These are very cool water level sensors. Simply stick them outside a non-metal container or tank and it will switch with a signal to a microcontroller. If you supply 5V from, let's say an Arduino Uno, it will output 5V to a digital pin so you can do your magick in your coding like starting a water pump. If you use two you can set a low and high water level to start a pump when the low water level is reached and stop the pump when the high water level is reached.
An easy to install and use water level sensor. Of cause, it will work on other liquids other than water too.
Input voltage: (InVCC) DC 5 ~ 12 V
Type: NPN
Current consumption: 5mA
Current consumption: 5mA
Output voltage(High level): InVCC
Output voltage (Low level): 0V
Output current: 1 ~ 50mA
Response time: 500mS
Working temperature: 0 ~ 100 degree
Induction thickness (Sensitivity) Range: 0 ~ 20mm
Cable length : 500mm
Communication: RS485
Humidity: 5% to 100%
Material: ABS
Waterproof: IP67