A BIPM/CIPM key comparison covering the calibration of ultrasonic hydrophones over the frequency range 1 MHz to 15 MHz

Abstract

A central objective of the Mutual Recognition Arrangement (MRA), signed by national measurement institute (NMI) directors in 1999, is the establishment of the degrees of equivalence of national measurement standards held by each institute. International comparisons, known as key comparisons, represent the sole mechanism for establishing these degrees of equivalence. In this paper we describe a key comparison, undertaken under the auspices of the BIPM/CIPM Consultative Committee for Acoustics, Ultrasound, and Vibration, related to the realization of the acoustic pascal in water at ultrasonic frequencies. This is most appropriately achieved through a comparison of calibrations of stable transfer standard hydrophones; 1 mm active element bilaminar membrane hydrophones, being chosen for this purpose. With NPL acting as the pilot laboratory, two hydrophones were calibrated using the NPL primary standard laser interferometer and circulated sequentially to participant NMI laboratories in Germany, China, The Netherlands, and Denmark. Laboratories were asked to report values for the hydrophone open-circuit free-field sensitivity over the frequency range 1 MHz to 15 MHz. The principal calibration methods used by the NMIs were optical interferometry and/or two-transducer reciprocity. The key comparison process, its results, and the analysis used to derive the key comparison reference values, are all described in detail.