I am looking for a way to determine the low level in a tank truck. Something along the lines of a capacitive prox switch that can be attached to the side of the tanker by the driver. He then starts starts the unload process and the switch would shut off the system. I am assuming the tanker would be metal, the product inside has a density a little more than water. We are also using nitrogen to push the product out of the truck and into the holding tank.