Fountain has a depth of about 2.5 meters, however it is never full of water - there is always up to 80 cm of space under fountain’s metal cover. I’m thinking about ultrasonic sensor, however I’m worry about signal reflections (firstly from the water after that from the plate and then again from the water to the sensor…), so it is not sure that ultrasound sensor would work. Does somebody have any experiences?
What do you want to do with the sensor?
Keep the fountain at a certain level?
If so then use a float switch to indicate when the water is not at the correct level and turn on a pump until the switch indicates the level is back to normal.
The simplest would be float switch as @TomGeorge suggested.
Another approach would be to use a pressure sensor. I use that to determine the depth of my coy pond. It is in the bubbler line that goes to the bottom of the pond. You could do something similar by using a small aquarium pump and only turn it on when you want a reading.
@J-M-L Which model of ultrasonic sensor do you have? Is A02YYUW maybe?
@TomGeorge The fountatin collects rainwater. This water will be used for the irrigation system and I would like to monitor how quickly it will run out, especially in the summer during longer periods of dry weather.
@gilshultz Do you have a sensor like the WNK8010-T in mind? I’m not sure if I understood what did you think that I can achieve with a small pump?
@kmin I meant that multiple reflections could be a problem, first reflection is a must to meausure anything But you are right. If I mount a sensor 10 cm under the cover, than there would be around 70 cm to the water when fountain would be full. Additional reflections would bring much longer distances which should be eliminated
Just a tip if using float type level switches.
The small ones with a hinged magnet have an M20 thread and nut for fixing through tank sides.
Get some 20-mm PVC conduit and a female M20 adapter. Glue the adapter and screw the switch into the adapter with a rubber sealing washer.
Extend the wires with small round cable.
Cut the conduit to length and fit another M20 on the open end. Fit an IP68 cable gland to seal the cable.
End result a level switch easy to fit at the right level and service.
But you are right. If I mount a sensor 10 cm under the cover, than there would be around 70 cm to the water when fountain would be full. Additional reflections would bring much longer distances which should be eliminated 
