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Not fully closing schrader-valve on Miflex inflator hose

Discussion in 'General Scuba Equipment Discussions' started by Oitzu, Dec 11, 2019.

  1. Oitzu

    Oitzu Regular of the Pub

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    I need to fact check against you guys.
    I bought recently (october) new Miflex inflator hoses for my regulators with standard QD coupling.
    Changed them, tested them under pressure, took them to a few dives. So far so good.
    After some dives i partly disassembled my second stages to clean them more thoroughly and gave them a negative pressure test after reassembly, just to be sure everything sits right.
    I realised could draw air through the 2nd stage fairly easy and quite a big amount, even though all connections should be "plugged".
    Searching for the leak in the system i found pretty quickly (you may have guessed it by the title) that the schrader-valve on the inflator hose is leaking and the air is coming from the inflator hose through the 1st to the 2nd stage.
    As this hose was pretty new i exchanged it at the retailer with a new hose, only to have the same problem with that one, too.

    I contacted miflex directly and they answered me in short: This should be expected, the hoses aren't made for negative pressure.

    Am i wrong with my opinion that the schrader-valve should fully close on the QD (by spring pressure)?
     
  2. The Chairman

    The Chairman Chairman of the Board

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    The quick disconnect is really designed to keep air from blowing through. It's not a true Schraeder valve in that respect. I test my second stages off of the hose by putting my finger over the inlet. I'm worried about missing o-rings or cracked bodies.
     
  3. rsingler

    rsingler Scuba Instructor, Tinkerer in Brass Staff Member ScubaBoard Sponsor

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    A Schrader valve is an upstream valve, meaning it closes under pressure (unlike downstream second stage valves, which will freeflow if IP gets too high).

    Therefore, by definition, a Schrader is a "downstream" valve when you're inhaling on a bcd hose. Theoretically, you could create enough relative pressure difference for the valve to open when you suck.

    But to be honest, I would never ever have guessed you could generate enough suck to overcome the spring on that seat that hides deep in the end of the valve.

    I'll try it tonight and let you know.
    I'm skeptical, but it's theoretically possible. Stand by...
     
  4. Oitzu

    Oitzu Regular of the Pub

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    Yeah sure with a strong underpressure it could be possible to overcome the spring pressure and open it.
    But i wasn't at any means sucking strong on the reg. Having the hose not on the reg and flooding the QD-side of the hose with water the water simply runs trough it.
     
  5. Oitzu

    Oitzu Regular of the Pub

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    Well there is a spring inside that should, in my opinion, push the valve close when no pressure is applied downstream.
    About taking the second stage of: Well there wasn't really a good reason to do that because normally the complete reg-set is airtight. Putting a finger over the QD stopped the flow.
     
  6. The Chairman

    The Chairman Chairman of the Board

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    I would check it while under pressure.
     
  7. rsingler

    rsingler Scuba Instructor, Tinkerer in Brass Staff Member ScubaBoard Sponsor

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    Just to confirm...you had the 1st stage tank inlet blocked, correct?
     
  8. The Chairman

    The Chairman Chairman of the Board

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    Great question! :D :D :D
     
    rsingler likes this.
  9. Oitzu

    Oitzu Regular of the Pub

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    Under pressure it closes because in that direction the air pushes it in position, but in the other direction the spring should hold in it place.

    ACD on the first stage, good luck creating enough underpressure against that spring. :wink:
    But yes it was definitely coming form the inflator hose.
     
  10. rsingler

    rsingler Scuba Instructor, Tinkerer in Brass Staff Member ScubaBoard Sponsor

    # of Dives: 500 - 999
    Location: Napa, California
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    That really sounds like a valve malfunction, unless the Miflex valve is a completely new design. Even the ones that aren't Schrader valves (thanks, @The Chairman , you're right), like Scubapro and Atomic, are still spring-loaded.
    If you take one apart, send pics!
     

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