ultrasonic distance sensor - wheaterproof?

Hi all,
I want to measure the level in my water cistern, that means the air is almost saturated with humidity. Does a ultrasonic sensor exist, which can tolerate these conditions? Of course I can put a sensor in a tight plastic case, but that would block the signals I assume. Do you have any ideas?
Richard

You better look for other ways to measure it.
Can you measure the pressure at a tube outside the cistern ? You could even measure it at a lower level, and recalculate what water level causes which pressure.

Does a ultrasonic sensor exist, which can tolerate these conditions?

Yes; my car has eight of them