The compensator and the densitometer are not the same thing. The compensator, which sits at the bottom of the tank and is always covered with fuel, is effectively an analogue device which injects a signal (in opposition) into the fuel quantity RC bridge. the denser the fuel, the more current it allows to pass which is used to offset the fuel quantity indication and compensate for volumetric variations due to fuel temperature (and therefore density) giving a reasonably accurate indication of available fuel mass (energy available from fuel is a value according to its mass not its volume). The densitometer is an electronic device, used in digital FQIS systems, which (far more accurately) adjusts the FQIS indication to provide an accurate indication of fuel quantity. ols analogue systems were maybe accurate to 5% and the digital ones with densitometers to 1-2%.