Abstract: This invention relates to an apparatus and process for detecting gas permeability in a main rupture disc which is in gaseous communication with a vessel. The process comprises: fitting a chamber over and in gas-tight association with the outside surface of the main rupture disc; selecting a burst presure for the main rupture disc and a set pressure; obtaining a gaseous pressure within the chamber which is greater than the normal operating pressure within the vessel, and equal to the difference between the set pressure and the burst pressure of the main rupture disc; and monitoring the chamber pressure for a decrease thereof. The apparatus comprises, generally, a chamber, a means for obtaining a gaseous pressure within the chamber, and a means for monitoring a change in pressure in the chamber.