How Did Bomber Navigators Calculate the Weight of the Fuel?

In many books I've read about US bombers the navigator needed to take into consideration how much fuel he has on board, and it's always given as a weight in lbs (not volume). How was this weight determined? Was there some gauge, and if so, what kind of sensor was used for measurement?

Here's an example I recently read in which the weight of fuel was used. On a mission to drop bombs on Germany from Britain, a B24 was damaged. The navigator knew how much fuel (in lbs) he had in the tanks, his fuel consumption, his location, his speed, etc. He determined that he's have to ditch in the English Channel four miles shy of England, so they radioed ahead and a rescue boat was dispatched to the specified location. Sure enough, the plane was put down in the water +/- 1 mile from the estimated target and the boat picked up the crew without them having to even get wet. This illustrates how accurately the measurement of the fuel was. So, does anybody know how this was determined?

Thanks!

 

They know the altitude of base they fueled at. They also know how many gallons of fuel is pumped into the fuel tanks. Just simple math to convert the known volume to weight, accounting for the volume differences at the altitude it was pumped at. Essentially, all 8th and 9th AF bases were at or close to sea level so the weight was easily calculated.

 

OP, I'm sure U.S. pilot manuals and notes on specific types of aircraft were the same as RAF ones....they contained details on engine usage of fuel at various speeds and altitudes, and navigators would have been trained to carry out ongoing calculations - remembering that the aircraft itself was getting lighter and thus less fuel required to be burnt to move it along at a given velocity as the hours passed.