Self-compensation methodology for ultrasonic thickness gauges

There are many causes for the reduction of the thickness in pipelines, tanks and other mechanical structures. Corrosion, erosion, and abrasive wear cause degradation of mechanical structures and decrease their lifespan. These can be very slow processes that are difficult to track over time. Thickness gauging monitoring is commonly used as a way of preventive maintenance. The pulse-echo ultrasound can be a suitable technique to measure the thickness diminution in industrial facilities. Although ultrasound is considered a robust technique, in this particular application it presents two main difficulties: the mechanical stability of the assembly and the variation of the ultrasonic speed over time. Both mechanical assembly and acoustic propagation speed are strongly influenced by the temperature. In this paper, the implementation of a methodology that compensates for the temperature influences on the ultrasonic speed and the mechanical assembly is presented. The methodology can be applied in metallic structures to evaluate corrosion over long time periods. The temperature compensation data is obtained from the analysis of the ultrasonic signals. In this sense, the method can be called self-compensated. As initial data for the determination of thickness changes, the ultrasonic speed in the material at a reference temperature must be known. All results are evaluated at this temperature. An analysis of the uncertainty sources and limitations of the methodology is also included. To show the experimental application of the proposed technique, a rigid sample was designed in order to avoid mechanical instability. The results show that the methodology can compensate for the temperature, detecting a thickness reduction in the order of a few micrometers.