Actually I wil be doing a bachelors degree in process chemistry this spring, so those are excactly the kind of problems I like :)
I have a mega myself.
Most common with flow rates is mesuring diffrential pressure drop when the fluid passes through an Orifice in the pipe, so you use a DP-cell to do this
Using an absolute pressure sensor you get the 1atm value
Specific gravity is mesured with pressure of a known volume:
you have a known volume or more specific a height of liquid over the sensor. So the pressure at the depth corresponds to weight at a specific volume + 1 atm, this is pressure on a cell minus the atmospheric pressure. This type of sensor is also a DP-sensor or dp-cell :)
So to sum it up:
You mesure the spesific gravity by having a dp sensor at a known depth of the liquid you can get spesific gravity. Or if you use another method to mesure the fluid level like capacitive sensing rod, sonar, or whatever then you combine that with the dp-sensor at the bottom of the tank to get specific gravity
You have a diffrential pressure sensor at the bottom of the tank, probably just before the oulet valve, this will give you the height of the liquid when combined with specific gravity.
Make a thin stainless steel box for your temperature sensor where the sensing elements touch the wall of the box, and submerge in the tank