How hard is WOB on rebreathers?

marekm · 12th February 2007, 10:21

Hi everyone.

I've just finished building an axial scubber for my homebuild project. Its dimensions are 100mm diameter by 300mm length, 2kg of 4 - 8 mesh sofnolime.

Some photos here New Page 1

When testing it out on a couch dive with 2kg of lime, I found the WOB breathing to be annoying, like breathing through a poor quality octopus regulator (eg: Sequest Mirage octopus). When I put only 1kg of lime in the scrubber, the WOB was quite comfortable, about the same as a Scubapro R190.

Is this WOB normal for rebreathers, or are there some rebreathers (with say radial scrubbers) that have a WOB as good as a high quality OC reg (eg: Scubapro G250/MK25).

Do I simply have to get used to the WOB being worse on a rebreather than OC, or should I start work on a radial scrubber?

Thanks in advance for your help.
-Marek

decoweenie · 12th February 2007, 10:55

IMHO, I believe your scrubber is too long for its diameter. Thus the higher WOB.

Woz · 12th February 2007, 10:57

That's what I would guess too. The dia is too small so you have to huff through the scrubber. The efficiency will be rubbish, too.

Nemo1957 · 12th February 2007, 11:22

I agree length vs. the diameter is wrong. Reminds me of a old poem:

Long and thin goes way in
Short and thick does the trick

Mark Seewitz · 12th February 2007, 11:30

How hard is hard?
I dive a YBOD with axial scrubber, after the grand total of
10 dives post course I have to say that WOB is not greater
than my Apeks OC regs, but is different.Gas is not forced
down the throat but drawn gently in. If you really have
to pull at your rig to get gas then maybe you are packing
too tight, or have a mechanical problem such as a low
perforation percentage in scrubber mesh,
too small a diameter hoses, small cross
section valves etc.
Don't give up, Mark Seewitz

marekm · 12th February 2007, 12:18

Quote: (Originally Posted by Mark Seewitz)

WOB is not greater
than my Apeks OC regs

Thanks Mark, that is what I was looking for. As I'm familiar with Apeks regs, I now know what to aim for in terms of WOB. Given the WOB is fine when the srubber is empty (and okay when filled with 1kg of lime), I suspect the problem is with the packing of the lime or the diameter of the srubber.

Although I could make a 2kg short stumpy scrubber that is 150mm wide and 150mm long, to lower the WOB, the drag characteristics would be worse (and it would not look as sleek with the 20cft tanks

)

So for now I'll make a slightly larger radial scrubber and test it with 8 - 12 lime.

paulraymaekers · 12th February 2007, 12:26

Quote: (Originally Posted by marekm)

Thanks Mark, that is what I was looking for. As I'm familiar with Apeks regs, I now know what to aim for in terms of WOB. Given the WOB is fine when the srubber is empty (and okay when filled with 1kg of lime), I suspect the problem is with the packing of the lime or the diameter of the srubber.

Although I could make a 2kg short stumpy scrubber that is 150mm wide and 150mm long, to lower the WOB, the drag characteristics would be worse (and it would not look as sleek with the 20cft tanks

)

So for now I'll make a slightly larger radial scrubber and test it with 8 - 12 lime.

already some years ago there were tests done by US navy i think, that showed an 'optimal' behaviour for axials when diameter and length were +/- equal

regards
paul

dave t · 12th February 2007, 13:29

I think 2.1 ratio is a good start point but its not just the scrubber, you can have a really free scrubber and crap mouthpiece, counterlungs or hose fittings

the whole loop is responsible

Dave

decoweenie · 12th February 2007, 13:42

Quote: (Originally Posted by dave t)

I think 2.1 ratio is a good start point

I am sure you meant 2:1 ratio...

And I might add 2:1 as max, and less is better, IMHO.

marekm · 12th February 2007, 14:01

Hi Dave T,

Firstly, thankyou for your suggestions. Next, the ratio I originaly used was 3:1, which is no good it seems. But when taken down to 1.5:1 (ie: less lime) it works ok in terms of WOB. However, I don't want to make a short and stumpy Inspo type scrubber, but rather find a way to spread the scrubbing material over a long and narrow scrubber while not increasing WOB.

The loop (seen here http://home.iprimus.com.au/marekm/rb/100_0548.jpg) has a minimum of 25mm internal diameter in all its parts (with 38mm for hoses). The DSV check valves are very soft so it has a minimal WOB. I've tried various size mouth pieces and they have had no effect on the WOB. Basically, when exhaling, the exhale CL fills up, and then the WOB becomes difficult. On the inhale, the inhale CL empites, and then again the high WOB kicks in. So unless I've missed something, all this seems to point to the scrubber being the problem.

I'm thinking of making a long radial scrubber and see what happens to the WOB then.

Cheers.
-Marek

12th February 2007, 10:21	#1 (permalink)
marekm Banned Current Rebreather/s: Not Bought Yet Other Rebreather/s: Not Bought Yet Join Date: Oct 2006 Location: Brisbane, Australia Posts: 196	How hard is WOB on rebreathers? Hi everyone. I've just finished building an axial scubber for my homebuild project. Its dimensions are 100mm diameter by 300mm length, 2kg of 4 - 8 mesh sofnolime. Some photos here New Page 1 When testing it out on a couch dive with 2kg of lime, I found the WOB breathing to be annoying, like breathing through a poor quality octopus regulator (eg: Sequest Mirage octopus). When I put only 1kg of lime in the scrubber, the WOB was quite comfortable, about the same as a Scubapro R190. Is this WOB normal for rebreathers, or are there some rebreathers (with say radial scrubbers) that have a WOB as good as a high quality OC reg (eg: Scubapro G250/MK25). Do I simply have to get used to the WOB being worse on a rebreather than OC, or should I start work on a radial scrubber? Thanks in advance for your help. -Marek
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12th February 2007, 10:55	#2 (permalink)
decoweenie Crash Test Dummy Current Rebreather/s: Other CCR Other Rebreather/s: Other CCR Join Date: Feb 2005 Location: Cairo Posts: 5,487	Re: How hard is WOB on rebreathers? IMHO, I believe your scrubber is too long for its diameter. Thus the higher WOB. __________________ "...after a while you get bored offering advice to a bull that like to keep butting the fence with its head rather than walking through the open gate..." - Rebreather World PM
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12th February 2007, 10:57	#3 (permalink)
Woz Custom Title Allowed! Current Rebreather/s: Home Build Other Rebreather/s: Home Build Join Date: Jun 2005 Location: Nottingham Posts: 186	Re: How hard is WOB on rebreathers? That's what I would guess too. The dia is too small so you have to huff through the scrubber. The efficiency will be rubbish, too. __________________ Still punting shiny diving tatt to gullible divers www.kitfondle.co.uk
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12th February 2007, 11:22	#4 (permalink)
Nemo1957 RBW Member Current Rebreather/s: Optima Other Rebreather/s: Join Date: Apr 2006 Location: New York Posts: 89	Re: How hard is WOB on rebreathers? I agree length vs. the diameter is wrong. Reminds me of a old poem: Long and thin goes way in Short and thick does the trick
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12th February 2007, 11:30	#5 (permalink)
Mark Seewitz New Member Current Rebreather/s: Inspiration Vision Other Rebreather/s: Join Date: Feb 2007 Location: Melbourne, Australia Posts: 2	Re: How hard is WOB on rebreathers? How hard is hard? I dive a YBOD with axial scrubber, after the grand total of 10 dives post course I have to say that WOB is not greater than my Apeks OC regs, but is different.Gas is not forced down the throat but drawn gently in. If you really have to pull at your rig to get gas then maybe you are packing too tight, or have a mechanical problem such as a low perforation percentage in scrubber mesh, too small a diameter hoses, small cross section valves etc. Don't give up, Mark Seewitz
(Offline)

12th February 2007, 12:18	#6 (permalink)
marekm Banned Current Rebreather/s: Not Bought Yet Other Rebreather/s: Not Bought Yet Join Date: Oct 2006 Location: Brisbane, Australia Posts: 196	Re: How hard is WOB on rebreathers? Quote: (Originally Posted by Mark Seewitz) WOB is not greater than my Apeks OC regs Thanks Mark, that is what I was looking for. As I'm familiar with Apeks regs, I now know what to aim for in terms of WOB. Given the WOB is fine when the srubber is empty (and okay when filled with 1kg of lime), I suspect the problem is with the packing of the lime or the diameter of the srubber. Although I could make a 2kg short stumpy scrubber that is 150mm wide and 150mm long, to lower the WOB, the drag characteristics would be worse (and it would not look as sleek with the 20cft tanks ) So for now I'll make a slightly larger radial scrubber and test it with 8 - 12 lime.
(Offline)

12th February 2007, 12:26	#7 (permalink)
paulraymaekers rEvo's daddy Current Rebreather/s: rEvo Other CCR Home Build Other Rebreather/s: rEvo Other CCR Home Build Join Date: May 2005 Location: belgium Posts: 1,640	Re: How hard is WOB on rebreathers? Quote: (Originally Posted by marekm) Thanks Mark, that is what I was looking for. As I'm familiar with Apeks regs, I now know what to aim for in terms of WOB. Given the WOB is fine when the srubber is empty (and okay when filled with 1kg of lime), I suspect the problem is with the packing of the lime or the diameter of the srubber. Although I could make a 2kg short stumpy scrubber that is 150mm wide and 150mm long, to lower the WOB, the drag characteristics would be worse (and it would not look as sleek with the 20cft tanks ) So for now I'll make a slightly larger radial scrubber and test it with 8 - 12 lime. already some years ago there were tests done by US navy i think, that showed an 'optimal' behaviour for axials when diameter and length were +/- equal regards paul __________________ www.rEvo-rebreathers.com .... the earth is flat, Elvis is alive, and radial scrubbers give longer dwell time than axials...
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12th February 2007, 13:29	#8 (permalink)
dave t RBW Member Current Rebreather/s: Inspiration Classic Inspiration Vision Evolution Sport Kiss Classic Kiss Dolphin Ray Azimuth Home Build Other Rebreather/s: Dolphin Ray Azimuth Home Build Join Date: May 2005 Location: UK Posts: 674	Re: How hard is WOB on rebreathers? I think 2.1 ratio is a good start point but its not just the scrubber, you can have a really free scrubber and crap mouthpiece, counterlungs or hose fittings the whole loop is responsible Dave
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12th February 2007, 13:42	#9 (permalink)
decoweenie Crash Test Dummy Current Rebreather/s: Other CCR Other Rebreather/s: Other CCR Join Date: Feb 2005 Location: Cairo Posts: 5,487	Re: How hard is WOB on rebreathers? Quote: (Originally Posted by dave t) I think 2.1 ratio is a good start point I am sure you meant 2:1 ratio... And I might add 2:1 as max, and less is better, IMHO. __________________ "...after a while you get bored offering advice to a bull that like to keep butting the fence with its head rather than walking through the open gate..." - Rebreather World PM
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12th February 2007, 14:01	#10 (permalink)
marekm Banned Current Rebreather/s: Not Bought Yet Other Rebreather/s: Not Bought Yet Join Date: Oct 2006 Location: Brisbane, Australia Posts: 196	Re: How hard is WOB on rebreathers? Hi Dave T, Firstly, thankyou for your suggestions. Next, the ratio I originaly used was 3:1, which is no good it seems. But when taken down to 1.5:1 (ie: less lime) it works ok in terms of WOB. However, I don't want to make a short and stumpy Inspo type scrubber, but rather find a way to spread the scrubbing material over a long and narrow scrubber while not increasing WOB. The loop (seen here http://home.iprimus.com.au/marekm/rb/100_0548.jpg) has a minimum of 25mm internal diameter in all its parts (with 38mm for hoses). The DSV check valves are very soft so it has a minimal WOB. I've tried various size mouth pieces and they have had no effect on the WOB. Basically, when exhaling, the exhale CL fills up, and then the WOB becomes difficult. On the inhale, the inhale CL empites, and then again the high WOB kicks in. So unless I've missed something, all this seems to point to the scrubber being the problem. I'm thinking of making a long radial scrubber and see what happens to the WOB then. Cheers. -Marek
(Offline)

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