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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
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    That's what I had originally thought, but I'm now struggling to see how having two O2 monitoring computers is beneficial, as if both computers are deriving their information from the same O2 cells, then surely they'll both just say the same thing?

    Therefore under what circumstances is having a secondary O2 monitoring computer beneficial?
     
  2. michael-fisch

    michael-fisch ScubaBoard Supporter ScubaBoard Supporter

    # of Dives: 2,500 - 4,999
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    If 1 sensor dies or doesn't display near what the other 2 are displaying, I can ignore its values and end the dive safely.
    If my controlling computer dies, I can safely finish the dive with the 2nd computer and manual adds.
    If the solenoid dies, the manual adds come into play.
    I'm set up to safely finish the dive as long as only 1 item craps out. If more than 1 interconnected item craps out I can always switch to bailout and safely finish the dive.
    Without 2 computers both of which have PO2 monitoring, I'd just be switching to bailout more often.

    Monitoring 5 sensors over 2 computers is an interesting thought process, at what point do you start excluding sensors and why? For me not a real safety advantage.

    Michael
     
  3. happy-diver

    happy-diver Skindiver Just feelin it

    # of Dives: 2,500 - 4,999
    Location: same ocean as you
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    LINEARITY

    Mate what are you after here, the OK to spend a few hundred dollars less

    Run the EXT wire next to the other one and bolt the Petrel to the back of the paddle
    How are you going to run the wire through the Disco anyway, it's all potted and stuff

    Find yourself a comfortable chair and read your thread ten times
     
  4. Gareth J

    Gareth J Manta Ray

    # of Dives: 1,000 - 2,499
    Location: UK
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    Early (AP) Inspirations where often modified to take a 4th cell which could be plugged into a VR3.

    The early Inspirations where only PO2 controllers (no decompression information). The 4th cell allowed a user to run the rebreather manually in the event the controller(s) failed, and it gave you 'real' decompression information based on the loop PO2. The negative was you did need to remember to calibrate the fourth cell, in a separate routine.

    This did mean you had two fully independent systems;
    • The CCR managing PO2 using 3 cell voting logic.
    • The VR3 running the decompression, with the possibility to run the rebreather manually, if the CCR controller failed.
    This solution has dropped a little out of favour. Most modern (including the AP range) of rebreathers have built in dive computers giving realtime information for the decompression.
    The only potential issue is a full loss of the rebreather electronics. Most attempt to solve this by dual batteries running each controller/dive computer separately. It does still leave you with the issue of cell failure, hence the 3 cell voting logic approach.
    Plugging a dive computer into the cells via a fisher connector does give you a fully independent display, calculating decompression requirements and displaying real time PO2. The weakness, as has been pointed out, is that you are reliant on the same cells. I guess there is always the possibility, space permitting, that an additional cell could be fitted as an independent cell. However, I would suggest the cells, even if shared with the primary controller, are a safer solution than one single cell. The only potential issue is if a failure in the main controller places an additional load on the sensor signals. This would distort the reading, giving erroneous information.
     
    Jheard89 likes this.
  5. Jheard89

    Jheard89 Angel Fish

    # of Dives: 500 - 999
    Location: Taiwan
    24
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    That's what I had wondered previously, as with the Perdix, if your primary computer were to die, then with no secondary pO2 monitor it would be prudent to switch to your bail out. So yea I agree redundancy would be better, I was just confused why I'd heard others elsewhere downplaying the need.

    Guess I'll stick with my original thinking and go for the petrel. Thanks for clarifying.
     
  6. rjack321

    rjack321 ScubaBoard Supporter ScubaBoard Supporter

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    You need to have 2 independent monitors - a handset can flood, a battery can die, screens can lock up or go black, cables can lose connection
    2 handsets
    handset & hud
    handset & fisher cable
    fisher cable & hud

    The Poseidon comes prewired with 2 ways to monitor so adding a petrel as a 3rd isnt needed.
     
  7. Jheard89

    Jheard89 Angel Fish

    # of Dives: 500 - 999
    Location: Taiwan
    24
    1
    3
    Oh I wasn't considering a third. My confusion was regarding the Perdix or petrel (via the Fischer) for the second. As I've said elsewhere in this thread I was planning to get the petrel, but was left confused by comments I saw elsewhere (cant remember which forum) saying the Perdix would be fine and redundant continuous pO2 monitoring isn't necessary.
     
    michael-fisch likes this.
  8. michael-fisch

    michael-fisch ScubaBoard Supporter ScubaBoard Supporter

    # of Dives: 2,500 - 4,999
    Location: Finally Lake City FL
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    Yeah for the longest time I could't figure out why Shearwater started making non PO2 monitoring computers, finally it hit me that there is a much bigger market of OC divers out there that they had been ignoring for the longest time. I believe that the Petrel and Teric were designed for this market.
    Hopefully Shearwater will not forget their original customer base as rebreather divers fade back to less than 5% of their total sales, but most of their service and support calls.

    Michael
     
  9. Gareth J

    Gareth J Manta Ray

    # of Dives: 1,000 - 2,499
    Location: UK
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    MOD1 courses always teach, if in doubt bailout.

    However, MOD2 and 3 try to avoid bailing out if at all possible, keeping you on the loop, extending available gas, and improved decompression. Techniques such as gas extending, flushing the loop, 4 cycles on the loop, flushing the loop again and repeat.
    With deeper diving, having enough bailout gas becomes more of a logistics problem.
     
  10. rjack321

    rjack321 ScubaBoard Supporter ScubaBoard Supporter

    # of Dives: 1,000 - 2,499
    Location: Port Orchard, WA
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    You already have 2 displays.
    The handset or M28 computer and the HUD. 2 independent readouts of the same cell.

    Can you even install a fisher cable onto a Poseidon? The shearwater is meant to have 3 cells feeding it. And the voting logic in the shearwater is NOTHING like how a Poseidon uses one cell as a primary with a backup cell.
     
    688ClassRebreather likes this.

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