Microballoons are tiny, hollow, thin-walled, glass spheres ranging in diameter from ~ 1 to 160 μm. They are commercially available in bulk for use as low cost, lightweight filler to be mixed with resins and other materials. As reported last year, the unique properties of microballoons have been exploited in a number of instances for recording the maximum of a pressure excursion [1]. To understand the technique, consider first what happens when microballoons are mixed in a carrier fluid, placed on the surface of an Acoustic Emission (AE) transducer and then subjected to a pneumatic pressure. Since the microballoons within a typical sample exhibit a random distribution of rupture strengths, the weakest burst first as the pressure increases. As the microballoons break they give rise to AE events which are easily detected by the transducer. A typical plot of the number of events versus pressure is presented in Fig. 1. If the mixture had been pre-pressurized to Po, little AE activity would have occurred on the ramp up to Po (the weaker balloons having already burst), followed by an abrupt onset in AE. This manifestation of the Kaiser Effect [2] is evident in the data of Fig. 2, where Po = 100 psi. In this manner, Po, the maximum pressure to which the mixture has been subjected, can be determined by detecting the abrupt onset of AE events upon repressurization of the mixture. A mixture, such as grease and microballoons, for instance, can therefore be placed at strategic locations on a structure to be “read” at a future date to determine the maximum pressure to which each location had been subjected.