>Mike, I dont get this; I think I understand the setup you
>describe, but the glass tube, with or without water, would
>be transparent, right? how would you determine the presence
>of water at a certain point in the tube?

Your original post indicated you needed to check the water level at two
points, a low point to turn on a pump to start filling, and a high point to
turn off the pump. You will probably need a manual override of some sort
(probably just a push button) to start filling from empty.

You then need two sensors, one each at the two desired switching points. The
meniscus of the water will direct enough light away from the photodiode part
of the sensor to allow detection of the level. When the water is above or
below the sensor then the light transmission through the tube will allow
enough light to illuminate the sensor to detect that the meniscus is not
there. Your electronics will need to keep track of where the water level is
at, and probably require some safety feature to ensure that it cannot start
something dangerous without a check of water level after power failure.

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