This question was posted earlier. Flexim ultrasonic transducers are IP 68 and work perfectly under water. A trial to test the performance was conducted at a water treatment plant on the raw water filter bed.
The directive was to ensure that NO coupling fluid, adhesive, ultrasonic paste, gel or any chemical was used- instead Flexim Viton coupling pads are utilised to ensure a lifelong maintenance free acoustic coupling.
Raw water is supplied through GRP pipes – 550 mm OD -Wall thickness 23 mm which are permanently immersed in the tanks. The first of 6 meters was installed 3 years ago and have operated flawlessly.