PDR1, as I recall it the B747 & B744 have a capacitive probe mounted sideways close, but not too close - water - to the bottom of each tank to provide a density calibration. This has led to problems, big ones, when refueling with fuel which tends to stratify because the fuel is then non homogeneous so your calibration will not be the average. The early B747-SP flights SYD-SFO & SYD-LAX caused us a lot of grief because the USA West Coast fuel tended to stratify and these A/C were dedicated to that run. Since we were usually at MTOW errors could not be tolerated and determining the actual fuel load became very difficult. Often we just took a deep breath and buggered off into the wild blue a bit unsure of actual weights. Undesirable of course, but no other option really.