Ensuring the validity of results of hydrophone calibration by a mutual comparison calibration method

Abstract

Hydrophone calibration is one of the most important basic metrology activities in ultrasound. Hydrophones are used in many different calibrations and testing methods in industry, for instance, the evaluation of medical electrical equipment basic safety and essential performance. Ensuring the validity of results of metrological services is a major concern for any laboratory. Primary calibration of measuring instrument or standard is a widely understood way to assure good results, despite it is not enough, it is not the unique way, neither it is easy to get in some circumstances. For instance, hydrophone calibration is a service restricted to a few laboratories worldwide, which demands a logistic effort and may be quite expensive. An alternative method to guarantee the traceability by in-house check is of interest. This paper presents a method to ensure the traceability of hydrophones after mutual calibrations. Eight previously calibrated hydrophones from different manufactures and characteristics (membrane and needle) were mutually calibrated, by comparison, all of them acting as standards and calibration items. Calibrations were performed from 1 MHz to 10 MHz and the results were evaluated by pair-difference chi-squared statistics. Uncertainties typically laid between 6% to 13% and sensitivities were between 33 to 318 nV Pa−1. After statistics analyses, 4 hydrophones were considered reliable to be used as standards, as their sensitivity keep on stable even after several years since the last calibration. The method can be used by the industry, accredited laboratories, or national metrology institutes to perform functional checks for virtually any measuring instrument or standard, being the limit the number of available devices to properly apply the statistical analysis.