What is the spread on the sensor?
That I do not know, I suppose I could download the datasheet.
I have a spare expansion tank and have knocked up a stand for it, and was going to do some tests. See how far from the tank the sensor works. I am more than sure it reads what is directly in front of it. The sensor sits below the seam on the tank and trigger at 5mm below the low level line, this looks to me like a perfect position, more luck than judgement. I have made some clips to hold it in place, but will add some sikoflex to keep it secured.
The sensor is an ultrasonic type.
The trigger state of the sensor is normally open, so will be turned on when water is sensed. This I will invert with a relay. This will give the failsafe operation of the sensor.
If the sensor fails, the warning lamp will come on.
I am due to fit a new binnacle cover soon, so was going to install it all at the same time.
Will post up some pics of the sensor later when I have made sure it works how I intend.