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Measure scrubber runtime?

Discussion in 'Rebreather Diving' started by Big-swede, Feb 20, 2021.

  1. wedivebc

    wedivebc CCR Instructor Trainer ScubaBoard Supporter

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    I once had a CCR student who was formerly in the Russian Military and had been taught that the usage of a scrubber was inversely proportional to the weight change caused by the moisture it had absorbed. I was skeptical but we tried an experiment and weighed our scrubbers before the dive then after. They had both taken on similar weight afterward and the change in weight roughly corresponded to amount predicted by the formula the student used. I am no chemist so I don't know the formula but it makes sense that water is a byproduct of CO2 absorbsion and would therefore add to the weight of the scrubber.
     
  2. Akimbo

    Akimbo Just a diver Staff Member ScubaBoard Supporter

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    This is an old and complex problem. I was only tangentially exposed to US Navy studies on different pure O2, semi-closed, and closed circuit HeO2 rebreathers. They spent a LOT of money to develop these charts for each rig with temperature and activity curves. For example, EOD (Explosive Ordinance Disposal) divers typically are not working as hard as combat swimmers.

    In many ways, the military's effort is simpler because the age and physical condition for each activity group was much more consistent than CCR users in the recreational market. They have breathing machines for inhalation and exhalation resistance that can be programmed across huge volume variations but I have never heard of one that can meter CO2 into the system for scrubber studies.
     
  3. Gareth J

    Gareth J Contributor

    # of Dives: 1,000 - 2,499
    Location: UK
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    Just to remember.

    Despite all the tools (temp stick etc), rules of thumb, etc. Scrubbers need to be treated with respect. Hard data, and manufacturer specifications should be full respected.

    Even then, there is always the potential for breakthrough, channeling, over packing. etc

    Better to change the scrubber early, and take an extra 30 minutes double checking your unit, than trying to save a buck, or get an extra 5 minutes of scrubber life.

    I don't want a CO2 incident, if I'm going to have one I much prefer it to be on the boat during the pre-breath than at the bottom of the shot in 50m!
     
  4. rjack321

    rjack321 ScubaBoard Supporter ScubaBoard Supporter

    # of Dives: 1,000 - 2,499
    Location: Port Orchard, Washington State
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    Unless someone beats me to it, sofnolime's net formula is readily converted to mass.
    Sofnolime-1025-NI-UK-DS-V3.pdf (molecularproducts.com)
     
    grantctobin likes this.
  5. sea_ledford

    sea_ledford Captain

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    The problem with strictly following the rated duration comes when you start doing 4, 5 or 6 hour dives.
    What is lower risk:
    Pushing the scrubber duration while in a shallow, low effort state (deco).
    Adding a second CCR.
    Carrying enough gas to do the last 2-3 hours OC.
    Buy a CCR that can use a Cislunar MK5P scrubber with a hydrophobic membrane and buy two of them so you can swap them out underwater.
     
  6. Tassi Devil Diver

    Tassi Devil Diver Rebreather Pilot

    # of Dives: 1,000 - 2,499
    Location: Tasmania, Australia
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    I don’t consider this a problem when the rMS is used in accordance with the manufacturer’s instructions as is covered in the initial MOD1 course. The rMS has been found to be very conservative as noted in the study provided by Dr Mitchell above, maybe even overly conservative as the paper suggest.

    Urrrr no, continue on the dive as normal, if you are coming to the end of the recommended scrubber duration when a failure occurs, I would start to think about rounding out the dive, complete any necessary decompression safety stops etc. I have never aborted a dive due to the rMS “crapping out”, rMS failure has happened to me plenty of times early on, I still have an old probe in one of my 5 canisters that throws up lost probe errors from time to time. No cause for alarm.

    I start any significant deep decompression dive that is likely to last longer than 2hrs on a new scrubber.

    No, not at all, continue on with the dive if inside the rEvo recommendation for scrubber time. I typically run my scrubbers down towards 00:00 RCT on a third or fourth dive on canister, this is a shallow no stop dive, and if the Scrubber “carps out” that late I casually end the dive. No need to bail out, no need to panic, not in any immediate threat of a CO2 hit or other calamity.

    I certainly don’t, it’s not a critical piece of rEvo like a PO2 censor, redundancy is the rEvo recommendations for scrubber duration for non rMS units. I will happily dive on with or without the rMS.

    I have found the rMS predictions of scrubber time to be very robust, I am satisfied there is more than adequate conservatism built into the scrubber time prediction algorithm. Whilst the hardware has been a source of frustration I am satisfied that this has been resolved.

    I have found the rEvo rMS to be a great tool for measuring and keeping track of scrubber runtime
     
    stuartv and Heat Miser like this.
  7. Heat Miser

    Heat Miser DIR Practitioner

    # of Dives: 200 - 499
    Location: Perth
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    I feel a lot more comfortable using the dual scrubber system "with RMS" as relative newbie there have been times when I haven't cycled properly, or if I did, I didn't mark the cannisters properly, sometimes this has been noticed before the dive, once after. Thankfully I can say this hasn't happened for about the last 50 hours or so eventually/slowly hopefully I'm learning.

    But I feel it does add redundancy and another level of safety, if you are close to time limit to cycle (RCT), and thus obviously getting closer to the limit of remaining scrubber time (RST), your much more a tune to the symptoms of hypercapnia, and I would think it's less likely to creep up on you unexpectedly.
     
  8. kensuf

    kensuf ScubaBoard Sponsor ScubaBoard Sponsor

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    A six hour dive where the last 3 hours are at 20' on deco is one thing, but the problem is when people think that means a scrubber is really good for six hours, so they do a 3 hour dive on day 1 then try to do another 3 hour dive on day 2. It's that portion on dive #2 when they're deep (and on hour 4 on the scrubber) that bed depth of the unused scrubber material becomes an issue.
     
  9. The Monkey

    The Monkey Registered

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    Commercial confidentiality means I can't elaborate, but this exists.
     
  10. Akimbo

    Akimbo Just a diver Staff Member ScubaBoard Supporter

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    It makes sense. It isn't technically difficult and data is a lot easier to collect from an automated system.
     

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