[en] The development of the UKAEA acoustic emission monitoring system has continued, directed towards its use to prevent pressure vessel failure in materials of general engineering practice. The acoustic facility, SWEL, can locate sources on large vessels to +- 3 inches and has a two-dimensional display, on line, of the emission sources. Measurements have been made of the location of artifical and natural defects in two experimental ductile steel vessels and on the US EBOR test vessel. Defects of a brittle nature are shown to emit strongly and can be readily detected. Defects in parent plate are shown to be quiet when significant crack growth is occurring, in accord with results from metallurgical specimens. No signals are detected during periods of yielding around defects in vessels in spite of indications from specimens that low level emission occurs. (author)