b. Use the ratio
when one area of the test part is accessible for direct measurement.
the thickness of an area of the test part as measured by mechanical or optical means
the thickness of the same area as indicated by the ultrasonic instrument calibrated on material
similar to the test part
Assume an area of a test part is measured with a micrometer and is 0.167 inch thick (d2 =
0.167 inch). This same area is measured with ultrasonic instrument and gives a reading of 0.133
inch (d1 = 0.133 inch).
the correction factor.
d. All ultrasonic readings on the test part are now multiplied by 1.25 to obtain the actual thickness. If
another area of the test part gives an ultrasonic reading of 0.200 inch, correct this by multiplying by the
0.200 inch x 1.25 = 0.250 inch = the actual test part thickness.
Flat surfaced reference standards may be used for measurements on convex radii of curvature as small as one inch and
concave radii of curvature as small as three inches. Test parts with radii smaller than one inch convex or three inches
concave, require reference standards with curved surfaces with radii equal to the test part radii, ±10%. In addition,
shoes are required (see paragraphs 188.8.131.52.2 and 184.108.40.206).
The surface finish of reference standards should be 63 microinches or better if maximum accuracy is to be obtained.
Surface roughness introduces errors as shown Table 5-7.
The thickness of reference standards shall be measured by mechanical or optical means. The maximum tolerance for
these measurements shall be one-half the tolerance permitted for the ultrasonic thickness measurements on the test part.
For example, assume a test part thickness will be measured within ±0.005 inch. The reference standards must then be
measured within ±0.0025 inch. If the test part measurement tolerance is not specified, the reference standards should
be measured within ±0.0005 inch or one-half the instrument tolerance, whichever is greater.
Table 5-7. Measurement Error Introduced by Surface Roughness of Reference
Standard or Test Part.
Surface Finish (microinches)
Measurement Error (inch)