| Ultrasonic clamp-on flow meters operating with the transit time method use an acoustical calibration factor, which determines the transmission angle in the fluid. This can be measured indirectly by a flow calibration. The result of a flow calibration, however, depends on parameters such as the pipe diameter and the flow profile that are not part of the clamp-on meter being calibrated. A new method is presented that allows for direct determination of this acoustical calibration factor. With this method, transducers can be calibrated for a designated flow range and pipe range independent of calibrated master flow meters, and respective pipes and calibration facilities. For the first measurements, different transducers were measured and the results were evaluated statistically. Reproducibility tests with one transducer (dismounted and mounted after each measurement) show a variation of the values significantly below 0.1 %. Tests with a batch of transducers show variation of material parameters with a value of 0.30 %. The sensitivity of this new method is good enough to detect temperature changes during a day which can be found as a change in the mean value of the acoustical calibration factor. The reproducibility of the measurements with relative standard deviations far below 1 % promises a stable and reliable calibration method. Especially for low frequency transducers designed for big pipes, this method can reduce factory calibration costs significantly. |