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

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

    OK I admit its a plant question to make the most of the Brain power on the board.....

    Axial / Radial......Donut or cylinder......Weight of material used.....Outside to Inside? Hyrdophobic or not......Lithium Hydroxide.....

    Classic Designs CIS Lunar / Mark 15.5 seem to still be king of the hill...

    Stuart

  2. #2
    Sebastian Chander Deep Smeg is a jewel in the rough Deep Smeg is a jewel in the rough Deep Smeg is a jewel in the rough Deep Smeg is a jewel in the rough Deep Smeg is a jewel in the rough Deep Smeg is a jewel in the rough Deep Smeg is a jewel in the rough Deep Smeg's Avatar
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    Deep Smeg

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    Balance

    Being a Meg diver and a student of Tom's, I've been gunning for the CIS canister as the holy grail of scrubber design.

    However the short time I had to talk to people like Kevin Gurr(Ouroboros), Gordon Smith(KISS) and Doug Mckenna(Micropore), last month, really chalanged my opinion on this.

    Looking at the canisters for the CIS and the Ouroboros I was struck by how much longer the grain boundry(I think that's the right term) was on the latter.

    The CIS being Long and narrow only has a boundry of i'm guessing about 20 grains, where was Kevin's was 4-6 times that.

    While the overall duration of the scrubbers maybe based on the total weight of material, I think the CIS has a lot more potential for cyclic breakthrough from over breathing, than Kevin's.

    In more general terms it seemed from the very limited discussions that I had with the designers that canister design is always a balance between DWELL time and Work Of Breathing. Tighter packing and more material might lead to longer duration and less chance of overbreathing but would eventually make the W.O.B. too fierce to sustain.

    This is something I noticed on the SPORT Kiss when using 812 granules. Gordon and Kim agreed that the design works much better with 408, since it was designed for it.

    Kevin's new idea for increasing DWELL time by adding s-shaped fins inline with the gas flow path seems very simple without increasing the WOB, and leading to I beleive a 10-20 % increase in performance.

    Both Kevin and Doug mentioned that some designers don't have the means to do chamber testing on their designs, which are apperently extremely revealing of the shortcomings of certain designs.

    While the hyrdrophobic properties of the CIS make it higly desirable for flood recovery, I'm now looking at adapting my 5.5lbs axial canister for that rather than purchasing a CIS canister when they become avaialble.

    Cheers

    Seb

  3. #3
    Decodiver
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    Quote Originally Posted by Pelagian
    While the hyrdrophobic properties of the CIS make it higly desirable for flood recovery, I'm now looking at adapting my 5.5lbs axial canister for that rather than purchasing a CIS canister when they become availabl
    Hi Seb,

    I also agree that the hydrophobic properties of the CIS are not the main attraction, if indeed an attraction at all, with the Meg.

    I will be getting one of the Cis scrubbers for my Meg, purely for the increased scrubber duration. We are looking at wrecks for next year in the 150 metre range.

    For me there is little point in a completely hydrophobic scrubber in the Megalodon.

    If the loop compromise is sufficient to introduce enough water for it to be a necessity, then it is too late anyway.

    Due to the absence of a method of pumping the water out of the scrubber itself, the problem will not be solved and the can will just fill up.

    The hydrophobic qualities of the Cis scrubber were developed in order to facilitate an underwater scrubber change on the Cis-Lunar.

    Cheers,

    Dave Cooper.

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    Sebastian Chander Deep Smeg is a jewel in the rough Deep Smeg is a jewel in the rough Deep Smeg is a jewel in the rough Deep Smeg is a jewel in the rough Deep Smeg is a jewel in the rough Deep Smeg is a jewel in the rough Deep Smeg is a jewel in the rough Deep Smeg's Avatar
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    Flood Recovery

    Hey Dave

    The CIS canister also carries about 5.5 lbs of scrubber, so even though the flow in a radial is supposed to be more effecient ithna an axial, I don't think it'll get that much more time. My main concern is overbreathing across such a short grain boundry any way, especially at the depths your talking about.

    We saw a protottype 7lbs radial for the meg at Inner Space. Essentially the same can design as the CIS but without the Hydrophobic walls, this increases it's volume.

    I do like the idea of being able to recover from a complete flood. I have deliberatley fully flooded Tom's meg with the CIS canister in it about 4 or 5 times in the pool and managed to get all the water out, leaving the solenoid a little damp, but still functional and nothing that couldn't be taken care off later.

    Cheers

    Seb

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    Seb,

    I was under the impression that the Cis scrubber was 7lbs, the axial scrubber was 5.5 lbs and the prototype was 10lbs. That is why you would get a longer life from the Cis as supposed to the standered axial.
    Martin


    Quote Originally Posted by Pelagian
    Hey Dave

    The CIS canister also carries about 5.5 lbs of scrubber, so even though the flow in a radial is supposed to be more effecient ithna an axial, I don't think it'll get that much more time. My main concern is overbreathing across such a short grain boundry any way, especially at the depths your talking about.

    We saw a protottype 7lbs radial for the meg at Inner Space. Essentially the same can design as the CIS but without the Hydrophobic walls, this increases it's volume.

    I do like the idea of being able to recover from a complete flood. I have deliberatley fully flooded Tom's meg with the CIS canister in it about 4 or 5 times in the pool and managed to get all the water out, leaving the solenoid a little damp, but still functional and nothing that couldn't be taken care off later.

    Cheers

    Seb

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    Sebastian Chander Deep Smeg is a jewel in the rough Deep Smeg is a jewel in the rough Deep Smeg is a jewel in the rough Deep Smeg is a jewel in the rough Deep Smeg is a jewel in the rough Deep Smeg is a jewel in the rough Deep Smeg is a jewel in the rough Deep Smeg's Avatar
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    I'll check the numbers again to be sure

    Seb

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    One of the nice features of using the cis scrubber is the ability to use it with lithium hydroxide. Not the ability to recover from a total loop flood. You do not need much water in the scrubber in order to cause major damage to the rebreather and yourself with lithium hydroxide, it not like normal sodium hydroxide where some water in the scrubber material and all you notice is a sour taste in your mouth and perhaps a higher WOB. I've been told, now i do not know if this is correct, but if i am wrong i bet someone in here is going to make me know about it... Anyway i have been told that you get 11hours out of the cis scrubber with lithium hydroxide while you get 6-7 hours perhaps even 8 with normal sodium hydroxide.

    /Jonny

  8. #8
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    PRISM Topaz & Sport KISS

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    Guys, when you hear about the extreme durations that have been reached with both the Cis-Lunar MK-5P and Halcyon RB80 you have to take into consideration that they were used with scooters. Not under own propulsion. And that the majority of the duration was spend on deco.

    I pulled this data from another post I made:

    The specs for the original Cis-Lunar MK-5p cannister give a liquid volume of 2.5 litrers and the following weight volumes and duration ranges:

    H.P Sodasorb: 2.5 lbs/1.1 kg - Duration: 3 - 5 hrs
    Sofnolime: 5 lbs/ 2.3 kg - Duration: 5-8 hrs
    Lithium Hydroxide: 2 lbs/0.9 kg - Duration: 10-15 hrs

    Don't know which grade sorb were used as a basis, or what testing parameters.
    Depending on those the duration advantages aren't all that much unless you switch to LiOH. The estimated cannister duration with Sofnolime at an O2 consumption of 1 ltr/min is four hours.

    The RB80 has a considerably larger amount of sorb available, somehwhere in the neighborhood of 8 lbs. Add to that the SCR function, expelling some of the exhaled gas. The fact that the WKPP don't use the stock 10:1 ratio, but rather 12:1 or higher shows that the metabolism should be fairly low. So should be the CO2 production. I believe the use of scooters (and later hanging on the line) are a major factor here.

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    Hello everybody. I would like to start by by saying that I've learned more from this site than any other RB site. Thanks to all you non wankers. But that being said, isn't the issue/specs as it relates to scrubber design, that which is ideal? After talking to someone who has dived a Cislunar, they thought that the WOB with the hydrophobic membrane was way too high. If I presume that the time benefits of using Lithium Hydroxide are worth the extra WOB, doesn't this still represent something less than ideal? It's only a small fraction of all dives that need this extra range of time. Not to mention that LH is not OK to fly with and thus only available to people who have the budget and time to set up such an expedition and ship the stuff ahead. As far as I can see, the best scrubber is the one which has the best duration with readily available materials, has the lowest WOB, has the most insulation(be it air or some other effective material), has the slowest breakthrough, and lastly is easiest to pack. Am I missing something fellows?

  10. #10
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    You have some good points there.

    Personally, I don't think there really is an ideal scrubber design, the design has to be ideal for the task at hand, much like the rest of the rebreather it's a part of.

    Having said that, some basics, as pointed out, should be there in any scrubber.
    The duration has to specified exactly by the manufacturer, during that duration one should not be able to 'overbreath' the scrubber. It should be easy to pack correctly, easy store is large enough for several dives and esy to clean.

    WOB should not be increased beyond reason, as that leads to higher CO2 production and is counter productive. The scrubber at least should be somewhat flood tolerant, so a slow leak doesn't force the diver off the loop.

    Each and every scrubber design out there has some compromise or other. The question is which one works the best for the diving you do? Is duration most important, small size for travelling, hydrophobic characteristics for loop recovery, what are you priorities. And how much are you willing or able to pay for it?

    The highly touted and anticipated Cis scrubber is a prime example:
    The ability to flood the loop, and if the leak can be fixed UW and the sensors survive to recover make it a good choice for overhead environs. Caustic cocktails are next to impossible.
    The disadvantages are the before mentioned increase in WOB, the reduced absorbant capacity (note that it takes less sorb than a smaller sized Meg container) and increased cost of purchase (the membrane ain't cheap) and maintainance (it needs to be replaced every so often). The gas flow makes it easier to recover, but less efficient.

    Another example the ExtendAir cartridge/cannister:
    A great idea due to its superb ease of use and much reduced chance of caustic cocktails. As long as the cannister seals are okay no channeling should occur, as the sorb is attached to a solid grid.
    But it costs quite a bit more, you get less absorbant per volume, it's harder to come by and bulkier to ship. The currently available sizes fall short of granular designs in terms of duration.

    And so it goes, as I said, for each and every design available.

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