Originally Posted by alien
A coloured float in the sight glass might help if you go this route.
If one was willing to use a float, you could wrap IR reflective tape (or paint) around a cylindrical float and use a Wiimote to calculate the position change:http://arstechnica.com/science/2010/12/scientists-wield-wiimote-to-measure-evaporation/http://onlinelibrary.wiley.com/doi/10.1029/2010WR009350/abstract
Wiimotes are pretty easy to interface with. The camera outputs the position the top 4 brightest IR points at 1024x768 resolution. Provided the heat source doesn't overpower the sensor, it might be possible to get the sight glasses of three keggles in a single level arrangement covered by one wiimote.
That might be just as cheap and easier than the OpenCV route.... hmmm....