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Benefits of redundant continuous pO2 monitoring?

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

  1. Jheard89

    Jheard89 Angel Fish

    # of Dives: 500 - 999
    Location: Taiwan
    24
    1
    3
    Hi,

    Was wondering if anyone could clarify something for me regarding the benefits of redundant external pO2 monitoring with a computer such as a Petrel w/Fischer etc?

    Am I correct in assuming that such a computer would only be providing a secondary reading of your O2 sensors, and not monitoring pO2 itself? Just trying to get my head round what would happen if one or more of your sensors had malfunctioned but didn't provide a faulty reading (if such a thing is possible?). Would the redundant computer pick this fault up, or would it also continue to read as if everything were fine?

    If so, then what are the actual benefits of having a computer that can continuously monitor pO2 (ie a petrel) vs. one that can't (ie a Perdix)?

    Forgive me if I'm missing the obvious, but the only thing I could think of, is that if your primary computer malfunctioned you would still be able to monitor your pO2/O2 sensors with your backup? I suppose if your primary computer failed then in the absence of a redundant pO2 monitoring source, the prudent thing would just be to bail-out in the unlikely event your sensors also malfunctioned?

    Basically just trying to decide between buying a Perdix or a Petrel.
     
  2. rjack321

    rjack321 ScubaBoard Supporter ScubaBoard Supporter

    # of Dives: 1,000 - 2,499
    Location: Port Orchard, WA
    9,786
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    What unit are you diving now?

    I use an external monitoring shearwater as my primary display on my mCCR kiss.
    Its a secondary display on my Meg.
    The fischer cable is connect to the same 3 cells as everything else.

    The fisher cable provides real time ppO2 data for deco, the predix assumes you are at whatever setpoint you tell it. If your not at that setpoint your deco is not going to match
     
  3. michael-fisch

    michael-fisch ScubaBoard Supporter ScubaBoard Supporter

    # of Dives: 2,500 - 4,999
    Location: Finally Lake City FL
    1,088
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    My understanding of the matter is:

    The Petrel II ext is best used while diving a rebreather monitoring all 3 O2 sensors in real time. The divecan version can be set up by the company that makes the rebreather to also display tank pressure for 2 tanks in real time.
    It's for rebreather divers, as is the NERD computer.

    The Perdix calculates according to how you set it up, w/o realtime O2 monitoring. The Perdix AI can also display tank pressure of up to 2 tanks in real time, again w/o realtime O2 monitoring.
    It's basically for OC divers and its really not for rebreather divers that want to monitor their PO2.
    Since at least 90% (probably well over 95%) of divers are OC divers there is a place for Perdix, Perdix AI and Teric computers. Since I don't dive OC anymore I have no use for them - they can't cover my needs.

    Michael
    If I'm wrong about the Perdix AI and Teric computers please post quickly.
     
  4. Jheard89

    Jheard89 Angel Fish

    # of Dives: 500 - 999
    Location: Taiwan
    24
    1
    3
    I'm diving Poseidon mk7. The Shearwater will be my secondary display.

    Right, but the Fischer provides real time pO2 data based on what the O2 cells are saying? It doesn't monitor the pO2 itself directly? So if your O2 cells were giving an incorrect reading, your computer would also be giving you the same incorrect reading?

    The only benefit then would be if your primary fails then you can still calculate your deco more accurately?
     
  5. rjack321

    rjack321 ScubaBoard Supporter ScubaBoard Supporter

    # of Dives: 1,000 - 2,499
    Location: Port Orchard, WA
    9,786
    3,961
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    I use a non-fisher petrel as a backup computer to the ext-predator I use at the primary display. I set the ppO2 to 1,2 and the deco is within a few minutes of my live deco predator.
     
    Doby45 likes this.
  6. rjack321

    rjack321 ScubaBoard Supporter ScubaBoard Supporter

    # of Dives: 1,000 - 2,499
    Location: Port Orchard, WA
    9,786
    3,961
    113
    Its using the same cells so yes if those cells are crap you get erroneous info
     
  7. Jheard89

    Jheard89 Angel Fish

    # of Dives: 500 - 999
    Location: Taiwan
    24
    1
    3
    That's what I had originally thought and was planning to buy the petrel. However, I saw some posts where people were saying having two computers that can monitor pO2 is unnecessary. So for someone using saying a Poseidon, which comes with its own display panel, a Perdix would be a sufficient secondary computer?
     
  8. Jheard89

    Jheard89 Angel Fish

    # of Dives: 500 - 999
    Location: Taiwan
    24
    1
    3
    So for someone using a Meg or a poseidon for example, the Perdix would suffice as a secondary computer?

    Just wondering if there are any real advantages for choosing the petrel in this case?

    Not having to deal with a connector cable is certainly appealing.
     
  9. michael-fisch

    michael-fisch ScubaBoard Supporter ScubaBoard Supporter

    # of Dives: 2,500 - 4,999
    Location: Finally Lake City FL
    1,088
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    113
    For many it's ok.
    I have 3 O2 sensors, 2 O2 monitoring computers, a solenoid and manual adds. Relying on only 1 thing to keep you alive is, for me, foolish.

    Michael
     
  10. michael-fisch

    michael-fisch ScubaBoard Supporter ScubaBoard Supporter

    # of Dives: 2,500 - 4,999
    Location: Finally Lake City FL
    1,088
    786
    113
    Sure using a Perdix or Teric is OK as long as you also have a 2nd way of monitoring your PO2, HUD comes to mind.
    Disadvantage is the decoinformation on the Perdix and Teric will not match up with whatever your PO2 monitoring and controlling computer displays, you could just use a bottomtimer instead of the the expensive Perdix or Teric computers.
    I had a Shearwater HUD and decided that I didn't like having blinking LEDs in front of my eye while cavediving, which forced me to use Predator/Petrel II ext. combos.
    So far, I haven't seen a advantage to move on from the Predator controller/Petrel II ext combo, since I really don't need continuous tank pressure information. And if I needed that I could get canbus Petrel IIs refitted to my rebreathers for a small fortune.

    Michael
     

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