A02YYUW returns incorrrect data

Hi, 

Using the ultrasoon sensor to measure water level in een closed water tank. 

Filing the tank i get correct data. After a while the sensor starts to produce incorrect data. Either the distance is 0mm, or the distance is far beyond the depth of the tank. All with correct checksums btw. 

By drying the surface of the sensors, for a while normal, expected readings are produced. But then i have to dry the sensor surface again. This is not workable for me since in a closed tank i can not prevent thw sensor surface to become moistly. 

Is there a way to prevent these incorrect readings? If not, for me this sensor is useless. 

Any ideas?