Check valve for speedo?

Discussion in 'OnBoard Electronics & Controls' started by Lawrence101, Aug 10, 2012.

  1. Lawrence101
    Joined: Oct 2011
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    Lawrence101 Junior Member

    I recently purchased a complete set of Teleflex instrument gauges. I have a question regarding the speedometer pitot tube assembly unit.
    The question is, do I need a check valve or ballast between the pick-up device and speedo gauge ?
    I read somewhere that water may siphon up into the tube and possibly get into the gauge housing. Does anyone have any idea of this ? or is this just an old fisherman's tale :)


    Lawrence
     
    Lawrence101, Aug 10, 2012
    #1
  2. flyfish
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    flyfish New Member

    I've never had a check or antisiphon valve in the speedo tube and have never had any problems. Speedometer is above the water line and my speedometer tube has been fastened up under the gunwale.
     
    flyfish, Aug 10, 2012
    #2
  3. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    They work with vacuum so there is no positive pressure. They sometimes have a small restrictor to keep the gauge from jumping.
     
    gonzo, Aug 14, 2012
    #3
  4. Petros
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    Petros Senior Member

    they actually work by differential pressure, it compares stagnation pressure with free stream pressure across a sealed diaphragm or drum. If you have a check valve in it, the pressure can only go one way, it will build pressure up and not be able to relieve it on the other side of the check valve, the needle will peg and be useless. It not only will not work, you do not want a check valve in the system at all.
     
    Petros, Aug 14, 2012
    #4
  5. CDK
    Joined: Aug 2007
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    CDK retired engineer

    Good thinking Petros!
    There will get some water in the tube at speed, but it will never reach the gauge unless it is installed in the bilge.
     
    CDK, Aug 15, 2012
    #5
  6. Frosty

    Frosty Previous Member

    NO Vacuum --no flow it just compresses the air in the tube --it wont travel more than 3 feet up the tube unless it has a leak at the guage.

    Aircraft Pito tubes are identical. Gonzo if you disconnect the pipe on your speedo guage you will get your feet wet.

    Suck on the guage and nothing will happen blow and it rises.
     
    Frosty, Aug 15, 2012
    #6
  7. pistnbroke
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    pistnbroke I try

    love gonzo
     
    pistnbroke, Aug 16, 2012
    #7

  9. Lawrence101
    Joined: Oct 2011
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    Lawrence101 Junior Member

    Just as i thought, thanks guys for the confirmation.
     
    Lawrence101, Aug 20, 2012
    #8
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