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Portable power standard for the traceability of measuring devices for ultrasonic physiotherapy equipment

15.12.2005

A portable power standard for the traceability of measuring devices for ultrasonic physiotherapy equipment has successfully been developed and tested in an international cooperation under EU funding.

Ultrasonic physiotherapy is a widely used medical application. According to written standards, the ultrasonic power determined in water has to be stated in order to characterize the emitted ultrasonic field. Measuring devices are mostly based on the radiation force principle and are subject to quality management. In order to improve the traceability, a portable ultrasonic power standard (PPS) has been developed and tested in an international cooperation and funded by the European Union. Participants were TNO/NL (coordinator), NPL/UK, CSIRO/Australia, and PTB. Four power standards were produced, one for each participant.

Each power standard consists of the following parts: 5 ultrasonic transducers derived from therapy treatment heads, an electronic driver (console) and a cavitation detector. The driver produces the excitation voltage for the transducers and includes the control software which enables a proficiency test at the user's site as follows: certain output power values are selected by the national institute and entered into the driver memory, and the user has to measure the power with his measuring device and report it without knowing the selected values.

In a series of fundamental measurements, the emitted ultrasonic power of all drivers and transducers was determined using the primary PTB radiation force balance. In addition, the power standard intended for PTB was measured over a period of about 18 months in order to check its temporal stability, and the device was circulated to three external testers in Germany (two manufacturers of therapy equipment, one test house) in order to check its proper functioning and robustness. Figure 1 shows as an example the ultrasonic power P emitted by one of the transducers at a selected power level of about 300 mW as measured by PTB, as a function of time. There obviously was an ingress of water (transducer leakage) at the first external tester and the low resulting value is considered to be an outlier. The remaining power values are within ± 1 %. The experience reported by the testers was taken into account in subsequent improvements.

The power standard is now being held available on loan to interested parties.

Ultrasonic output power P of a small 3 MHz transducer at nominally 300 mW as measured by PTB, as a function of time. The vertical arrows indicate the travel trials to three external testers in Germany.

Figure 1: Ultrasonic output power P of a small 3 MHz transducer at nominally 300 mW as measured by PTB, as a function of time. The vertical arrows indicate the travel trials to three external testers in Germany.

Contact person:

K. Beissner, FB 1.6, Se. Sc. 1.60, E-Mail: klaus.beissner@ptb.de