A Differential Microcalorimeter for the Measurement of Ultrasonic Powers at the Milliwatt Levels

A microcalorimeter system has been developed to evaluate ultrasonic power outputs, especially of pulsed diagnostic transducers. The system is composed of two basically identical calorimeters, interwoven by an extensive network of thermocouples serving as an averaging temperature comparator. Borh calorimeters are mounted into a common housing with heavy aluminum walls so geometrically as to ensure a well-balanced thermal equilibrium. Either calorimeter may be used as an energy recipient, while the other serves as a temperature reference. Their filling consists of a silicon oil that matches the acoustical impedance of water. Each calorimeter contains a heating wire for calibration and a cooling system to achieve a fast temperature equilibrium following the measurement. The thermocouple battery is formed by 30 pairs of junctions, made of 20-μ-diam copper and constantan wire. It permits temperature evaluations at the millicentigrade level. The overall sensitivity of the system is approximately 0.08°C per calorie. Transducers to be measured are inserted into one of the calorimeters and are energized for about 30 sec and then pulled out to prevent the heat created in the transducer from reaching the area of measurement. Both pulsed or CW diagnostic transducers have been measured to ultrasonic power levels as small as 1 mW and with an accuracy of about ±4%.