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Thread: Reduced scrubber size in RB-80 clone

  1. #1
    Underwater Journal Walt Stearns is a name known to all Walt Stearns is a name known to all Walt Stearns is a name known to all Walt Stearns is a name known to all Walt Stearns is a name known to all Walt Stearns is a name known to all Walt Stearns is a name known to all Walt Stearns is a name known to all Walt Stearns is a name known to all Walt Stearns is a name known to all Walt Stearns is a name known to all Walt Stearns's Avatar
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    Reduced scrubber size in RB-80 clone

    I am looking at couple the RB-80 clones (STDE EDO-4, GDS AH1, RON, etc.) type Passive Addition Semi-closed Rebreathers.

    I understand, compared to constant mass flow systems, the RMV-keyed passive addition concept (used in the Halcyon RB-80 rebreather) is reported to be effectively more more efficient method of insuring a relatively steady FO2 over a range of breathing rates.

    Pros:
    No electronics - no worries about water leakage.
    No 100% O2 addition - less risk of O2 toxicity.
    Intuitive recognition of gas addition failure. ("I can't get a breath!" alarm mode)
    Integral water / condensation removal pump.
    Counterlung enclosed inside shell.

    Cons:
    No electronics - no direct monitoring of PO2, but that can fixed with a simple PO2 meter.
    Decreasing PO2 on ascent - so I need to exhale through my nose to vent expanding gas to allow makeup with fresh gas.
    Deep dives require one or more gas switch - not big deep dives (greater than 150 feet).
    Lower back counterlung location may cause high work of breathing.

    One of the models that has caught my eye is one called the AH1-SP built by German Diving Systems - http://www.gds-techdive.com/Produkte/Rebreather/rebreather.html which features a shorter size scrubber (holds approx. 2 Kg of soda lime), reducing the RB’s height/size to that of a 63 cu.ft. tank (which also brings the counterlung few inches higher up the back).

    What I can decipher from the specs on this model, scrubber duration in warm water should be 4 hours. My question is with this reduced scrubber size, which is roughly half the length of most RB-80 clones, will it still work or is the pathway through the sorb too short to be effective?

  2. #2
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    Re: Reduced scrubber size in RB-80 clone

    Don't know the unit and can't comment on actual scrubber performance, but aside from the half capacity the SP model also substitutes the anodized aluminum of the lower assembly (watertrap/injector housing/bellows housing etc) fpr POM/Delrin.

    Cutting scrubber capacity by half doesn't necessarily mean you still get half the duration. Best example are APD units, the YBOD with 5.4 lbs tested 3 hrs during CE while the Evo with 4.4 lbs tested 2 hrs. another example was the attempt by SMI to make a smaller scrubber for the PRISM. They knocked about a third off the original 5.5 lbs basket and ended up with a prototype that did so bad in cold/water and depth that it never went into production.
    The Dräger Dolphin has 2.25 kg absorbant and it rated between 3 and 4 hours, and also an SCR (blowing more CO2 bubbles than a passive unit). Maybe read their press release and take it into consideration.

    2 kg is just not much ... 4 hours is pushing it in my opinion.

  3. #3
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    Re: Reduced scrubber size in RB-80 clone

    Isn't there a noise issue with substituting plastic over alu in PSCR? Don't they say plastic deadens the noise of dump/addition? It isn't that loud to start with so if you can't hear it then you lose the big benefit of a mechanical SCR.

  4. #4
    Underwater Journal Walt Stearns is a name known to all Walt Stearns is a name known to all Walt Stearns is a name known to all Walt Stearns is a name known to all Walt Stearns is a name known to all Walt Stearns is a name known to all Walt Stearns is a name known to all Walt Stearns is a name known to all Walt Stearns is a name known to all Walt Stearns is a name known to all Walt Stearns is a name known to all Walt Stearns's Avatar
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    Re: Reduced scrubber size in RB-80 clone

    Your right Stefan about the YBOD’s 5.4 lbs scrubber capacity testing out to CE standards at 3 hrs, I have used Dräger Dolphins numerous times and have found the scrubber (with a full 2.25 kg absorbent) will last a good fours in warm water (80 to 69 F.) before breakthrough. The same unit with the spacer/reducer (cuts the amount of sorb roughly 1 kg ) lasts maybe a little over an hour in the same conditions. What my concern with the shorter scrubber on the AH1-SP Sorb capacity being roughly 2 kg is the depth the of the filter action of the sorb falling between 4 and 5 inches for gas to pass through for recycling.

    I also looked at your site Lizardland, and found it highly interesting, especially in regards to trying out one of STDE’s EDO4s. From what I could decipher, you had mixed feelings about the system. Would like to hear more, but first I should outline what I looking to do with it, maybe the both of you can help me out on this.

    Primarily what I am looking to use it for is photographing marine life that tends to be easily up set by noise. I have used both Drager Dolphins with (50 and 60% dosage) as well as the Inspiration CCR with equal success. While CCR’s are certainly the most quite, having anything putting out less than 80% of the noise of OC does just fine. Contrary to what some believe, rebreathers do not make you invisible to marine life, rather something that is more tolerable to them.

    The majority of my working depths range between 30 and 150 feet, with 60 to 90 the most prevalent. I like the design approach of the passive SCR’s. Compared to mass flow SCR’s, if you are good breather, especially in resting situation you are somewhat rewarded, expending less gas. My understanding of how the work is that the reduction in PO2 from the drive gas to the loop is roughly 4% (40 percent in the tank becomes 38% in the loop.). O.K. depth and time range there will be the same as diving OC with 38% nitrox. See where I going with this.

    I would greatly appreciate your feedback.

  5. #5
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    Re: Reduced scrubber size in RB-80 clone

    I was going to order an EDO-04 but decided to wait because I had the opportunity to dive with Michael and Arnaud who make the rebreather. I got the chance to try it out and to see it being used on some serious diving.

    My experience of it... I found it a very difficult rebreather to use. It was extremely heavy and very uncomfortable in the water. I found the WOB just too much for me. When the counterlung bottoms out you really feel a lot of effort to make the ADV fire so that you can get a full breath. For me it was very tiring. I would not like to have an emergency while using it because I think the breathing resistance could cause a feeling of panic. Also it is not that easy to hear the ADV. This is essential with this type of rebreather otherwise you have no signal as to whether or not it is functioning properly. Also the sound of gas being dumped is very confusing as it sounds just like breathing a flooded loop. I was also worried about getting stones or gravel stuck in the bellows as I would mainly be using it in caves.

    The other big problem is gas logistics. I cannot remember the figures exactly but I think that if you are diving beyond something like 25m then you must carry two drive gases. If you were to try to ascend without switching gases then the loop can become hypoxic very easily. From talking to other people, deco can be hard to plan too. With CCR you can have one single diluent bottle that will work at any depth, with PSCR you need a whole range of gases depending on depth.

    For me, I chose the KISS instead. It is so much simpler and easy to use, the WOB is much better and I think much more flexible. I have no regrets. The PSCR idea is nice but in practical terms I think full CCR is a better option.

  6. #6
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    Re: Reduced scrubber size in RB-80 clone

    Quote Originally Posted by UWshutterbug

    What my concern with the shorter scrubber on the AH1-SP Sorb capacity being roughly 2 kg is the depth the of the filter action of the sorb falling between 4 and 5 inches for gas to pass through for recycling.
    To be honest, I don't know.

    The scrubber bed in its entirety must be considered. Annular axial and radial scrubbers have only a few inches for gas to pass through for example. They make up for it by having a broad area for the gas to spread across, rather than a long way through it. Furthermore, the gas velocity through the scrubber is a major factor. Both of these affect dwell time and hence ability of the scrubber to absorb the CO2.

    Quote Originally Posted by lizardland
    I cannot remember the figures exactly but I think that if you are diving beyond something like 25m then you must carry two drive gases. If you were to try to ascend without switching gases then the loop can become hypoxic very easily. From talking to other people, deco can be hard to plan too.
    Isn't the gas management and deco calculation the pretty much same as open circuit?
    A passive SCR just extends the gas supply for a longer duration.
    Since they are RMV keyed they tend to keep the loop gas composition closer to the gas(es) in the tanks than cmf units, which should ease deco calculations. And gas management shouldn't be more complicated than either OC or OC bailout for CC divers.

    Either way, I would be very cautious buying a rebreather without trying it out first. Even more so in the price range that most pSCR units are. The Lizards's experience seems to reinforce that.

    Another potential problem to consider is lack of training and certification on all units I'm aware of, save for the RB80. Depending on where you dive you may have to show proof of that and end up sidelined or on OC instead of your unit.

  7. #7
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    Re: Reduced scrubber size in RB-80 clone

    Quote Originally Posted by caveseeker7
    ...I would be very cautious buying a rebreather without trying it out first...
    That's the girly-man way... ;)

  8. #8
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    Re: Reduced scrubber size in RB-80 clone

    I'll take girls over boys any day. And night. :D

  9. #9
    Underwater Journal Walt Stearns is a name known to all Walt Stearns is a name known to all Walt Stearns is a name known to all Walt Stearns is a name known to all Walt Stearns is a name known to all Walt Stearns is a name known to all Walt Stearns is a name known to all Walt Stearns is a name known to all Walt Stearns is a name known to all Walt Stearns is a name known to all Walt Stearns is a name known to all Walt Stearns's Avatar
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    Re: Reduced scrubber size in RB-80 clone

    Caveseeker7 is correct, gas management and deco calculation the pretty much same as open circuit. “Since they are RMV keyed they tend to keep the loop gas composition closer to the gas(es) in the tanks than cmf units, which should ease deco calculations. And gas management shouldn't be more complicated than either OC or OC bailout for CC divers.” A passive SCR just extends the gas (less a 4 percent drop in difference between mix in the bottle and the loop), supply for a longer duration.

    Now to WOB. I have had a good look at the RB80 and Lizardland is certainly right about the size and weight of a full size RB80 and EDO. They are quite large - similar dimensions to an aluminum 80, weighing not much less when packed with sorb. Add tanks, especially those prescribed for cave diving and you have one heavy mother. From those who dive them, WOB can be a bitch in a head down position, annoying in the vertical head up postion and perfectly fine in the horizontal position.

    Now the million dollar question is, in the shorten version (overall height 22 to 23 inches) the counterlung is four to five inches higher up the back, which should (with the exception of head down) lessen WOB.

  10. #10
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    Re: Reduced scrubber size in RB-80 clone

    Intresting thought regarding the hydrostatic differences.
    The shorter scrubber certainly should impact WOB.

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