Application of t-Test Statistical Analysis to Assess the Need for Calibration Verification in NDT Ultrasonic Thickness Measurements under Varying Environmental Conditions

Abstract

Ultrasonic thickness measurement is widely used in non-destructive testing (NDT) to monitor pipeline integrity and detect wall thinning or corrosion without damaging the component. However, measurement accuracy can be influenced by environmental conditions such as temperature variations, couplant condition, and operational shifts. This study investigates the effect of environmental conditions associated with day and night operational shifts on ultrasonic thickness measurements using statistical analysis. A total of 120 ultrasonic thickness readings were analyzed over three consecutive days. For each day, two independent groups were collected, consisting of 20 measurements during the day shift and 20 measurements during the night shift. The mean thickness values recorded during the day shifts were approximately 10.003 mm, 10.001 mm, and 10.000 mm for Days 1, 2, and 3 respectively, while the night shift measurements produced higher mean values of approximately 10.089 mm, 10.095 mm, and 10.096 mm. To determine whether these differences were statistically significant, an Independent Samples t-test was applied at a significance level of α = 0.05. The calculated t-ratios were 10.056, 12.867, and 12.724 for Days 1, 2, and 3 respectively, with corresponding p-values less than 0.0001. The combined dataset showed a mean difference of approximately 0.092 mm with a 95% confidence interval ranging from 0.083 mm to 0.101 mm. Graphical analyses using box plot distributions and normal quantile plots further supported the statistical findings. The results indicate that environmental conditions associated with operational shifts may influence ultrasonic thickness measurements. Therefore, periodic calibration verification during different operational shifts may help improve measurement reliability in industrial inspection applications.