Ultrasound power measurement : a microprocessor based device utilising thermal expansion of a total absorber
A new type of ultrasound power meter is described which is robust, portable, easy to operate and therefore suitable for use in the work place by non-specialist personnel. The device should be of particular value in the field of ultrasonic therapy, where a lack of suitable instrumentation has discouraged performance monitoring of ultrasound machines in the past although numerous surveys have found the calibration of such equipment generally to be poor. The mode of operation is based on absorption of the ultrasound beam within a liquid, which expands as the energy becomes degraded to heat. The rate of this thermal expansion is monitored by means of a capacitative liquid level sensor, the output signal being suitably processsed and passed to a microcomputer for analysis. Correction for interchange of heat with the surroundings is carried out automatically by the micro, by recording the 'background drift' in liquid volume for a few seconds prior to insonation and subtracting this from the expansion rate observed during input of ultrasound. Calibration is achieved by measuring the (corrected) expansion rates brought about by known powers supplied from an electrical heating coil. When using the new power meter for measurements of therapeutic ultrasound the performance compares favourably with that of other techniques: the reproducibility is on the order of 5% above about 0.5W, becoming less good below this (reaching about 20% at the minimum measurable power of 25-50mW). Design modifications which may improve performance at low power levels and possibly allow measurement of dignostic ultrasound are suggested.