It is possible that sonar would work though I have never tried it. IR definitely doesn't. I've had some success with using a touch sensor to detect when the water hits a certain height but that's really only useful for one particular depth.
I will test out a sonar sensor today and see how well that works.
In general Phidgets does not have any sort of sensor that is well suited for this.
burley wrote:It is possible that sonar would work though I have never tried it.... ...I will test out a sonar sensor today and see how well that works.
Did this ever get tested? I also want to measure liquid in a tank. But, in my case the liquid is fuel oil and the only access I have to the (buried) tank is from above - and the 1128 Sonar Sensor would be perfect for this application.