Quote Originally Posted by Joe S. View Post
.. . Here is an optical method. Water is a pretty good difuser of light, way better than air, if you make up a vertical pipe with clear lucite pins penetrating the pipe and covered on the pipes inside with black tubing so light will not penetrate from their sides, when they are covered with water the light should be able to enter through the ends, or just use fiber optics.
Looks good. In fact, I guess one could use IR detectors directly in place of the lucite pins, of course covered with black tubing so that only a small aperture at the tip is open.

Need to try this approach.