Hi @emirhanuzum, welcome to the forum 
Our Ping Sonar requires a strong reflection to have confidence in its distance measurements - it’s best suited for detecting large surfaces (e.g. the sea floor, a wall, etc), rather than small obstacles like the cylinder you’ve described. It sounds like the Ping360 would be a more appropriate sensor for your application - it’s been designed for this kind of use-case.
Most likely it’s being detected as a weaker signal than the wall, so your options are either
- process the profile data directly to make your own distance estimate (instead of the sonar’s distance estimate)
- reduce the range so the pool wall is not able to be detected, in which case the sonar may register the cylinder as the most likely distance
The 30 degree beamwidth is a fixed property of the sonar’s transducer, and so can’t really be changed without changing the transducer itself, although it may be possible to physically restrict the sound path. Be aware that reducing the beam width would also reduce the directions where your object is detectable.
If using sonar is not vital, you may be interested in a computer-vision based approach