Not sure of the brand you wish to buy/install. The external tank monitor that I installed had two foil strips that are placed about 3 inches apart on an external side of the tank. There is a cube with wires that connects to the foil strips and provides the signal to the display. The foil strips are cut to length and there was way more length than I needed for my installation. Once installed and powered up you calibrate the instrument to read empty and full. I would guess than most external sensor types are similar.