• Welcome to ScubaBoard


  1. Welcome to ScubaBoard, the world's largest scuba diving community. Registration is not required to read the forums, but we encourage you to join. Joining has its benefits and enables you to participate in the discussions.

    Benefits of registering include

    • Ability to post and comment on topics and discussions.
    • A Free photo gallery to share your dive photos with the world.
    • You can make this box go away

    Joining is quick and easy. Login or Register now by clicking on the button

Loop Volume Question

Discussion in 'Rebreather Diving' started by AdamSa, Dec 2, 2019.

  1. AdamSa

    AdamSa Angel Fish

    # of Dives: 200 - 499
    Location: Malta
    12
    3
    3
    I have noticed that when I am holding a stop at a fixed depth, the loop volume steadily slowly increases until I have to vent some gas out. I cannot understand why this is the case - the electronic system is adding in enough oxygen such that it replaces the oxygen that i have metabolised. So why is the volume increasing? When at depth, this becomes a non issue, the shallower I get the more noticeable it becomes. I have tried to close the flow stop just to make sure the ADV was not putting in any gas without me noticing - no difference.
     
  2. taimen

    taimen ScubaBoard Supporter ScubaBoard Supporter

    # of Dives: 200 - 499
    Location: Europe
    768
    377
    63
    If this is during ascent, it is your body off-gassing.
     
    RainPilot and kensuf like this.
  3. AdamSa

    AdamSa Angel Fish

    # of Dives: 200 - 499
    Location: Malta
    12
    3
    3
    I have only ever noticed this on ascent, i don't typically stand still at a constant stop during the descent/bottom period of the dive so it would be harder to notice. Do we possibly off-gas that much?
     
  4. taimen

    taimen ScubaBoard Supporter ScubaBoard Supporter

    # of Dives: 200 - 499
    Location: Europe
    768
    377
    63
    That is a good question. If it is off-gassing, it would be inert gas. Then solenoid would add more oxygen to compensate pO2 for the inert gas entering the loop and the resulting volume increase is much more than just the inert gas off-gassed into the loop.
     
    FreeFlyFreak likes this.
  5. kensuf

    kensuf Cave Instructor

    2,053
    2,674
    113
    Depends on the bottom time, the inert gas, and dive, but yes. If you've got helium in the mix you can clearly see it off-gassing by watching your PO2 decay rapidly at the first few stops.
     
    Doby45, OceanEyes and Caveeagle like this.
  6. Caveeagle

    Caveeagle Rebreather Pilot

    # of Dives: 500 - 999
    Location: High Springs, FL
    1,533
    1,029
    113
    Thanks for this explanation. Even with only ~20% He, it can be a challenge to keep my PO2 >1.4 at 20’.
     
  7. kensuf

    kensuf Cave Instructor

    2,053
    2,674
    113
    20 feet in fresh water is an ambient P of 1.58 -- maximum PO2 you will ever be able to achieve will be just a hair under P.

    Where I'm going with this is try venting a bit of loop gas, then see if it's easier to maintain your PO2. As you're deco'ing, and inert gas is going into the loop, you may need to vent a little more. The rate of off-gassing slows down as your stop time progresses -- the first few minutes will have a rapid amount of off-gassing, which will impact your PO2.

    I teach my CCR students that their rebreather is a fancy rum and coke blending machine. To blend the perfect rum and coke when you are starting with a glass full of coke, you will first need to take some out before pouring the rum in. Otherwise, the rum will just spill over the edge of the glass and be wasted.
     
  8. JonG1

    JonG1 Nassau Grouper

    # of Dives: 2,500 - 4,999
    Location: Glossop UK
    137
    41
    28
    Could the solenoid be passing, ie dirt on seat, does the po2 change at all?

    Or if you have a BOV is it fully closed to oc?
     
  9. tbone1004

    tbone1004 ScubaBoard Supporter ScubaBoard Supporter

    # of Dives: I'm a Fish!
    Location: Greenville, South Carolina, United States
    15,885
    7,256
    113
    short answer is yes, you do off-gas that much and since you're on a closed loop, it has nowhere to go. It's also something you can do to check your decompression status at the end of a dive since you'll see your loop volume stabilize again. Usually quite a bit after your computer has cleared, but it's a nifty thing to notice.
     
  10. rjack321

    rjack321 Captain

    # of Dives: 1,000 - 2,499
    Location: Port Orchard, WA
    9,134
    3,041
    113
    Not that much (a few liters) but for every bit of inert gas offgassed the CCR has to add O2 to compensate and keep the ppO2 up so there's that much more volume.
     
    taimen likes this.

Share This Page