Epson USN 52R, USN 52L manual Assessing flaws using ultrasonics, Effect of temperature variations

Introduction Important notes

Effect of temperature variations

The velocity of sound within the test object also varies as a function of the temperature of the material. This can cause appreciable errors if the instrument has been calibrated on a cold calibration block and is then used on a warm or hot test object. Such errors can be avoided either by warming the calibration block to the same temperature before calibrating, or by using a cor- rection factor obtained from tables.

Measurement of remaining wall thickness

The measurement of the remaining wall thickness on plant components such as pipes, tanks and reaction vessels of all types which have been corroded or eroded from the inside requires a suitable test instru- ment and special care in handling the probe.

The test operator must be told the nominal wall thick- nesses and the likely amount of the loss due to erosion or corrosion.

Assessing flaws using ultrasonics

In present-day test practice there are basically two methods of assessing flaws.

If the diameter of the sound beam is smaller than the extent of the flaw, then the beam can be used to scan the boundaries of the flaw and thus determine its area.

If, however, the diameter of the sound beam is greater than the size of the flaw, the maximum echo response from the flaw must be compared with the maximum echo response from an artificial flaw at the same depth provided for comparison purposes.

Flaw boundary scanning method

The smaller the diameter of the sound beam from the probe, the more accurately the determined flaw area corresponds to the actual flaw area when scanning the flaw boundaries with the sound beam from a probe.

If, however, the sound beam is relatively broad, the determined flaw area can substantially differ from the actual flaw area. Care should therefore be taken to se- lect a probe which will give a sufficiently narrow beam at the position of the flaw.

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