A device relative to a delta indexing method for a hierarchy file storage including a from-end side file server and a back-end side file server is provided. Analysis of the spectrum of ultrasonic waves provides the operator with an indication of the size, thickness, location, dynamics of formation and quality of the spot weld. The arrangement further includes a spring-loaded mechanism disposed within the plug-receiving cavity. A multi-layer display is used having at least a coarse and fine layer with granularities corresponding to a time slot resolution.