efforts to model the whole integrated function, from the grass roots up have been, at best, marginally successful. None that I know of have yielded a simple set of parametric factors which are usable, even by divers who are especially skilled in the arts, for making on-site, pre-dive estimates of probable scrubber duration under prevailing dive conditions. This many-faceted scrubber duration challenge is, in many ways, analogous to the complex thermo-chemical and physical function factors in a solid-propellant rocket motor. In dealing with these types of complex problems, especially those which defy comprehensive detailed understanding and modelling of each functional contributor,.. One often can, by reverting to simplified parametric correlation of the total observable effect, deduce simplified but valid functional factors which arch-over the complex interactivity and yield reasonable computational factors which are useable by skilled, working-level users (on site divers). Thats a principal goal in the studies I spoke of in my previous post, but this remains to be documented and validated to the Rebreather diver community. Till then I believe that the 14 % per ata of depth is a reasonable approximation of the depth-dependent effect on a scrubber of a given charge weight (given a previously validated scrubber duration, at analogous dive and work conditions, but another depth). Re the red-highlighted statement above .... Au Contrare !! There are substantial bodies of both lab and manned testing data showing 'irrefutibly', in principle, that scrubber breakthrough times decrease as a function of depth, at least down to 100 m. I know of no credible scientific test data to the contrary. The question, as above, is simply how great is the effect, and can reasonably reliable computational factors be deduced to support on-site, pre-dive estimates. Anything would be welcome to fill the vacuum that exists now. ============================ Lithium Hydroxide has seen significant use in rebreather diving equipment, including SCUBA-type rebreathers.... just not significantlly in the recreational or civilian technical diving arenas. Get um, Danno.... __________________ www.deeptek.net Inspiration / Evolution, O2ptima Instructor
(Offline)

28th October 2007, 21:17	#27 (permalink)
DanDunfee Custom Title Allowed! Current Rebreather/s: Other CCR Other SCR Home Build Other Rebreather/s: Not Bought Yet Other CCR Other SCR Home Build Join Date: Sep 2005 Location: Virginia, USA Posts: 204	Re: Why does depth matter for scrubber lifetime Quote: (Originally Posted by devilgas) Get um, Danno.... ============== Bill Good to see ya up on line...You goin to DEMA ?? I'll be there nearly all the time. My cel is 703 220 4336. Lets split a beer. __________________ There are 'Bold' divers and old divers, but NO old, ' Bold ' divers
(Offline)

30th October 2007, 00:48	#28 (permalink)
Shawn Custom Title Allowed! Current Rebreather/s: Megalodon Other CCR RB80 / Clone Other Rebreather/s: Other CCR RB80 / Clone Join Date: Oct 2007 Location: Kansas City Kansas Posts: 142	Re: Why does depth matter for scrubber lifetime All good information...the Lithium Hydroxide question came from the Extend air link on the ISC web page under options. So I started reading everything I could about Extend air. Back to the radial scrubber thing for a minute.... I have seen a lot of talk about a radial scrubber especially the CIS lunar. If break through is easier (??) with a radial scrubber then why all of the hub bub? or is it more that the Cis Lunar had a water resistant scrubber canister? I get to these posts kinda of late so sorry about all of the slow responeses. thanks __________________ WWW.DiveOne.net
(Offline)

30th October 2007, 07:19	#29 (permalink)
York probubbly not Current Rebreather/s: Megalodon Other Rebreather/s: Join Date: Mar 2005 Location: Bristol, UK Posts: 118	Re: Why does depth matter for scrubber lifetime Quote: (Originally Posted by Shawn) All good information...the Lithium Hydroxide question came from the Extend air link on the ISC web page under options. So I started reading everything I could about Extend air. Back to the radial scrubber thing for a minute.... I have seen a lot of talk about a radial scrubber especially the CIS lunar. If break through is easier (??) with a radial scrubber then why all of the hub bub? or is it more that the Cis Lunar had a water resistant scrubber canister? I get to these posts kinda of late so sorry about all of the slow responeses. thanks as usual, there is no free lunch the radial scrubbers offer you longer duration for normal breathing, but less safety once the scrubber is closer too its end. That still leaves you with 1.5 to 2-times longer performance. The water-resistance is independent of the scrubber's performance. E.g. you can buy a radial scrubber for the Meg but without the hydrophobic membrane Golem Radial Scrubber for the Meg Overall, IMHO, the radial scrubber is the way to go J Last edited by York : 30th October 2007 at 07:23. Reason: link added
(Offline)

30th October 2007, 08:23	#30 (permalink)
Abbo Nicholas Smith Current Rebreather/s: Megalodon Other Rebreather/s: Join Date: Aug 2006 Location: Tokyo Posts: 449	Re: Why does depth matter for scrubber lifetime Quote: (Originally Posted by Shawn) ...the Lithium Hydroxide question came from the Extend air link on the ISC web page under options. So I started reading everything I could about Extend air. To be clear, the ISC link takes you to Extendair, a company that makes products for all kinds of uses. Extendair uses lithium hydroxide for subs and mines. ExtendAir If ISC comes up with a way of obviating the risk of caustic cocktails and thereby making lithium hydroxide viable in its rebreathers, you are likely to get crushed flat in the rush to buy them.
(Offline)

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