Electronic Sight Glass

A capacitive sensor can sometimes distinguish between liquid and gas through a plastic or glass tube. The ones with an adjustable sensitivity are best.

Do a Google search for [capacitive proximity sensors].

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Some more:

all i can think of is a capacitive detector. that of course may not work if the fluid is erratic, i guess you could also detect for sudden changes. if its a glass tube that should do it. then there is the optical method, but that involves a complex twisted alignment . induction is also another possible... bubbles can cause electrical property changes that can be used different ways. also theirs the system we use where we have a Sonic Transducer on a glass rod we in a boiler system to detect levels and bubbles. we have it set so the receiver will pick up its own signal if there is bubbles or lack of water.. this works because the water acts as a cushion to not allow the sonic to reach the other end. the more dense the water is the less the tube vibrates. of course this takes place on the high end of the sonic spectrum. there are many possible ways, i think they already make devices that you can attached that are of a capcaitive method.

Thanks for the input, actually the tube carrying the liquid will probably be copper and possibly stainless steel. under pressures of 125 to 275 psi. There is a device on the market that has two sensors that attach to the tube about 3 inches apart and the bubbles are detected and indicated as an audible noise.

depending on bubble size, an optical method could work. add a light source and an off-axis detector to pick up the light scattered by the bubbles.