Hydrophones are an indispensable part of procedures for the determination of output parameters of ultrasound machines for any diagnostic, therapeutic or technical application. Following the current trend of increasing application frequencies in ultrasound NPL and PTB extended the frequency range of their primary calibration facilities to and beyond 50 MHz. Although both laboratories already offer quality management assisted and/or accredited measurement services at higher frequencies, an evaluation of the calibration results on an international level is still missing.
In this project a key comparison on hydrophone calibration will be carried out in a frequency range between 1 MHz and 50 MHz. Both laboratories will use their primary interferometric techniques for the determination of the frequency response of two hydrophones. Two hydrophones, one provided by each laboratory, will be used during the comparison. They will be chosen to provide different performance characteristics. Several parameters, such as the spot geometry and diameter, layer thickness and manufacturing process will be varied. This is important to clarify possible differences or inconsistencies of measurement results. A final report will summarize all results. The rules of bilateral comparisons for RMO or BIPM key comparisons will be applied.
Final Report 2014-05-31
Hydrophone calibration services are commonly offered until frequencies up to 20 MHz. The highest frequency addressed in the CCAUV.U-K2 key comparison was 15 MHz. Because of the tendency of using higher frequencies in technical and medical applications increasing demands in industry and calibration practice exist to extend the calibration frequency range to at least 40 MHz. To support research acivities, a comparision between the primary hydrophone calibration facilities of NPL and PTB was carried out with the intension of extending the upper frequency limit to 50 MHz.
A hydrophone with a small active diameter of 0.2 mm was circulated to both laboratories and measured with the primary calibration systems independently. Both laboratories used an interferometric technique detecting the displacement of a pellicle in the sound field. In this way the sound field pressure is determined on the basis of a length and a time measurement. In the labs two different set-ups were established. At NPL the pellicle is submerged in the water tank and at PTB the pellicle is mounted at the surface of the water. Both methods provide reliable measurements.
Measurements were carried out first at PTB up to 50 MHz using two transducers with different focal lengths. At low frequencies the values obtained agree well within the declaired uncertainties. At high frequencies a systematic difference was observed which could not be coherently explained in all details. In the next step NPL calibrated the hydrophone using both, the primary interferometric set-up and a secondary technique. Unfortunately, in case of the primary calibration problems with noise occurred and investigations were necessary to reduce this noise. Finally, the upper frequency of reliable results was identified to be limited to 30 MHz for the time being. Within this accepted frequency range the agreement of the values obtained by both participants is satisfactory. Now further steps are to be taken to improve the higher frequency capabilites in both laboratories. After another research periode another comparison should be started.