The article considers the problems of diagnostics of reinforced concrete structures and creation the generalized calibration dependence for the use of ultrasonic devices. The results of the study of the influence of some factors on the results of measuring the dissemination time of ultrasonic vibrations are presented. The study was conducted using devices "Pulsar 1.1" and "Novotest IPSM-U" in compliance with regulatory requirements. Statistical processing of the obtained results (with visualization) was performed using the software package EXEL. Processing of the obtained data confirmed the multifactorial dependence of ultrasonic measurements on the concrete surface. The lack of clearly defined conditions for measurements reduces the reliability of the results. The use of ultrasonic measurements to obtain absolute values of the properties of concrete is not adequate. It is proposed to make changes to the regulations regarding the limits of application of ultrasonic measurements.