Abstract

The traditional method of thickness gauging using a single time of flight path and the first echo received (known as 'single echo' method), precludes reliable and safe measurements on the materials used widely in today's industry and engineering.

In my paper 1 would like to point out the problems of the single echo method. These problems caused many well known companies to ban the use of single echo gauges in their territory. One main disadvantage is, that the coatings are measured with the metal together, causing large errors. For this reason, the manuals of these gauges ask the user to remove coatings before measurement. Removing and restoring the coatings needs time as well as material and tools to do this. Taking into account the extra time and material used, the operation proves to be slow and expensive for the user. Since the single echo method has no built-in error check, the reading on the display may be erranous, but in an industrial environment it is often impossible to make sure of it. An incorrect reading may cause false alarms and expensive testing, while missing to detect a corrosive damage in time can have even more serious consequences.

Using the so-called 'multiple echo' method, it is possible to measure through the coating, while the method provides a built-in safety check, ensuring that the result on the display is the correct one. The use of the instrument is very simple, training of operators is minimum. The inspection time and cost is greatly reduced.

These advantages of the method are confirmed in Lloyd's Register Approval of Thickness Measurement of Hull Structure, which states in item 2.4.1. (Grade 1 Approval), (iii) (equipment): Firms must possess equipment that is capable of giving accurate readings of plate thickness whether or not the measurements have been taken through protective coating. This means that the equipment must use or be capable of using the multiple echo technique in order that the protective coatings are discounted from the displayed readings. 'A scan' flow detectors, multiple echo direct reading gauges or a combination of both may be used."

Keywords: Ultrasonic Testing, Multiple Echo Method

Source: Conference Proceedings of the 4th International Conference of the Slovenian Society for Nondestructive Testing, 24 - 25 April 1997, Ljubljana, Slovenia.
Pages:389

Publisher:
Slovenian Society for Nondestructive Testing, Faculty of Mechanical Engineering, Askerceva 6, SLO 1000 Ljubljana, Slovenia. Tel +386 61 177 12 03, Fax +386 61 218 567, Email: janez.grum@fs.uni-lj.si.