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Radiation Force Balance (RFB) is the approach recommended in IEC 61161 for the primary measurement of spatially integrated, time averaged, total ultrasonic power generated by ultrasonic transducer. In this approach, an absorbing or reflecting target is used and attached to a highly sensitive microbalance to detect the changes in apparent mass due to radiated ultrasound. Various influencing factors that significantly contribute to the deterioration of uncertainty of power measurement are taken into consideration and explained in this paper. The error in estimation of RF voltage fed to the transducer under study becomes critical relatively at higher frequencies due to impedance mismatch. Experimentally, it was observed that the length of cable used to feed the transducer must be as shorter as much as possible. In order to optimize and minimizing the effect, the use of cable should be avoided and an alternative arrangement should be made to perform the measurements at the transducer node only, if possible. In addition, external disturbances also become the factor for the movement of the target which is comparatively more significant at low power level (in few mW range). In case of high power (watts), the absorbing targets expand due to local heating of the target. The expansion of the target results into change in the buoyancy which in turn results into over estimation of power. The amount of over estimation depends on the excitation duration of the ultrasonic transducer. At CSIR-National Physical Laboratory, India All the above mentioned influencing factors have been considered in such a way that the effects of RF voltage measurement, target movement and thermal expansion of the target are minimized which finally improves the performance and the estimated measurement uncertainty of the power measurement system. For the target movement, the detection of the output of microbalance is continuously acquired and stored in the computer at the rate of 10 reading / second. Such continuous acquisition of microbalance output enables us to clearly visualize the effects such as target movement; thermal expansion etc. Further, in order to improve the RF voltage measurement accuracy, a unique DC convertor module has been developed. In this paper various aspects of the improved primary facility of total ultrasonic power measurement have been discussed.
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