Abstract: A distance measuring tool is provided for measuring the length of a drill pipe maintained under tension within a well. The tool includes a unitary instrumentation and instrument drive module which is sealed within an instrument housing and which may be removed and replaced as a single unit when malfunction occurs. The moving parts of the module are sealed in lubricating fluid, and a pressure equalization system is provided to equalize the pressure of the lubricating fluid relative to the external drilling mud pressure. The instrumentation and instrument drive module includes a mechanical distance counter and a drive mechanism which drives the counter only when the tool is descending into a drill pipe. The instrumentation within the tool is protected against impact by a shock absorbing nose assembly which includes both mechanical spring and hydraulic shock absorbing units.