Stream does not (or should not) be seen by the sensor. I am more worried about debugging the sensor for the temp change and the nonreflective surface of the foam. Haven't gotten it yet...
I have used ultrasonic for many years for measuring levels of various materials. Most are not affected by dust or turbulance. I would think that foam would not be an issue but if your using it to measure level in HLT or MLT, foam shouldnt really be an issue. Im in the process of puting mine together with a Micrologix 1500 PLC and plan on using ultrasonic for the HLT. Plus this is just my 2¢.