Procedure designed to determine a characteristic variable to assess a measuring arrangement, comprising a clamp-on ultrasonic flowmeter and a pipe on which the clamp-on ultrasonic flowmeter is fixed, and/or to assess the operation of said measuring arrangement, wherein the characteristic variable determined is the angle at which the sound primarily propagates through the fluid located in the pipe, wherein said procedure is characterized by the following steps: A) Provision of characteristic values with regard to the excitation signal generated by the clamp-on ultrasonic flowmeter; B) Provision of sensor-specific data with regard to the acoustic characteristics of one or more ultrasonic transducers of the clamp-on ultrasonic flowmeter; C) Entry of data regarding the pipe material and the pipe wall thickness, particularly by a user; D) Determination of the characteristic variable from the characteristic values and data of Steps A to C using a mathematical model comprising the following steps: i Calculation of a system response using the mathematical model; ii Automated evaluation of the system response in order to determine the characteristic variable; F3) Initiation of fault diagnostics on the basis of the characteristic variable, whereby the compatibility of the data of Steps A-C is checked. wherein the characteristic variable determined is the angle at which the sound primarily propagates through the fluid located in the pipe, wherein said procedure is characterized by the following steps: A) Provision of characteristic values with regard to the excitation signal generated by the clamp-on ultrasonic flowmeter; B) Provision of sensor-specific data with regard to the acoustic characteristics of one or more ultrasonic transducers of the clamp-on ultrasonic flowmeter; C) Entry of data regarding the pipe material and the pipe wall thickness, particularly by a user; D) Determination of the characteristic variable from the characteristic values and data of Steps A to C using a mathematical model comprising the following steps: i Calculation of a system response using the mathematical model; ii Automated evaluation of the system response in order to determine the characteristic variable; F3) Initiation of fault diagnostics on the basis of the characteristic variable, whereby the compatibility of the data of Steps A-C is checked. A) Provision of characteristic values with regard to the excitation signal generated by the clamp-on ultrasonic flowmeter; B) Provision of sensor-specific data with regard to the acoustic characteristics of one or more ultrasonic transducers of the clamp-on ultrasonic flowmeter; C) Entry of data regarding the pipe material and the pipe wall thickness, particularly by a user; D) Determination of the characteristic variable from the characteristic values and data of Steps A to C using a mathematical model comprising the following steps: i Calculation of a system response using the mathematical model; ii Automated evaluation of the system response in order to determine the characteristic variable; i Calculation of a system response using the mathematical model; ii Automated evaluation of the system response in order to determine the characteristic variable; F3) Initiation of fault diagnostics on the basis of the characteristic variable, whereby the compatibility of the data of Steps A-C is checked.