A method for detecting a fuel leak in a compartment is provided. The method deposits an insulator overlying a substrate and etches a trench in the insulator. A gap of the wound conductor of the coil body varies as the charging area of the case varies. A drive roller is spaced from the feed roller and from the plate for drawing the substrate from the spool on the feed roller over the plate edge to separate the sample chips from the substrate and apply the chips to the cards.