The ESTI cartridge is virtually the first safety device which can be tested quite simply for working efficiency without being destroyed. When the ESTI cartridge is heated slowly and carefully the contained fluid expands the air bubble becomes visibly smaller. As the liquid is heated so that the internal pressure increases, and if there were a crack in the glass, even hair-thin and invisible to the human eye, the liquid would leak at this point and the air bubble would expand. If no fluid leaks, the ESTI cartridge is in order.