Are the bubbles composed only of CO2 (or mostly)?
If so - and this may not be the answer you want, but it might be cheap to do - since you have mentioned that you can see the bubbles - then a camera could as well.
Place a camera above the vat to observe the bubbles. Then apply some machine vision algorithms to it - perhaps using Python and OpenCV (or something similar - RoboRealm would be a good choice if you are using Windows); you'd have to do some edge detection, and some kind of contrast filtering to only get the bubbles and nothing else - then some kind of blob measurement (and isolation, maybe) - so you can determine when a bubble occurs, just how much gas was in the bubble (when it "pops"). Assume a bubble is a half-sphere with a diameter that of the bubble.
Not a perfect nor accurate solution - but it would be relatively cheap.