Equipment and Materials.
Some instruments are designed specifically for thickness measurements. However, a basic ultrasonic instrument can
also be used for thickness measurements. Most instruments of both kinds have a digital display to provide the thickness
being measured. Detailed operating instructions shall be obtained by consulting the manual for the specific instrument
Search units recommended by the instrument manufacturer shall be used with the thickness measurement instruments.
A narrow dead zone and a good resolution are required for measurements in the lower ranges (thin materials).
Therefore, dual search units and/or search units with plastic delay lines are used (see paragraphs 188.8.131.52 and 184.108.40.206.).
Instruments dedicated to thickness measurements often are supplied with compatible search units. With a dual search
unit, the ringing of the transducer element is not detected by the instrument. Therefore, received signals close to the
initial pulse can be clearly resolved. A plastic delay line bonded to the face of the transducer element separates the
initial pulse from the front surface signal; this improves near-surface resolution (shortens the dead zone). For
measurements in the higher ranges (thick materials), a conventional straight beam contact search unit may be used.
Reference standards are required to calibrate the instruments prior to the inspection. This calibration is performed by
the instrument operator; calibration by laboratories is not generally required. For the thickness measurement
instruments, check the operator's manual to see if one or two reference standards are required. If two are required it is
best to have one 50-90% of the nominal thickness to be measured and one 110-150% of the nominal thickness to be
measured. Only one reference standard is required when using a basic pulsed instrument for thickness measurement.
Direct, accurate readings of thickness can be obtained only when the acoustic velocity in the reference standards is
equal to the acoustic velocity in the test part. For this reason, the material and heat treat condition of the reference
standards should be the same as the test part. If reference standards of a different material or heat treat condition is
used; the resultant thickness readings must be corrected by a correlation factor. The correlation factor may be
established in two ways:
Use the ratio
when the velocities of the test part and reference standard are known.
acoustic velocity in the test part material
acoustic velocity in the reference standard material
Assume the calibration blocks are made of 2014-T4 aluminum and the test part material is 410
longitudinal wave velocity in 410 stainless steel
2.91 x 105 inches/sec from Table 5-2.
longitudinal wave velocity in 2014-T4 aluminum
2.46 x 105 inches/sec from Table 5-2
29 1 1 0
the correction factor.
All readings on the test part are now multiplied by 1.18 to obtain the actual thickness.
If a test part reading is 0.110 inch, correct this by multiplying by the correction factor:
0.110 inch x 1.18 = 0.130 inch = the actual test part thickness