The most accurate maturement of liquid level I have dealt with in the Petro 
Chemical Industry is the long wire float method, where a float is guided on 
one or two vertical wires which run from the top to the bottom of the tank.

The float then either has a flexible wire of some sort  which runs from the 
float up to a spool on the top of the tank so the height of the liquid is converted 
into a rotational position which can be read using a potentiometer. (some
gearing may be used.) (This can also be done digitally.)

The top of the float may also be used as a reflector and a simple beam of 
light or Infra Red or laser etc can be reflected of the top of the float giving a 
very accurate reading once the height of the float above the liquid is subtracted.

If the whole thing is put into a 'still tube' the effect of liquid sloshing around is eliminated.

This method enables you to very accurately read liquid levels in tanks tens of 
meters high containing just about anything.

MikeW