Your assumption is way of the mark. its an steel beer keg, which has no opening on the top and bottom..Item #2 makes a great deal of difference! My original understanding was that the transducer was looking down at the surface of the water in the metal container. Now I understand that is not the case, that the signal must first drive the material of the container which in turn drives the air above the surface of the water, and then the return echo must vibrate the container material thus driving the detector.That is not how industrial ultrasonic level detectors work. The probe on those devices penetrates the top of the container so that there is nothing in the way of the ultrasonic energy. While in theory it is possible to make the system as desired provide adequate results, it is not reasonable to expect it to happen without a whole lot more effort and expense. So I suggest an alternate approach if there is not a way to give the probe a direct view of the water surface. That would be to use an insulated capacitive probe that enters the container through the same top opening, presuming that there is such an opening. It could also enter through a bottom opening if that were the only one available. The probe would sense the level of the water as a difference in capacitance between the insulated probe and the conductive container material, which the capacitance would affect the frequency of an oscillator. measuring the frequency of the oscillator is straightforward and easily done, and would provide a high resolution analog of the fluid height in the container..
AND, from the description of the container size, it may be the water tank of a water pump well system installed for home use. At that point I offer a caution, which is that those tanks include a diaphram to separate the water from the air, and so the challenge becomes different. One more reason for the TS to give all the details with the initial questions.
