A previous question reveals that the Shuttle orbiter was weighed before it was moved to the Vehicle Assembly Building. Presumably, the other dry components of the launch stack (the solid rocket boosters and the empty external tank) were weighed as well before assembly.
However, the liquid oxygen and liquid hydrogen were pumped in at the pad. How were these amounts measured (e.g. the integral of the flow? a sensor inside the tank?), and more importantly, to what precision were they known?
