A02YYUW / SEN0311 Field of View / Beam Pattern

Hi,

I am working with a A02YYUW ultrasonic distance sensor - I am trying to use it to measure the level of a liquid inside a tank. So far, my measurement doesn't work. I believe I am getting reflections from the wall of the access port through which I am trying to do the measurement. I wonder if the field of view of the sensor is too large.

On https://wiki.dfrobot.com/_A02YYUW_Waterproof_Ultrasonic_Sensor_SKU_SEN0311, there are two different plots for the paperboard beam patters. Why are there two? Which one is correct?

Also, on a more general note, does anyone have an idea how this could be solved?

Thanks,

Patrick