Previous hydrophone comparisons between European laboratories have covered the frequency range 0.5 to 15 MHz. Due the growth in medical applications involving the application of high frequency ultrasonic fields (above 15 MHz), there is a need to compare calibrations. As the only two laboratories currently providing this Service to industry, a bilateral comparison will be carried out between NPL and PTB, with the former acting as the pilot laboratory. It is proposed to circulate at least two membrane hydrophones, which are a capable of characterizing ultrasonic fields within this frequency range.
A world comparison of hydrophone calibration methods had previously been completed under the auspices of the BIPM, covering the frequency range 1 to 15 MHz (CCAUV.U-K2: Free-field open-circuit sensitivity of ultrasonic hydrophones). There was a strong requirement to extend this comparison up to 40 MHz and, as only two laboratories world-wide had such capability, EUROMET project 745 was planned as a bilateral comparison which would at a minimum cover frequencies up to 40 MHz. Two suitable hydrophones were donated to the project, one each by the participant laboratories, PTB and NPL. Two significant problems were encountered during the course of the comparison. The first involved NPL’s new primary standard (based on optical interferometery), which malfunctioned. The new interferometer would have provided significantly enhanced signal-to-noise ratio for calibrations, allowing the attainment of improved measurement uncertainties – key for calibrations at elevated frequencies. The calibrations had therefore to be completed after the re-commissioning of the old NPL primary standard. The second issue related to one of the hydrophones used for the comparison, a commercially available hydrophone design which incorporated an integral amplifier. Whilst being calibrated at NPL, the device appeared to develop an instability, and this was confirmed when the hydrophone was returned to PTB for checking. The (irreversible) changes were caused by water ingress. Consequently, comparison was only possible between for one hydrophone – a Marconi 0.2 mm active element bilaminar device, made from 9 micron pvdf film. As required by the protocol, calibration data were returned by NPL and PTB covering the frequency range 1 to 60 MHz, at a minimum of nine spot frequencies. Full uncertainties budgets were provided, showing a variation in the expanded uncertainties (k=2) varying between 4% at low frequencies, to 20% at 60 MHz. Conclusions from the comparison were:-
although agreement between NPL and PTB was within the stated uncertainties, differences were larger than anticipated (15% at 40 MHz and >30% at 60 MHz);
a follow-on comparison project should be organized, utilizing at least two hydrophones, with different performance characteristics.
Subject to timing and interest, it may be possible to include other laboratories interested in assessing the capability of the systems, in an informal way.