What is this proximity sensor?

I need a water/weather proof proximity sensor and sensors in this kit is what I need (round shape things below the display)

I want to use them with arduino but I've no idea what type of sensor are these. Is there anyone knows what are these sensors or how to use with arduino?

Thanks.

Ultrasonic proximity sensors - each transducer is pinged to send out a pulse of ultrasound and then switched to listening for the echo.

mtD, Did you find that on the web? Could you send me a link or a name and model no.?

http://velocity.net.in/

what are you building exactly? if you are thinking of building a robo that walks on your pool floor, that'll narrow it down :D

try some of these - http://www.robotshop.com/senix-tspc-21s-waterproof-ultrasonic-sensor-1.htmland one morehttp://www.robotshop.com/senix-tspc-30s1-waterproof-ultrasonic-sensor-1.html