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Distribution block question.....

Discussion in 'Sidemount Diving' started by Mitchell Teeters, Feb 21, 2012.

  1. Mitchell Teeters

    Mitchell Teeters ScubaBoard Supporter ScubaBoard Supporter

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    I'm eventually going to dive side mount and I have a question about the distribution block system. Since I will be using the block which will essentially equalize the pressure between the tanks, do I need two SPGs attached to each tank? I think one will suffice unless I'm totally wrong. TIA.
     
  2. ianr33

    ianr33 Solo Diver

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    Distribution block??
     
  3. TSandM

    TSandM Missed and loved by many.

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    I'm assuming he means the Z-system that UTD makes. Do they have a port off the block for a HP hose? I think the block is just an LP distribution system.
     
  4. Mitchell Teeters

    Mitchell Teeters ScubaBoard Supporter ScubaBoard Supporter

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    Yes it is only LP, I was wondering if there would be equalization between the two, thus only one SPG. If this would work it would greatly simplify gas management.
     
  5. Teller

    Teller Tech Instructor

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    Gas managment is already simple.
     
  6. TSandM

    TSandM Missed and loved by many.

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    You're not thinking it through. There's no place on the block to take a HP feed from. So you'd put one SPG on one of your tanks, and nothing on the other? How will you know when to swap tanks? (Because the block is a passive conduit, you have to run your tanks just like any other sidemount diver, running one open and the other closed, and then switching -- this is because any IP mismatch between the two tanks would result in you preferentially draining one, if you didn't use that technique.)
     
    Dive-aholic likes this.
  7. Mitchell Teeters

    Mitchell Teeters ScubaBoard Supporter ScubaBoard Supporter

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    That is why I'm asking. I would think that as one bottle donates air the other would also to equalize. If they equalize then the PSI would be the same in both tanks. I would think any differential would be minor. If you are drawing from a central location both would draw. I dunno. I've tried to glean out the z mount system.
     
  8. TSandM

    TSandM Missed and loved by many.

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    The problem with the block is that, as you inhale and lower the pressure in the system, the side with the higher IP will open first and refill the system. The first stage on the side with the lower IP will never open at all, so you will be drawing all the gas from one tank. For that reason, the Z-system is dived the same way as a standard sidemount setup -- when you are breathing off one tank, the other is kept closed.

    It's not like a transfill whip, where the two tanks equalize, because both tanks have first stages on them. With a whip, there is no first stage, but just a passive connection between the two gas sources, so pressures will equalize. Does that make sense?
     
    aquaregia likes this.
  9. DaleC

    DaleC Solo Diver

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    Lynne is correct about the IP's. The lower IP side will not open until the high IP side reaches the lower IP. It won't transfill like you think.
    You have two tanks, both with their own first stage. From the first stage of each tank a LP whip runs to the gas block. From the gas block you run your reg and DS/BC whips. The LP air at the block would have to move through a higher pressure gradient to make its way from one tank to the other. Which it won't.

    When you transfill, you use HP-HP pathways, not HP-LP-HP pathways.

    With one SPG on the higher IP tank you will get an accurate read of that tank till it nearly empty, then you dive the second tank blind.
    With one SPG on the lower IP tank your gauge will read full until the higher IP tank is drained and then it will work for the second half of the dive.

    I tried this for a project linking multiple tanks and worked through the same thinking you are having with a group. Here I have two tanks connected at one first stage but I also tried it with a gas block like the Z system as well. It was fun but, like the Z system, it violated the KISS principle too much for my liking.

    robosock036.jpg

    Just food for thought: You could use one SPG if you connected two tanks using a transfill whip and DIN fittings: from a single post on tank A to post one of an H valve on tank B. Your traditional reg set goes on post two of the H valve.

    I wouldn't recommend it but it would work.
     
    Last edited: Feb 22, 2012
  10. nwbrewer

    nwbrewer Barracuda

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    I've seen this a few times and I don't get it.

    My level of experience in SM is admittedly very small, as is the number of SM divers I've actually dove with, but I'm not aware of a good reason in traditional SM to turn on/off your tanks when switching regs. It seems that if something goes wrong with the tank I'm breathing from, or I'm in a mixed team and need to donate, I don't want to be screwing around trying to get the other tank turned on.

    Somebody more experienced, help me out, do you really do this? If so, why?

    Jake
     

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