Carlo Gavazzi CD50CNF Series Capacitive Flat-Style Proximity Sensor, 10 mm Detection, NPN Output, 10 → 30 V dc,

The Carlo Gavazzi Capacitive proximity sensor is a 3 wire DC sensor with open collector NPN or PNP transistor output. The sensor is available in a 5 VDC version and a 10 to 30 VDC version. The sensing distance is fixed and works through a non metallic wall.

For water storage tanks
IP67 degree of protection
Basic capacitive water level sensor
Detection of excess water in the whirlpool tub