CALIBRATION OF ULTRASONIC FLAW DETECTORS

The horizontal and vertical scales on a flaw detector display are only quantitative when they have been calibrated. The horizontal scale, more commonly known as the timeless, can be calibrated to give depth values for different materials and sound velocities. The vertical or amplitude scale can be calibrated to give information on defect size. The method normally employed to obtain quantitative information about a test piece is to compare the screen signals with those from specially machined blocks. These blocks are classified under two headings

Calibration Blocks

These are produced from material, of specified composition, and heat treatment and are machined to specified shape and surface finish. The calibration block may be a simple step wedge to allow the time base to be calibrated for accurate thickness measurement, or it may be the more complex 'M" International Institute of Welding Calibration Block which allows for calibration of time base, plus determination of probe parameters.

Reference Blocks

These are produced from the same material and to the same geometric form as the object to be inspected. Typically a reference block is made up so the ultrasonic operators can gain familiarity with sectional changes and the standard display patterns so formed. The reference block may also contain artificial defects from which the gain to be used in the actual inspection can be determined.

CALIBRATION BLOCKS

Three of the most widely sad calibration blocks are

The International Institute of Welding IIW-V1" Block

The block is machined from steel but has a perspex insert at one end. Because the velocity of sound is less in perspex, the time taken for an ultrasonic pulse to travel through the Insert is greater than for the same thickness of teal. The Perspex insert is machined to such a thickness that the pulse travel rim, is the awe as for 50= of steel. The insert can therefore be used as a 50= steel calibration block.

The "VP' block can be used for each of the following assessments: -

a Calibration of the time base in terms of thickness.

b Assessment of the dead zone.

c Checking linearity of time base.

d Checking linearity of the amplifier's gain.

e Assessing overall sensitivity of probe and amplifier.

f Checking resolution.

g Determination of beam characteristic

V2- Block

This is a more compact form of the "VI' block, suitable for site use, although somewhat less versatile in its functions.

CALIBRATION BLOCKS

Institute of Welding (IOW) Beam Profile Calibration Block

The IOW block is designed primarily for hem profile measurement. Beam profile is the variation in intensity of the ultrasonic beam for various beam lengths.

IOW BLOCK