I'm not testing wort but i am having that problem. I only need it to stay calibrated for 8 hrs at most. I read somewhere, perhaps this thread, that injecting air till it fills the tube again is a solution. I thought an aquarium pump and a check valve might do the trick.
I've been having good results measuring changes in specific gravity using a mpx7002dp with a tube immersed into the liquid and calibrated with a bulb.
here is some preliminary code.
https://github.com/rayvenwalker/mysensors-distillation/tree/master/SpecificGravity
