I have a swamp cooler that does not turn off automatically when the water level evaporates below the safe level, and if I fail to turn it off even once I could damage it.
The sensor simply needs to know if it is A) covered in liquid or B) not covered in liquid. I am not interested in how much liquid is left in the swamp cooler.
I have heard that people have used sensors with the Arduino to measure the frequency of bubbles by using a sensor to detect liquid.
What options are available for the RPi?
The first result from Google is a eTape Liquid Level for $40 which can measure how much water remains in the swamp cooler. This would be interesting, but its $40 and I would be paying for a feature I do not need.
This project on Fritzing looks cool and very cheap. It requires the construction of a homemade sensor using copper. I was however hoping to purchase a sensor, but I'll default to this option if necessary.