This article was prepared with the feeling that corrosion engineers may use the instruments described to minimize loss of operating time normally required for inspection purposes. In a few cases it may be possible to make all necessary measurements while a unit is in operation, and avoid shut-down until loss of metal necessitates parts replacement. In a still larger number of cases it may be possible to appreciably reduce shut-down time by increasing the speed of inspection. These are possibilities for saving money, and are attractive to industry1 . However, they require a judicious choice of instruments because there is no all-purpose instrument for the many inspection problems found in industry today. Most instruments have definite fields of application and definite limitations, and these are usually due to the physical principles employed.
Thickness gages have been built that utilize the following physical phenomena. Ultrasonic vibration, thermal conductivity, magnetism, electrical...
