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Thread: Number of O2 cells

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    IDA72 - sorrta

    Number of O2 cells

    I am finding as I go through the design process that there are always new ideas that can be incorporated. This idea may be worth while, being as the double head (scrubber) solution allows for duplicate electronics to be incorporated into each head. I was thinking that 3 O2 cells in each head would be redundant and allow for separate wiring for independant computers. Picture a Shearwater and a VR(3 or X) or an OSTC or a LECCIE set-up in any configuration on independant loops. I can invision a combination of any computer, HUD solution that an individual wants. This could boil down too 2 primary and seperate computers with a secondary and a HUD on two completly independant wiring solutions. I understand that this invloves the purchase of 2 more cells (being that 4 is the relative norm) but the trade off would be pure Redundant Array of Inexpensive Sensors and Monitors (RAISM).

    2 Main computers useing separate DECO algorithms with there own backup secondaries and two independant HUD's and all on there own wiring path.


    Just pondering,
    Hunter
    Last edited by 19Hunter68; 20th September 2009 at 00:55.

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    Re: Number of O2 cells

    If you won't trust three cells, why 6? Why not 9? These questions can't be answered without some data about failure modes, mean time between failures, etc, etc.

    4 the norm? I am happy with 3. If I add one cell that stays seperate who do I trust? Every new part adds a failure point. The issue is weather the extra risk from the failure point is out weighed by the reduction in risk elsewhere.

    I am also comfortable with the two controllers on my viison. I have a shearwater plug in only to take my diving history for deco computations back and forth from the unit to OC. (plus that way I don't suffer from handset envy when there's a rEvo or Meg guy around. )

    Seperate deco algorithms I assume means two copies of the same algorithm. Actually one feature of the Shearwater back up that is comforting is the 99 GF contingiency plan.

    I think the inovatin in RBs to come is going to be from increasing reliability of the systems most are already using not adding more copies of the same. Also usefull would be a renewed comittment to educating the world that just because PADI teaches the world to dive, not everyone should move on to RBs.

    But if you can make a BOV with a snorkle I think we can get PADI on board

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    Re: Number of O2 cells

    one cell is enough if you could validate it continuously. add a second for redundancy and to cross check during validation. Add a third to help you mentally vote out the bad one. Add a fourth so you always have 3 when one dies. Add a fifth...and so on

    Point is that logic behind the system is what would make or break it. Personally, for mCCR applications, I feel that 2 cells are sufficient if you have a practice of validating the cells during the dive. one of them dies, and you are then bailing out. 3 cells would give me enough confidence to remain on the loop with one bad one and my internal voting logic

    for eCCR, 3 required, no question

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    IDA72 - sorrta

    Re: Number of O2 cells

    So a 3-cell rig (as is current, 4 as an option) with a Primary, secondary and HUD were too say fail in any regard would cause a bailout scenario. My thought was a completly seperate solution from cell to display (including separate HUD's). Too simplify this I would think that a single display on each arm (as is now) but Left would be an O2 comp/HUD (left eye) on a single circuit, on right a carbon copy of the left on a completly independant circuit. Now if using a single cell on each separate circuit would work is good (2,3,4,5,6) I was only going off the current use of 3 and so proposed 6. This would still be considered primary and secondary but truely be independant from one another.

    Thanks,
    Hunter
    Last edited by 19Hunter68; 20th September 2009 at 03:09.

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    Re: Number of O2 cells

    3 cells is more than sufficient, 4 max IMHO. Cells are checked predive. I think it'd be more improtant to ensure that there are isolated display circuitry in case a display fails. This has been done in several ways for example:

    -KISS mCCR: 3 independent cells with each its own display
    -revo, factory config: 4 cells, two displays sharing a pair, but each cell is independently monitored (no averaging)
    -shearwater + SW HUD: each device monitors 3 cells, providing independent info on each cell

    I think 6 cells is too much to process mentally, too expensive, and way overkill. Consider location of the cells also. They should always be on the inhalation side of the scrubber as this is what is being inhaled by the diver. If placed on the exhalation side of the scrubber, your PO2 will read a bit lower as there is still CO2 taking up some volume before it is removed

    as a general design consideration-add gas pre-scrubber to aid in mixing through teh scrubber plenum. Add monitoring after the scrubber so cells are sniffing what is being breathed

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    IDA72 - sorrta

    Re: Number of O2 cells

    So what you are suggesting is that it would be possible to have a CO2 sensor made up of O2 cells? would the readings drop significantly? If in a twin radial scrubber, exhaling thru the center and inhaling from the outside, the cells located in both housings at the inhale side. Having two separate circuits and comparing the first to the second, if the first dropped in its readings vs the second, this would indicate a breakthru of the first scrubber. when both displays come in-line with each other again then a breakthru of the second scrubber would be known.
    If on the other hand, there is no significant drop in the readings, then the positioning of the cells really wouldn't matter.

    Cheers,
    Hunter
    Last edited by 19Hunter68; 20th September 2009 at 15:01.

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    Re: Number of O2 cells

    in looking at the math only, yes this would indicate CO2 levels, where:

    pre scrubber: Total P = PO2 + Pinert + PCO2
    post scrubber: Total P = PO2 + Pinert

    At the surface, we inhale 21% O2 + inert gas, and exhale 14-16%O2 + inert gas, with the balance CO2 (~5%)

    In principle, once the scrubber breaks through, pre and post scrubber O2 measurements would be the same. However, at this point its too late. With a dual scrubber as you mention, perhaps it would be more effective.

    However, this does not account for gasses mixing in the rig and flow from scrubber to scrubber. Fresh gas might affect the readings depending on where it is entered into the rig. you'd have to experiment with known gasses

    Measuring CO2 gas itself is probably not as effective as some might think in a RB. You could measure it on exhale and inhale, noting the delta, but this indication would only be significant at breakthrough, where its likely too late. This is why many mfgs are going to monitoring scrubber use itself, with temperature. This gauges the scrubber, rahter than CO2 specifically. This has its own inherent limitations, but its probably the best we'll get for now.

    Personally, I'd like to see something like a pCO2 monitor used in hospitals via an ear clip or finger clip. Changes in blood saturation are probably more indicative of a hit coming on than what we are breathing
    Last edited by OceanOpportunity; 20th September 2009 at 15:23.

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    Re: Number of O2 cells

    Quote Originally Posted by OceanOpportunity  View Original Post

    Personally, I'd like to see something like a pCO2 monitor used in hospitals via an ear clip or finger clip. Changes in blood saturation are probably more indicative of a hit coming on than what we are breathing
    That brings up a different, but related, point. I wonder how a clip on Pulse Oxymeter would react to a diver when the pO2 was above .209?

    And also if it would be a reliable early indication of low pO2?

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    Re: Number of O2 cells

    What are the thoughts of number of cells relating to the HUD vs. display?

    One of my RBs has 2 cells, each with totally independent electronics and batteries for the displays. The HUD electronics, with it's own batteries, reads those same two cells, isolated by 1 megohm resistors, for controlling the HUD.

    One of my other RBs has 2 cells, each with totally independent electronics and batteries for the displays. The HUD electronics, with it's own battery, reads a third cell.

    These are both (as are all of my RBs now) CMF mCCRs.

    I used to have an Rebreather with 3 cells and common electronics to run both the displays and the HUD and a 4th cell with it's own electronics and display.

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