An insertion assembly (1) includes a frame (11), having a longitudinal passage (13) for the thermocouple (3); a device (15) for blocking the pipe (5) in position relative to the frame (11); a device (17) for guiding the thermocouple along the passage (13); and a device (19) for driving the thermocouple (3). The driving device (19) includes at least a first drive belt (21) having a first longitudinal segment (25) pressed against the thermocouple (3), and a motor member driving said first segment (25) of the first belt (21) longitudinally relative to the frame (11).