Olympus 27MG, User Manual

Page: 69 / 98

DMTA-10043-01EN, Rev. C, July 2016
Application Notes 61
10. Application Notes
This chapter covers factors that can affect the performance and the accuracy of the
27MG Ultrasonic Thickness Gage, the selection of transducers with regard to the
minimum material thickness for valid measurements, and special considerations
when measuring corrosion at elevated temperatures.
10.1 Factors Affecting Performance and Accuracy
The following factors can affect the performance and the accuracy of the 27MG
Ultrasonic Thickness Gage:
• Surface Condition
Severe pitting on the outside surface of a pipe or tank can be a problem. On some
rough surfaces, the use of a gel or grease rather than a liquid couplant will help
transmit sound energy into the test piece. In extreme cases, it will be necessary to
file or grind the surface sufficiently flat to permit contact with the face of the
transducer. In applications where deep pitting occurs on the outside of a pipe or
tank it is usually necessary to measure remaining metal thickness from the base of
the pits to the inside wall. The conventional technique is to measure unpitted
metal thickness ultrasonically, measure pit depth mechanically, and subtract the
pit depth from the measured wall thickness. Alternately, one can file or grind the
surface down to the base of the pits and measure normally. As with any difficult
application, experimentation with actual product samples is the best way to
determine the limits of a particular gage/transducer combination on a given
surface.
• Transducer Positioning/Alignment
For proper sound coupling, the transducer must be pressed firmly against the test
surface. On small diameter cylindrical surfaces such as pipes, hold the transducer