Abstract The successful assessment of structural integrity by means of AE depends on the quality of the recorded AE data and on correct post-test data analysis. AE monitoring and signal analysis of tests on two large, spherical tanks were carried out using a 32-channel AE system (Spartan, from PAC, USA). Water pressurization was used on one, whilst the other was tested on-line. The effects of both the residual and loading stresses on the surface-opening cracks produced by multiple welding passes were assessed by means of AE. AE measurements generally take a rather long time and background noise cannot always be restricted during the test period (a major problem with on-line testing). It is possible to improve the detection of AE in the field by varying the pressure of the operating medium by 10% P ( P = operating pressure of tank). The relationship between the source (a crack growing with load changes) and its associated AE parameters is also discussed.