@chosken, welcome. Sounds interesting. I need such a tool today, as I'm on stage in the open, for four hours, at an arts festival, where I'm to be the token expert in the corner. It will be hot.
For mass measurement use a load cell and an instrumentation amplifier. For example;
Mass measurement could work, but it would be hard to compensate and adjust for all the variables. The water container would have to be suspended so that the mass falls entirely on the sensor. If any mass is suspended by something else, the result will be different. Movement would have to be filtered out in software.
I agree with @VK7HSE, there are other ways to solve this. Here are some suggestions;
- a water flow sensor on the outlet,
- a capacitive sensor on the outside of the bottle,
- a resistive sensor on the inside of the bottle (though as it is in contact with the water it has to be certified for that use; human safety and health),
- infrared reflectance level sensors, every 100ml distance within the bottle,
- a pair of accelerometers to compare the response to movement of the bottle in relation to the rest of the assembly, assuming it is for someone who is moving,
- a tiny speaker and microphone at the top of the bottle, with software to slide a tone to find the resonant frequency of the air above the water.
Hope that helps!