Aktek AT300 Panduan Pengguna - Halaman 15

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Appendix B Applications Notes

Measuring pipe and tubing.

When measuring a piece of pipe to determine the thickness of the pipe wall, orientation

of the transducers is important. If the diameter of the pipe is larger than approximately 4

inches, measurements should be made with the transducer oriented so that the gap in the

wearface is perpendicular (at right angle) to the long axis of the pipe. For smaller pipe

diameters, two measurements should be performed, one with the wearface gap

perpendicular, another with the gap parallel to the long axis of the pipe. The smaller of the

two displayed values should then be taken as the thickness at that point.

Measuring hot surfaces

The velocity of sound through a substance is dependant upon its temperature. As

materials heat up, the velocity of sound through them decreases. In most applications with

surface temperatures less than about 100℃, no special procedures must be observed. At

temperatures above this point, the change in sound velocity of the material being measured

starts to have a noticeable effect upon ultrasonic measurement. At such elevated

temperatures, it is recommended that the user perform a calibration procedure on a sample

piece of known thickness, which is at or near the temperature of the material to be

measured. This will allow the gauge to correctly calculate the velocity of sound through the

hot material.

When performing measurements on hot surfaces, it may also be necessary to use a

specially constructed high-temperature transducer. These transducers are built using

materials which can withstand high temperatures. Even so, it is recommended that the

probe be left in contact with the surface for as short a time as needed to acquire a stable

measurement. While the transducer is in contact with a hot surface, it will begin to heat up,

and through thermal expansion and other effects, may begin to adversely affect the

accuracy of measurements.

Measuring laminated materials.

Laminated materials are unique in that their density (and therefore sound-velocity) may

vary considerably from one piece to another. Some laminated materials may even exhibit

noticeable changes in sound-velocity across a single surface. The only way to reliably

measure such materials is by performing a calibration procedure on a sample piece of

known thickness. Ideally, this sample material should be a part of the same piece being

measured, or at least from the same lamination batch. By calibrating to each test piece

individually, the effects of variation of sound-velocity will be minimized.

An additional important consideration when measuring laminates, is that any included

air gaps or pockets will cause an early reflection of the ultrasound beam. This effect will be

noticed as a sudden decrease in thickness in an otherwise regular surface. While this may