Development and Study of Electronic Delay Reference Standard for Ultrasonic Pulse Velocity Measurement Devices

Concrete is the fundamental material used for the construction of civil infrastructures and seems essential. The strength of concrete is directly proportional to the Ultrasonic Pulse Velocity (UPV). UPV particularly in concrete is measured using the device called UPV tester, and concrete structures are graded according to Indian Standard 516 (Part 5/Sec 1): 2018 and American Society for Testing and Materials C597 (2009). The UPV tester requires reference delay blocks to ascertain the accuracy, as the instrument within the range of $20 \mu \mathrm{s}$ to $10 \mathrm{~ms}$ be within $1 \%$ accuracy. These blocks are fabricated from perspex, steel, and aluminium. Currently, perspex delay blocks of specific delays serve as the reference. Perspex is lightweight and easy to handle up to $100 \mu \mathrm{s}$. However, larger time delay requires longer blocks, making them difficult to carry during site testing. Additionally, the material exhibits significant temperature dependency. For instance, a perspex block with a length of $100.92 \mathrm{~mm}$, it was observed that the transit time variation from $38.94 \mu \mathrm{s}$ to $40.59 \mu \mathrm{s}$ for the temperature change from $5^{\circ} \mathrm{C}$ to $45^{\circ} \mathrm{C}$. The UPV measurements were performed in climate chamber. This resulted in change in velocity from 2591.6 $\mathrm{m} / \mathrm{s}$ to $2486.3 \mathrm{~m} / \mathrm{s}$, leading to an inaccurate instrument calibration under different weather conditions. To address these issues, an electronic delay based on microcontroller in interrupt mode was developed to mimic the functionality of the delay block and applied for UPV tester calibration. The electronic delay is lightweight, and its accuracy depends on the accuracy of the crystal. In present case a 16MHz crystal used in microcontroller for the electronic delay has an accuracy of 10 ppm over the measurement operating temperature range from $-20{ }^{\circ} \mathrm{C}$ to $70{ }^{\circ} \mathrm{C}$. This introduces an error of 100 ns in the measurement of $10 \mathrm{~ms}$ transit time within the temperature range from $20^{\circ} \mathrm{C}$ to $70^{\circ} \mathrm{C}$.