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Understanding Fuel Quantity indicating systems in large aircraft

Discussion in 'Electronic Basics' started by Jeremy D. Grotte, Jul 31, 2003.

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  1. I work B-52's, which use a DC fuel quantity indicating system, capacitance
    sensing probes, indicators, wiring, C/B's, etc.etc.
    I guess over the years, we've gotten so 'stupid' that now all we do is
    troubleshoot down to bad parts and swap them out, and when that fails, we do
    'swap-tronics' until it's fixed. Anyways, I want to learn more about this
    type of indication system. So here goes...
    I remember reading somewhere that the system sends a negative going DC
    square wave pulse of 20v down the wires at 6200hz, 50% duty cycle, very low
    current. So, I can assume that charges the capacitors (probes). The
    indicator takes the pulse on the return line (reading the probes discharge?)
    and does some 'magic' on it, and basically what you end up with would be
    some sort of wheat-stone bridge type of circuit using phase-shift and
    capacitors on the motor driven re-balance side instead of resistors and
    motor driven re-balance resistors. The probes themselves have 3 wires going
    to them, I assume a center, a positive return and a negative return, or a
    ground of some sort, and on the circuit card attached to the probes are a
    couple of diodes and resistors (if that helps anybody).
    Am I even close on this subject? Does anybody have any better ideas on how
    this works? I'm getting up in years in my career and need better info to
    pass along to my youngsters just getting started in the job and of course,
    knowing the guts of the system would sure help for troubleshooting. (i.e.
    one leg open/shorted will cause the indicator to drive one way, the other
    line open/shorted, drives the other way, etc.etc.)
  2. John Larkin

    John Larkin Guest

    I worked with (not for) Simmonds Precision on a couple of fuel
    measurement systems, and I got to see their standard capacitance
    gauging stuff. They did, probably still do, lots of aircraft systems.
    Their gauge was a coaxial probe, one pipe inside another with vent
    holes to let the liquid in/out. They used a sinewave oscillator (at a
    few KHz, as I recall) to poke a sinewave onto the outer tube. A cable
    came back from the inner tube to the summing point of another opamp,
    so the AC current induced to the inner electrode was measured, and
    that's proportional to the capacitance between the outer and inner
    pipes. The returned AC current is lowest when the tank is empty,
    highest when the probe is full of fuel with its higher dielectric
    constant. They did all this with ordinary IC opamps, with a bunch of
    intrinsic-safety networks in the cables (resistors, fuses, zeners) to
    ensure that no electronic failure could create a spark in the tank. It
    was all very simple circuitry.

    I interfaced their stuff to a PDP-11, measuring LNG in huge tanks on a
    barge-mounted gas liquefaction plant in Indonesia. I calculated how
    much LNG was in board, based on the shapes of the tanks and the roll
    and pitch of the barge. Nasty work, in assembly language.

  3. Fred Abse

    Fred Abse Guest

    But pretty damn' satisfying when it finally works the way you want it to.

    What's a PDP-11? I hear people saying.
  4. The son of a PDP-8, of course! ;-)
  5. PaoloS

    PaoloS Guest

    it's an old, big Digital Equipment computer.
    Try a search on google.

    "Speak softly and carry a big stick"
    Theodore Roosvelt
    IRAN libero!
  6. PaoloS

    PaoloS Guest

    I'm not an expert. I've only an idea of what was the PDP-11... :)
  7. I worked around one in the 80's. It was part of a Teledon videotex
    system. Now who has heard of that? If you have...did I spell Teledon
    correctly? :)

  8. Yes, the instruction set was beautiful because it was orthogonal.
    I used to play around (software) with a PDP11/23 which was a PDP11 in a
    DEC103 terminal chassis.

    Dana Raymond
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