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Manual Injection Valves

Discussion in 'Rebreather Diving' started by v101, Dec 25, 2017.

  1. JohnnyC

    JohnnyC Divemaster

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    Interestingly enough, I dove a unit today with no MAV, only an ADV, with an ADV shutoff, not my unit. My friend dove his with a MAV only, no ADV. I realized how annoying it was to have an ADV, and how I would much prefer MAV only. I may modify my unit and remove the ADV guts, seal the top, and run a MAV to the now guts-less port. Lungs get tight, smash the MAV. Easy, and no needing to remember to flick on or off the ADV.

    Hmmm.....

    As for your ADV/MAV, if it's a hydrostatically triggered ADV (second stage on the t-piece, as opposed to a plunger style like the current Meg ADV) then you can likely run it like a MAV by jamming a finger onto the diaphragm. However, a stuck open ADV (think first stage IP creep causing a freeflow) is much more likely than stuck closed. In fact, I don't think I've ever seen a reg fail closed. So, what you currently have is a potential for the ADV to freeflow, which is easy to handle, but no way to shut it down and still maintain the ability to put dil into the unit. Adding a MAV and an OPV on the first stage would allow you to shut off the freeflowing ADV and still maintain the ability to dil flush the unit. Sure you'd be pissing out the OPV on occasion, but you'd lose far less gas than a full on freeflow, and if you bail out to SCR mode, depending on the speed of the IP creep, you might not even trip the OPV since you'd be using the MAV quite a bit.

    That being said, I don't dive a JJ and don't know what configuration of flingdingers they've got. I would assume adding a MAV would require two ports on the unit/t-piece/wherever, so that they're independent.
     
  2. doctormike

    doctormike ScubaBoard Supporter Staff Member ScubaBoard Supporter

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    Other than feathering the valve, assuming that the free flow is internal.

    Yeah, it's an interesting thing to try to think through. In training, we did dil flushes by just pushing the button on the ADV diaphragm.

    The dil MAV is pretty simple to add - the single LP hose out of the dil reg goes to a three way gas distributor block, with one of the ports plugged off in the International (non-CE) version. LP hoses from the other two ports go to the ADV and the wing. If you add a dil MAV, it just goes into the third port.

    I guess an internally freeflowing ADV could be dealt with by shutting off the dil and feathering the valve as needed. Not quite as easy as a MAV button, but certainly easy to reach. Since dil is used for descents and for dealing with problems (dil flush, sensor verification, dewatering, mild hyperoxic loop, SCR mode...), it's unlikely that you would have a one of these issues along with a free flowing ADV, but in that case the feathering option would exist. Still, I see how it would be nicer to have the ability to shut off the ADV with a flowstop and just use the dil MAV in that unusual situation, which I guess is why those are required on the CE version.

    Thanks for your insights!
     
  3. joshk

    joshk Nassau Grouper

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    The ADV on the SF2 is inside the loop so it's not accessible during a dive.
     
  4. rjack321

    rjack321 Captain

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    You have a scooter? You probably want an ADV, especially scootering in a cave. And yea you need a shutoff on it
     
  5. JohnnyC

    JohnnyC Divemaster

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    I do, but I just don't descend that fast. I'm hugely protective of my ears, probably to the point of paranoia, so even on a scooter I'm descending slowly enough that I've always been able to manage loop, wing, suit, etc. even while on the trigger. With my Meg I'd pre-empt the ADV most of the time anyway.

    As for the shutoff, I don't necessarily agree 100%. If you're using a plunger-style like the Meg, I think a shutoff is a wise decision. However, with the hydrostatic ADV on the head of the Pelagian, you simply adjust cracking pressure until it only fires when you want it to. However, it's one more reg to deal with essentially, and I've already got enough to maintain. Interestingly enough, because of its position on the top of the head, to do a dil flush you can simply turn turtle and loose lip it. Fun trick, impractical in a cave or really anywhere.
     
  6. rjack321

    rjack321 Captain

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    On the trigger with a light in the other hand I don't really have a free finger to hit a MAV. I can, its just the ADV is world's easier and lets me have better light discipline
     
    DA Aquamaster likes this.
  7. JohnnyC

    JohnnyC Divemaster

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    Taking the ADV out of the equation, are you not adding suit gas or wing gas on a scooter descent? Equalizing? Hitting a MAV is essentially the same as hitting a wing or suit inflator. If you're concerned about light discipline, you're already breaking it by adding suit or wing gas or equalizing your noggin.

    Personally, I throw the light on my scooter hand after a couple shallow laps to get everything "situated." Then, on descent, left does the adds and ears, right does the driving. Even after initial descent, you've still gotta do it in the cave, so either way one hand is on the scooter, one to do your adds if it's any sort of significant depth change. If it's minor, or up and back down or vice versa, I'm usually not futzing with anything because I'll be back at nominal depth momentarily.

    Either way, you're either driving with your light as well, or you're moving the light to inflate/deflate anyway.

    Curious how you're dealing with all of it on a scooter descent, regardless of ADV or MAV.

    And of course swimming it's no issue at all.
     
  8. rjack321

    rjack321 Captain

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    I hit wing and suit a few times depending on how straight down-ish the descent is; it isn't that hard to thumb the lighthead and use my left hand. I only break light discipline if I absolutely don't have time or space to move my lighthead, which is fairly rare. I tend to overadd and then use the scooter to pull me down to the next level (assuming I know it keeps going down). I got a electric BC in my right hand - I don't futz with my wing or suit that much from say the park bench all the way to the Hinkle in Ginnie, or really anywhere after the trash room in JB. I do end up using my CLs for some buoyancy changes. And suit since I can dump with a shoulder roll. I like having the ADV to remove at least one manual add, although I also shut it off sometimes.
     

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