So what makes a good scrubber design.....

Steve

Yes it is correct, and it has been demonstrated via testing. As I mentioned before we kan kick around simple maths and compare how big our degrees are until the cows come home but Stuart has hit the nail on the head with this great question. The boris scrubber is a great example here. Same scrubbers in just about every way apart from the fins that legnthen the gas path, This does a few things; lengthens the gas path slightly, the increased friction increases the WOB a little but the increased dwell time gives a better duration. Thanks have some green as some form of compensation for me getting sucked into another one of these sort of threads and lowering the tone of RBW .

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silent running

Come on now Mike, don't be so cranky, it's a tricky subject with lots of room for misunderstanding. And it's even harder to tackle by typing with the others, some of whom are not native english typers... I always learn something from your posts, so I for one wish you wouldn't drop out just yet. Thanks for the diagrams, which made it easier to understand the radial flow dynamics. If all this stuff was so easy to understand, there'd be no reason to discuss it...

Back to the original title of the thread for a moment though, does it seem fair to say that one of the things which makes a scrubber design good is how well matched it is to the other aspects of the unit? It might seem kind of obvious, but since we are on the subject of the Boris/Mks, let's take them for example. They both seem to have made the best choice by using a wide, shallow radial scrubber, which works well with the physical dimensions of those BM CL units and give long duration and good WOB. How about the scrubber choices in other units as they relate to their overall unit design. Do the scrubber types chosen for other units make sense or confuse you? -Andy

caveseeker7

Steve, I don't think this thread lowers the tone of RBW (yet, anyway).
Andy summed it up nicely, scrubber designs is one of the toughest aspects of Rebreather to understand and master. Otherwise every unit would have a great scrubber.

Actually the two designs are quite different, the Ouroboros has an odd radial scrubber and the MK has an odd axial scrubber. The latter is called annular axial, the gas comes in the bottom and exits on top. The odd part is that the scrubber bed is wider than it is deep, the opposite from the usual bucket design.

The same is true with the Ouroboros, usually radial scrubbers have a wide area but shallow depth in a bucket shaped design. Just look at your PRISM's basket. The Ouroboros reverses that, the distance from the inside entry to the outside exit is larger than the heights of the scrubber, even without fins.

That's what makes them so intresting.

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Cheers
Stefan



"Political Correctness is a doctrine, fostered by a delusional, illogical minority,
and rabidly promoted by an unscrupulous mainstream media,
which holds forth the proposition that it is entirely possible to pick up a turd by the clean end.!"

paulraymaekers

ok, I let every-one believe what he wants....

back to the scrubber: why is every-one convinced that the MK15/Boris scrubbers perform so much better then axials??? (altough the MK15 axial??)

paul

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.... the earth is flat, Elvis is alive, and radial scrubbers give longer dwell time than axials...

Fred Evans

While I do believe that scrubber design has an impact on ‘duration’, I believe that the impact is much less marked than many claim it is. I suspect that the key difference is in time to break through under heavy workload. Unfortunately this is a darn site harder to evaluate.

The fact is that you can get similar per kg durations out of a KISS or Meg, as a CIS Lunar or 15.5 if you actually attempt it (easy and of ‘manageable’ risk to evaluate). But the issue here is how does your exposure to breakthrough under heavy workload compare throughout this period (difficult and highly risky to evaluate). Maybe we could get KD to test it.

So, do the fins make a difference? … probably, but I suspect its only going to be meaningful when your life depends on it (what better time). On a routine basis I suspect the durations would be effectively the same, pretty much as reflected in the overall duration CE rating from what I can see.

Note the total absence of any reference to Physics.

Regards, Fred

silent running

[quote=caveseeker7Actually the two designs are quite different, the Ouroboros has an odd radial scrubber and the MK has an odd axial scrubber. The latter is called annular axial, the gas comes in the bottom and exits on top. The odd part is that the scrubber bed is wider than it is deep, the opposite from the usual bucket design.[/QUOTE]



Ooops, thought they were both radial. Thanks for the correction Stefan. -Andy

paulraymaekers

hello Fred, where do I find those results of the CE tests??
I found some lines about the Boris, what about the others???
and that under the same circomstances!!

regards
paul

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.... the earth is flat, Elvis is alive, and radial scrubbers give longer dwell time than axials...

silent running

Stefan, the really dumb part is that I knew this already and then forgot it, more than once. You even reminded me of it again when you sent the MK16 manual... I blame excessive high school marijuana use. -Andy

caveseeker7

I'm not. At least not in general, or in all aspects.

- duration:
That's a tough one as different scrubbers have different absorbant capacities and are tested to different protocols. But I think the difference is less than many if not most people believe.

The size issue is major as what seems like small differences have a big impact. Take the YBOD and Evo, for exaple. Same design, 1/2 kg difference in capacity but a full hour in the CE ratings. So the Evo scrubber is 20% smaller but looses 33.3% duration under the same test protocol.

From what Pete Ready told me the experiments with a small radial were likewise, duration was reduced by a higher percentage than absorbant. On the other hand increasing the capacity of the radial scrubber from 2.5 kg to 2.75 kg added almost an hour in duration.

Now throw in the Boris, which has a different design, 1/4 kg more sorb than the YBOD but lasts 30 mins longer. Who is to say that the difference is the cannister design, not the sorb capacity (and the 0.5 ata lower ambient pressure at that rating). Or a combination thereof.

And while the PRISM has the same capacity, it lasts 90 mins longer. But at a different protocol with a lower CO2 addition rate (1.35 lpm vs 1.6 lpm). As I have no idea whatsoever how much of a difference the flow rate has on the maximum absorbtion capacity of the absorbant I can't know which one is really better. My guess would be the PRISM, as 90 mins is a big difference, but I don't know. Only CE testing could get that data.

Same with the 1600 scrubber: Different size, a whopping 3.7 kg. Different design, annular axial. Different protocol, tested to 1.0% CO2 in the loop rather than 0.5%. In other words, impossible to compare.

And just for fun throw in the cartridge, which is entirely different. :

- WOB:
That's where the radial shines, WOB is low. Since the annular axial should have like properties (due to the width/depth ratio) my guess is that WOB is also better than a normal bucket shaped axial. Imagine the MK series with a 3.7 kg axial bucket, in addition to the rearmounted CL. You'd probably need a safety diver to punch you in the chest on exhaltion and in the nuts on inhalation just to move the gas around.

Divematics' first Shadow Pack had a 3.7 kg axial, and he's had plenty of MKs to play with. I'll ask Tracy how they compare regarding WOB. My guess is that the radial is still ahead as the MK's have fairly small surface area for gas to enter and exit the cannister compared to the radials I've seen.

And the Boris'? Absolutely no idea, it is an odd duck.

- breakthrough (end of life)
IIRC the radial buckets have more gradual breakthrough. CO2 sneeks past the scrubber and rises slower, the curve is not quite as steep as in axial cannisters. Which one's better depends on the dive, I guess.

My 2 ... 4 ... 6¢.

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Cheers
Stefan



"Political Correctness is a doctrine, fostered by a delusional, illogical minority,
and rabidly promoted by an unscrupulous mainstream media,
which holds forth the proposition that it is entirely possible to pick up a turd by the clean end.!"

Mdemon

Yes, I see the colour change. Only I was expecting, wrongly, that the majority of the absorbtion would take place on the concave side of the fin, not the convex. You can see more colour in a straight line from the centre to the part of the outside where there is no fin. There is a slight deviation along the convex side of the fin - would that be due to Bernoulli(sp)?

That said, there still seems to be maximum colour change where there is no fin on the outer edge. So that would be where breakthrough will occur first.

Question: If that's the case, then why not have fins running so there is no straight line to the outer edge?

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