Significant errors in ultrasonic thickness measurement can occur if:

Significant errors in ultrasonic thickness measurement can occur if:

• A. The test frequency is varying at a constant rate
• B. The velocity of propagation deviates substantially from an assumed constant value for a given material
• C. Water is employed as a couplant between the transducer and the part being measured
• D. None of the above should cause errors

Answer: (B)

Ultrasonic thickness measurements convert the measured round-trip travel time into a thickness using the known sound velocity of the material. If the velocity value used in that conversion is not the actual velocity in the material, the resulting thickness will be systematically wrong. A faster assumed velocity makes the calculated thickness too small, while a slower assumed velocity makes it too large. This velocity mismatch is the primary reason significant errors occur. Varying the test frequency at a constant rate does not inherently change the travel time or the velocity used for the conversion, so it generally does not cause large errors in thickness. Using water as a couplant is a common practice and, when the coupling is stable and the appropriate path is accounted for, does not by itself produce significant thickness errors. If the velocity value is correct and coupling is proper, the measurement remains accurate.

Ultrasonic thickness measurements convert the measured round-trip travel time into a thickness using the known sound velocity of the material. If the velocity value used in that conversion is not the actual velocity in the material, the resulting thickness will be systematically wrong. A faster assumed velocity makes the calculated thickness too small, while a slower assumed velocity makes it too large. This velocity mismatch is the primary reason significant errors occur.

Varying the test frequency at a constant rate does not inherently change the travel time or the velocity used for the conversion, so it generally does not cause large errors in thickness. Using water as a couplant is a common practice and, when the coupling is stable and the appropriate path is accounted for, does not by itself produce significant thickness errors. If the velocity value is correct and coupling is proper, the measurement remains accurate.