AVR-Chat

All of these are good ideas.  When thinking about this, I ruled out 
weight becasue you never said it would be the same liquid each time.  
If so, weight is the best (easiest) option in my opinion.  

Camera and IR are good ideas, however sensitive to environmental 
conditions.  Is this bottle always going to be at the same place, and 
are the conditions always the same during the measurement?  I am 
assuming that you want to do this without human interaction, because 
that would be 

What about having a second container of liquid, putting the plastic 
bottle on a sled that has a linear encoder on it (could be as simple as 
black lines on a white background) and reading the encoder with an 
enclosed IR sensor as the plastic bottle floats in the other liquid.  
The less empty the bottle, the higher it will float?  This is more of a 
mechanical idea I guess.