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Benefits of redundant continuous pO2 monitoring?

Discussion in 'Rebreather Diving' started by Jheard89, May 31, 2019.

  1. hroark2112

    hroark2112 Tech Instructor

    # of Dives: 0 - 24
    Location: Raleigh, NC
    You have 2 options for a secondary computer.

    First a hard wired or Fisher/DiveCan computer that provides you with a reading of what your actual PO2 is by reading your cells. It won’t control the unit but it will give you accurate deco information based on your actual breathing gas in the loop, etc. The benefit is if your primary poops the bed, you can manually fly the unit based on the info your backup provides.

    Second option is a computer that does not read your actual PO2 but you set to your set point manually and it computes your deco based on your depth and the set point you input. You can’t manually control the unit based on this one, since it isn’t reading your loop. That being said, if you maintain your loop close to the setpoint you enter in the backup computer, the deco information on the backup will be really close to the primary, so the deco schedule should be pretty close.

    When I’m diving my O2ptima I have a Petrel as the controller and a NERD as a backup. When I’m diving my Kiss units, I have a Petrel or NERD as primary and a Teric or Perdix as the backup. On dives under 90 minutes the primary and backup computers usually are within a few minutes on the deco schedule.

    Hope this helps!!
    Caveeagle and Jheard89 like this.
  2. sea_ledford

    sea_ledford Captain

    A stand alone backup is primarily a backup computer, not PO2 monitor. On my SF2 setup I have the primary shearwater controller and a hud that reads the cells, as well as a standalone (was a predator, now a teric). If my primary controller dies, I can get the PO2 off the hud and have a backup computer for deco purposes. Still calling the dive, but on the loop.

    Two controllers would accomplish the same thing.

    A secondary computer that reads the cells would do that same thing as well, but the secondary won't have control of the solenoid, you would have to go manual.

    But as rjack said, can you even put an additional controller on a Poseidon? If I were to do that I'd go with the M28 if you don't already have one so you have the option of adding the SS sensor.
  3. NAND

    NAND Angel Fish

    # of Dives: 200 - 499
    Location: Germany
    ...i would like take this very interesting base question back to the engineering roots of any unit. Failure of any component in terms of risk management can be seen in various ways, the most common approach would be to multiply the likelyhood with the impact to determine the overall risk.

    Having a hardwired backup computer may be seen equal to having a standalone computer and external additional (Eg. independant HUD) PO2 monitor. Instead of the wire transmitting the PO2 data its simply the user in such case.

    However, taking this into concideration different manufacturers came up with different approaches to either eliminate the failure probability or the impact.

    Eg. Poseidons Se7en units tried for a long time to only work on the probability- developing even semiconductor sensors and coming up with their linerarity checks- blowing gas onto the sensors surface.

    Others, Eg. rEvo, rather worked on the impact portion of the equation- creating redundancy by using fully independant electronics with own sensors each- allowing for full failure of either component.

    Both approaches are cost intensive, have their individual advantages/disadvantages and must be chosen and followed trough with much user awareness.

    So- given all above, I think its crucial to look into the engineering roots first and then take informed decisions based on what the scope of modification is. Creating additional redundancy makes no sense if not the entire fault-tree is backed up. Therefore any additional computer only makes sense if any form of additional PO2 monitoring exists in addition that during operation can be dedicated to the unit.

    Dont forget: You already backed up your computer's deco algorithms by means of a runtime during predive...
    Jheard89 likes this.
  4. Jheard89

    Jheard89 Angel Fish

    # of Dives: 500 - 999
    Location: Taiwan
    I've only ever dived on the Poseidon, so I don't know how the HUD of other rebreathers compare, but on the poseidons HUD you only get a light/vibration telling you if/when to switch to OC.

    Yes it is possible to get a 3rd cell holder for use with a Fischer.
  5. Jheard89

    Jheard89 Angel Fish

    # of Dives: 500 - 999
    Location: Taiwan
    Oh right, wasn't aware that other systems had HUDs that could display pO2. Have only used the Poseidon.

    I had considered getting the M28, but had decided on a sheerwater that can also be used for when I dive OC.
  6. Caveeagle

    Caveeagle Rebreather Pilot

    # of Dives: 500 - 999
    Location: High Springs, FL
    I dive an O2ptima that has 4 cells. The petrel as a controller and monitors cells 1,2,3.
    Originally I had the LeD HUD that monitored cells 1,2,4. That gave me redundant monitoring of cells 1,2 and and independent look at cell 4. I used a Perdix as a stand alone backup deco computer that has a CCR mode where you manually change the po2 setpoints to keep it close to your actually. Deco times were typically very close.

    Recently I replaced the HUD with a Nerd2. It gives me live monitoring of cells 1,2,4 just like the HUD, but also calculates deco and gives me tank pressures via AI.

    **somebody mentioned using a Fischer-Petrel to monitor onboard tank pressures. Perhaps a mistake, but I don’t see how this would be possible without AI and no other way to convert a tank psi into a digital input to the computer.
  7. michael-fisch

    michael-fisch ScubaBoard Supporter ScubaBoard Supporter

    # of Dives: 2,500 - 4,999
    Location: Finally Lake City FL
    I mentioned using a canbus Petrel to also monitor tank pressures, as is done on the XCCR. No AI but direct plumbing to the head. Not Fisher either, the XCCR uses their own
    connectors which are hard to get.

  8. KenGordon

    KenGordon Rebreather Pilot

    You can have one of these CPOD incl. SSO2 and Cable

    I had guessed that you could have a second M28 fed by just that but don't see it as an option.

    Personally, on my JJ, I use a stand alone Perdix as backup to the Petrel which actually sees the ppO2. The backup is set to a GF high of 99 for bailout purposes and the difference between the two computer's idea of ppO2 is dwarfed by that.

    I think there is a good chance more people have been got by incompetent rewiring for 'redundant' monitoring than by primary computers failing.
    Gareth J and sea_ledford like this.
  9. rjack321

    rjack321 ScubaBoard Supporter ScubaBoard Supporter

    # of Dives: 1,000 - 2,499
    Location: Port Orchard, WA
    The shearwater is intended to have 3 cells connected not 1. You can use one is SCR mode I think
  10. JohnnyC

    JohnnyC PADI Pro

    # of Dives: I'm a Fish!
    Location: United States
    My Predator works with one cell in my Pelagian. Not that voting logic matters in that case since it's entirely manual, but 2 cells are on the Pelagian readout (just PO2, nothing else), and 1 cell on the Predator. I'm not sure if the newer ones allow it.
    rjack321 likes this.

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