What are the main methods to ensure that the ultrasonic thickness gauge measures the metal thickness value?
1. The ultrasonic thickness gauge can measure the thickness of various metals through coatings, with a representative range of 1mm to 50mm in steel, and only requires one echo. It can measure the smallest thickness in pitting conditions, but the coating is as thin as 0.125mm, and the outer surface of the coating should be smoother. The ultrasonic thickness gauge requires the use of one of two specific probes, with the highest outer surface temperature being approximately 50 ℃ or 51.67 ℃; Echo echo measurement can be used with various ordinary probes, which can often penetrate rough outer surface coatings. With appropriate probes, it can work at high temperatures close to 500 ℃ or 498.89 ℃, but multiple bottom echoes are required. In heavily corroded metals, there may not be multiple bottom echoes, and the thickness range of the coating thickness gauge is wider than that of through coating measurement.
The representative longitudinal wave velocity in steel is 5.900m/s (0.2320in/us), however, in paint or similar coatings, the velocity is usually lower than 2.500m/s (0.1000in/us). Conventional ultrasonic thickness gauges will erroneously measure the coating at the speed of sound of steel when measuring the total thickness of painted metal, which means that the coating will display up to 2.35 times its true thickness value (the ratio of the two speeds of sound). In situations involving thick coatings and tight tolerances, this error introduced by the coating can be considered a significant portion of the total thickness measurement. The solution to this problem is to use the mature technique of echo echo measurement, which conveniently utilizes the time distance between two adjacent bottom echoes. This time distance represents the continuous round-trip time of sound waves passing through the detection material.
In the case of coated metals, these multiple echoes can only occur within the metal rather than the coating, so the distance between any pair of echoes (bottom echoes 1 to 2, bottom echoes 2 to 3, etc.) only represents the thickness of the metal after removing the coating thickness.
Through coating measurement, a software is used to determine the time distance represented by a reciprocating stroke in the coating. This time distance is used to calculate and display coating thickness, and by subtracting this time distance from the total measurement value, the ultrasonic thickness gauge can also calculate and display the thickness of the metal substrate.
