A test plug assembly (10, 50, 90) includes a shaft (24, 56, 100) on which an annular elastic seal (22, 68, 94) and compression elements (32, 34, 62, 106, 110) are mounted. The compression elements (32, 34, 62, 106, 110) confront opposite axial sides of the annular seal (22, 68, 94) and are movable relative to one another for axially compressing and radially expanding the seal (22, 68, 94) into fluid-tight sealing engagement with an inner peripheral wall (38, 70, 96) of a pipe (16, 52, 92). A transducer (42, 80, 122) is mounted on the distal end of the shaft (24, 56, 100) and measures a condition within the pipe (16, 52, 92) at a location beyond the seal (22, 68, 94). As one example, the transducer (42, 80, 122) is a temperature compensation pressure transducer for obtaining pressure measurements within the downstream portion of the pipe (16, 52, 92) beyond the test plug (10, 50, 90). The transducer (42, 80, 122) produces electrical signals and is in communication with a control unit (982) located externally of the pipe (16, 52, 92). The control unit (982) converts the electronic signals of the transducer (42, 80, 122) for driving a display apparatus (983) to provide visible or audible information and/or warning signals concerning the measurements. The control unit (982) is responsive to transducer signals for automatically terminating welding of a pipe when the signals indicate that pressure at the distal end of the shaft (24, 56, 100) beyond the seal (22, 68, 94) is at a prescribed undesired level.