Measuring Fuel in the Tank

The advantage of having little, or just boring stuff, to do is that one's mind wanders. Eg. Could I measure the fuel in my tank by the "note" is makes when I tap it with something hard? We all know an empty tank makes a different sound to a full one so if we had a way of measuring the frequency of the sound made by the tank at different capacities we'd know how much was in it.
I wanted to test the theory using this...

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But unfortunately the basin where "Emily" is moored has been "locked down" and I wont be able to get to her for the foreseeable future  

Are there any electronics experts out there who could come up with something better than a guitar tuner? 

In theory a sufficiently sensitive and "tuned" instrument could tell how many liters there were in the tank.