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  • Q4449

    OK, If the tanks of your aeroplane only contain water, the capacitor gauges indicate:

    a) a mass equal to zero
    b) a mass of water different from zero, but inaccurate
    c) the exact mass of water contained in the tanks
    d) a mass equal to the mass of the same volume of fuel

    answer b)

    Surely, if the reference unit is also in the water, it will use the capacitance value of the water to work out the mass of the volume of water enclosed ???

  • #2
    Hi Anthony

    The unit is calibrated to measure the difference between the relative permittivity (capacitance) of fuel (about 2) and air (about 1) so it will be in error when covered in water which has a value of about 80.



    • #3
      Re: Q4449


      What about this one:

      If a fuel tank having a capacitive contents gauging system is empty of fuel but has a quantity of water in it:

      a) The gauge will show the mass of fuel equal to the same volume as the water
      b) The gauge will show the volume of the water
      c) The gauge will show the mass of the water
      d) The gauge will show full

      (c) marked correct but what about (d)

      Since water has a co-efficient of capacitance much more than air and fuel, would'nt the gauging system interpret it as high calorific value and show full scale deflection? Or it depends on the quantity of water present?



      • #4
        Re: Q4449

        Hi Haroon
        Where did the question come from, obviously its different from Q4449

        It will take in to account the quantity of water in the tank and compare the water/air capacitance relationship in the same way that it measures the fuel/air capacitance. I think as far as correct indications are concerned the gauge calibration will be be for fuel with a permittivity level of 2.1. The change in permittivity level for water at 80 will cause the gauge to over-read by as much as 8000%, so in my view it will read full.
        That's my opinion, I'm happy to be corrected.



        • #5
          Re: Q4449

          Thanks for the response Collin

          The question is not from BGS question bank.

          I think both options (c) and (d) are correct. Do you agree?

          Do you have any information (or web link) on a surge box inside a fuel tank that prevents sloshing of fuel away from pump inlet during abnormal manoeuvres.

          I've heard about the surge tanks only.



          • #6
            Re: Q4449

            No I don't think you can say that the gauge will show the mass of water. It will be totally inaccurate.

            I think your reference to a surge box around the fuel pumps relates to the tank construction where baffle check valves are installed in various ribs to reduce the rate of fuel flow towards the wingtips during airplane manoeuvring and keep the fuel booster pumps immersed. Low fuel states can also impose limiting pitch attitudes to ensure the pumps stay immersed.



            • #7
              Re: Q4449

              Based on Q4449 answer (b) a mass of water different from zero, but inaccurate

              I meant it will show an inaccurate (full indication) mass of water.

              Anyway got your point, Thanks a lot for all the answers.


              • #8
                Re: Q4449

                Hi haroon

                This question is to draw your attention to the fact that water when allowed to settle will cause problems with a capacitance fuel gauging system. A/C using this system tend to be the larger jet aircraft and jet fuels have grerater problems with water than piston fuels.

                The capacitance gauging system uses AC electrics. A capacitor placed in the circuit effects current flow. The highest current flow being when the capacitor is fully charged. This is when the tank is full of fuel, and the gauge is calibrated to this value. As the fuel level goes down and air covers part of the capacitor plates the current flow reduces and a torque moter drives the fuel indicator down. if a density compensator is fitted the gauge will indicate mass of fuel as fuel density effects the value of capcitance.

                Water between the plates will cause a short circuit to the system ! Short circuits have very high current flows, greater than the current flow in the circuit when full of fuel. The torque motor will drive the indicator all the way around past the full mark until it hits the stop ? it would do this if the tank was full of water or just sufficient water to short circuit the capacitor plates ?

                Moral of the question ? Make sure you do water drain checks ?

                Hope this helps

                John H


                • #9
                  Re: Q4449

                  Thanks John



                  • #10
                    Originally posted by John H View Post
                    if a density compensator is fitted the gauge will indicate mass of fuel as fuel density effects the value of capcitance.
                    I need your help!
                    Suppose the temperature decreases. Thus, the volume will decrease, and as a result the density will increase. Doesn't this mean that the dielectric constant of the fuel will increase? Since, the dielectric constant will increase, doesn't this mean that more currect will flow to the circuit and the indication , which decreased because of lower volume, will be compensated automatically?or not?
                    Also, could someone explain in simple words how the compensating capacitor works? I can understant the quoted sentence of John, neither the description in the notes - book.
                    A simple description could help me!
                    Thanks in advance!