Flood & Freeze Sensor - Sump Pump Overflow

I purchased a Flood & Freeze Sensor specifially for my basement to monitor possibility of the sump pump overflowing. By design, the sensor would sit on the floor next to the sump pit. However, that will only detect water if the pit actually overflows. I was considering placing the sensor inside the pit, roughly 12" from the top. This way it would activate if the water was rising, but before it reached the top so (hopefully) something could be done to prevent the overflow. Note: I had a "float" in my last home, and it is precisely what it did last winter when my sump pump died. I was able to run to the store and install a new pump without having an overflow. My thought would be to rig up a small "shelf" that the sensor would sit flat upon. A couple of questions: 1) How does the sensor actually get activated? Is it by water submerging the bottom part of the sensor, thus closing the circuit of the small silver probes? Or based on humidity or even splashes (in which case my idea would likely not work)? 2) Has anyone tried this? How is it working out for you? Thanks in advance, John