Ultrasonic testing (UT) is one of the most widely used techniques for non-destructive testing. This is an effective method to provide information of the test material like dimension, geometry, and shape. The precise thickness measurement is necessary for many industries to observe the deterioration, destruction and damages in metals, composite structures, pipes, soldering, and welding. In UT, the pulse-echo (PE) method is the conventional ultrasonic method used for testing and acquiring information about the specimen. In PE method, the ultrasonic wave is transmitted by the single transducer and the reflected signal from the specimen is received back by the same transducer. The duration between transmitted and received signal, e.g., time of flight is then measured. In this article, a comparative study of the dimensional measurements (thickness measurements) of an ASTM (American Society for Testing and Materials) reference block is carried out using ultrasonic contact and immersion methods. Longitudinal ultrasound was generated in the reference block by a piezoelectric broadband transducer. The responses of the ultrasonic echoes were recorded using an ultrasonic flaw detector. For the ultrasonic testing, the uncertainty associated with thickness measurement of a standard ASTM reference block is found as ± 0.035 mm while using immersion method. However, the estimated uncertainty is higher as ± 0.088 mm in case of contact method. The study clearly shows that the immersion testing system is capable to precisely measure the thickness with relatively lower uncertainty than the contact approach.

超声测试（UT）是无损测试中使用最广泛的技术之一。这是一种提供测试材料信息（如尺寸，几何形状和形状）的有效方法。精确的厚度测量对于许多行业来说是必要的，以观察金属，复合结构，管道，钎焊和焊接中的劣化，破坏和损坏。在UT中，脉冲回波（PE）方法是用于测试和获取有关标本信息的常规超声方法。在PE方法中，超声波是由单个换能器发射的，而来自标本的反射信号则由同一个换能器接收回来。然后测量发射信号和接收信号之间的持续时间，例如飞行时间。在本文中，使用超声波接触和浸没方法，对ASTM（美国测试和材料学会）参考试块的尺寸测量（厚度测量）进行了比较研究。纵向超声是由压电宽带换能器在参考块中产生的。使用超声探伤仪记录超声回波的响应。对于超声测试，使用浸没法时，与标准ASTM参考试块厚度测量相关的不确定度为±0.035 mm。但是，采用接触法时，估计的不确定度更高，为±0.088 mm。研究清楚地表明，浸入式测试系统能够以比接触法更低的不确定度精确测量厚度。