user4115: Do ultrasonic sensors sense through transparent materials?
In my case, I have to read if there's something inside a transparent acrylic bottle.

user2967: Ultrasonic sensors detect echoes of high frequency sounds bouncing off of a boundary between  air and some other substance. 
As others have pointed out, the light transmission properties of an object (transparency to light) has nothing to do with its transparency to ultrasonic sound waves.
It's possible to project ultrasonic sounds through solids or liquids, but that's not what these sensors are designed to do.
Trying to send ultrasonic pulses through an acrylic bottle will no doubt mess up the signals. Try pointing one of these sensors at an acrylic bottle in a controlled environment (fixed distance, mounted firmly and the same way each time, a solid back-stop at a fairly large, fixed distance behind the bottle to avoid varying echoes from behind the bottle) and check the distance measurement with the bottle full and empty. If you detect a difference in readings, then use those 2 different distance values for full and empty. (But make sure your production environment is the same as your test environment, including the area around and behind the bottle you're measuring)

user56781: Wanting to use HC-SR04 to measure water level, but protect the SR04 from moisture I tried covering with cling film, the most minimal film I could find. No - the sensor sees it and reports 1cm distance.  So these sensors have to be left exposed - probably won't last long at top of my water tank.

user37: I'm assuming you mean a "normal" sensor, like this:

Ultrasonic sensors work by sending out sound waves. Obviously, sound waves don't care whether a material is transparent or opaque, so they are blocked just as easily by acrylic as by a brick wall.
Side note: If you look on the front of an ultrasonic sensor, you will notice that it is covered by a screen, not glass or plastic. This is why.

4115: <p>Do ultrasonic sensors sense through transparent materials?</p>

<p>In my case, I have to read if there's something inside a transparent acrylic bottle.</p>
 2967: <p>Ultrasonic sensors detect echoes of high frequency sounds bouncing off of a boundary between  air and some other substance. </p>

<p>As others have pointed out, the light transmission properties of an object (transparency to light) has nothing to do with its transparency to ultrasonic sound waves.</p>

<p>It's possible to project ultrasonic sounds through solids or liquids, but that's not what these sensors are designed to do.</p>

<p>Trying to send ultrasonic pulses through an acrylic bottle will no doubt mess up the signals. Try pointing one of these sensors at an acrylic bottle in a controlled environment (fixed distance, mounted firmly and the same way each time, a solid back-stop at a fairly large, fixed distance behind the bottle to avoid varying echoes from behind the bottle) and check the distance measurement with the bottle full and empty. If you detect a difference in readings, then use those 2 different distance values for full and empty. (But make sure your production environment is the same as your test environment, including the area around and behind the bottle you're measuring)</p>
 56781: <p>Wanting to use HC-SR04 to measure water level, but protect the SR04 from moisture I tried covering with cling film, the most minimal film I could find. No - the sensor sees it and reports 1cm distance.  So these sensors have to be left exposed - probably won't last long at top of my water tank.</p>
 37: <p>I'm assuming you mean a "normal" sensor, like this:</p>

<p><img src="https://i.stack.imgur.com/TaZjQm.jpg" alt="enter image description here"></p>

<p>Ultrasonic sensors work by sending out sound waves. Obviously, sound waves don't care whether a material is transparent or opaque, so they are blocked just as easily by acrylic as by a brick wall.</p>

<p>Side note: If you look on the front of an ultrasonic sensor, you will notice that it is covered by a screen, not glass or plastic. This is why.</p>
