Rainman said:I think so. . I little dry, but it makes sense to me. . .
capacitance type meaning an AC signal is sent from one "plate" to another usually cocentric metal tubes in a probe. The "dielctric" is the fuel, or more accurate the fuel / air mixture (as fuel is burned becomes more air). Basic engineering (if you took any college physics) dictates that C = εA/d. Where ε is the permittivity of the dielectric. A is the area of the plates and d is the thickness between the plates. These are both constants in this setup.
As the amount of fuel in the probe decreases from burnoff, the amount of air in the probe increases, thus continuously changing the permittivity of the dielectric as a different electrical value. Capacitance type sending devices are typically less suspectible than float type devices (that are in your car for example) in regards to the movement or sloshing of fuel--so work better in a dyanmic aircraft environment. In general, the overall accuracy is really dependant on the consistency of the fuel (JP8 or JP5 work well, but civilian 100LL for example often contain different amount of additives that throw them off [changes the dielectric]) and more importantly the quality of the calibration that is done.
hope that helps. . . . . . .
flashypants said:"I believe, I believe!! Where's that damn button?!"
ENSsquid said:the fuel quantity indicators are on the same PFM bus as the FCU. So try looking there.
Rainman said:I think so. . I little dry, but it makes sense to me. . .
capacitance type meaning an AC signal is sent from one "plate" to another usually cocentric metal tubes in a probe. The "dielctric" is the fuel, or more accurate the fuel / air mixture (as fuel is burned becomes more air). Basic engineering (if you took any college physics) dictates that C = εA/d. Where ε is the permittivity of the dielectric. A is the area of the plates and d is the thickness between the plates. These are both constants in this setup.
As the amount of fuel in the probe decreases from burnoff, the amount of air in the probe increases, thus continuously changing the permittivity of the dielectric as a different electrical value. Capacitance type sending devices are typically less suspectible than float type devices (that are in your car for example) in regards to the movement or sloshing of fuel--so work better in a dyanmic aircraft environment. In general, the overall accuracy is really dependant on the consistency of the fuel (JP8 or JP5 work well, but civilian 100LL for example often contain different amount of additives that throw them off [changes the dielectric]) and more importantly the quality of the calibration that is done.
hope that helps. . . . . . .