sounds like youre attempting to assert composition of a solution based on buoyancy. its not the worst plan ever, but your hydrometer isn't going to have the same precision as an instrument designed to give you compositional analysis. in a sealed vessle you could use differential pressure monitoring to get a level, or ultrasonic detection tuned to the vapor composition (sound waves through vapor) to derive density princely the same as an acoustical wave sensors. you could shine a laser through your vessel or a glass tube then use the fluid's refractive index. The refractive index, which is related to a substance's optical density, can then be used to calculate the physical mass density.
** lots of assertions in these sorts of assessments, that one is measuring a specific thing, and not something functionally similar. personally for the application I'd think your be more interested in specific gravity.
