Abstract: A self propelled device which travels along a reformer tube or other tube to detect variations in the tube diameter that may indicate impending failure of the tube. A two piece frame opens and closes about a hinge axis to allow application of the device to a tube and removal from the tube. When closed, wheels on the frame are held against the tube by a spring latch which allows the wheels to move in and out radially when diameter changes are encountered. One set of wheels is driven by an electric motor to propel the device along the tube. The diameter changes are sensed and measured by a linear variable differential transformer which monitors the gap between the ends of the frame sections. A chart recorder records the tube diameter measured at different positions along the tube length.