Small, hermetically sealed objects are often leak tested after subjecting the finished device to an environment of tracer gas under high pressure. Parts with very large leaks are often missed during testing because of very fast signal decay, necessitating employment of an auxiliary gross leak detection technique, such as one which discriminates normal and-abnormal pressure decay during evacuation. Such techniques require careful consideration of variables, including the presence of water vapor in the system, the condition of the mechanical vacuum pump, and electronics response time lags, and are not always precisely predictable in operating environments. However, if a large leak discrimination system is designed to operate at test pressures close to atmospheric, and if a gas expansion method is applied comparing the volume of the test object to a fixed volume, such variables are eliminated, yielding a highly reliable and rapid large leak discrimination method. Combining such a method within a helium mass spectrometer leak detector (HMSLD) results in a system capable of detecting leaks from very large leaks down to the limits of HMSLD testing.