uses a thin plastic tube to establish a vertical column of air inside the tank. From the panel you pump to remove all the liquid in the column then it relevels itself to measure the inches in the tank based on the air pressure difference in the tube. The air displaced exterts presure read from a guage. It's up to you to convert inches to the % full. On a regualr shaped tank it's simple but can be calibrated to any shape tank.
There are no wires or electrical
connections used on anything. You only have to route
the tubing from the meter to the tank. Anything else will require electrical
connections and electrcicity to read the guage.