As a student, a long time ago I did a course work on determining the level of liquid in a metal closed vessel.
I used the principle of determining the natural resonant frequency.
The knocker from the bell tapped on the barrel and then the frequency of the sound was measured.
It worked very well, I could determine about the difference of 200ml in a 50l barel.
But to ensure accuracy, you will need to additionally measure the temperature, of course.
For stepwise measurement, you can use the usual old way of determining the gas level in a ballon - tap your finger at different levels - the empty part will have a different sound effect.
This can be automated by installing multiple sensors.
These methods do not interfere with the requirements of safety and cleanliness, but in any case, installation of equipment will be required.
If you cannot approach the balloon with a screwdriver and soldering iron, then only an X-ray remains.