Ultrasonic Thickness Gauges are commonly used to measure the thickness of coatings, and their performance is critical to the accuracy of the measurement results. In this paper, combined with JJF 1126-2004 "Calibration Specification for Ultrasonic Thickness Gauge" and CNAS-CL07:2011 "Requirements for Measurement Uncertainty", the indication error and uncertainty of ultrasonic Thickness Gauge in coating thickness measurement are evaluated, a calibration method based on standard thickness blocks is proposed, and the key sources of uncertainty are analyzed. The results show that the error of the standard thickness block, the uncertainty propagation, and the repeatability of the device are the core factors affecting the measurement uncertainty. In order to achieve high-precision coating thickness measurements, this article further clarifies the representation of Calibration and Measurement Capability (CMC).
Measurement and evaluation methods
Measurement principles and methods
Ultrasonic Thickness Gauges use the principle of ultrasonic pulse reflection to measure the thickness of a material. The test uses a standard thickness block, and the average value is calculated by averaging multiple measurements. The formula is as follows:
H = H - H standard
where H is the mean value of the instrument, and H standard is the nominal value of the standard thickness block.
Uncertainty rating
Repeatability rating – the range is calculated from multiple measurements, and the uncertainty is assessed using the range method.
Effect of error of standard thickness block——Considering that the error of standard thickness block obeys uniform distribution, the root mean square method is used to evaluate the uncertainty of standard thickness block.
Resultant Uncertainty – The total uncertainty is calculated based on the uncertainty synthesis of different sources.
Extended uncertainty
The extended uncertainty is calculated by using the inclusion factor k=2. For a Thickness Gauge with a resolution of 0.01mm, the extended uncertainty range is 0.02 to 0.06mm.
Calibration & Measurement Capability (CMC)
The 0.01mm resolution ultrasonic Thickness Gauge has a Good Calibration Capability (CMC) of 0.02 to 0.06mm based on an analysis of the calibration capability of the standard thickness block.
Application & Significance
Ultrasonic Thickness Gauges are widely used for coating thickness measurement, and their calibration methods and uncertainty assessment directly affect the reliability of the measurement results. The analysis in this paper provides reliable data support for the application of ultrasonic Thickness Gauges in industrial coating inspection, and guides the optimization of equipment calibration and measurement capabilities.
