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Thread: wiring hobbs hour meter

  1. #11
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    Default Re: wiring hobbs hour meter

    Get a single pole double throw oil pressure switch. Use common for ground and
    have both idiot light and hobbes constantly powered but seeking ground. Go
    into the switch "normally closed" with oil pressure, and "normally open" with
    the hobbes. Attach "common" to airframe ground (-12vdc)

    When the switch is static (i.e. no pressure) normally closed will conduct
    to common, thus grounding the idiot light, turning it on. Vice versa for the
    hobbes when the switch gets pressure, normally open will then conduct,
    which will apply ground to the hobbes.

    The one in the link I posted previously in fact does that I believe (has
    a COM with, NC/NO contacts)

    Regards,
    Jeff Hays

  2. #12
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    Default Re: wiring hobbs hour meter

    Great, thanks.

  3. #13
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    Default Re: wiring hobbs hour meter

    The oil pressure sender and the spdt switch are now mounted to the firewall. Using an 1/8" copper line to exit the engine (same as Cessna) freed up some space at the oil pump. The switch I used is a NAPA S352. I buzzed out the switch Jeff recommended, but it showed everything open.
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  4. #14
    Senior Member Av8r_Sed's Avatar
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    Default Re: wiring hobbs hour meter

    Copper? It's well known to work harden and though it has some history in aircraft applications, I wouldn't fly with it. I believe braided flexible stainless is the way to go.
    -- Paul S
    Model III SN910
    582 IVO Med

  5. #15
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    Default Re: wiring hobbs hour meter

    I strongly +1 Pauls comment. The attach point at the engine is subject to considerable vibration which will over time, harden the copper and it will eventually fail/split and that will also be accelerated by the continual heating/cooling cycles of the copper from the fluid.

    It could be 10 years from now, but it might be considerably sooner and the result might not be pleasant. In an emergency I would consider using copper with pressure rated rubber isolators/connectors at the terminations, but that's about it.

    If you decide to keep it, I would include it in the preflight checklist and plan on preventative replacement every couple hundred hours.

    Greg

  6. #16
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    Default Re: wiring hobbs hour meter

    Ah, sorry about that I looked at it and I guess made a bad assumption. Glad
    you found a switch that worked. It's a pretty simple way to do it.


    Regards,
    Jeff

    Quote Originally Posted by mr bill View Post
    The oil pressure sender and the spdt switch are now mounted to the firewall. Using an 1/8" copper line to exit the engine (same as Cessna) freed up some space at the oil pump. The switch I used is a NAPA S352. I buzzed out the switch Jeff recommended, but it showed everything open.

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