Reeta Gupta

Evaluation of acoustical characteristics of ultrasonic transducer backing materials at high hydrostatic pressures

Abstract The present work reports on the measurement of acoustical characteristics in commonly used transducer backing materials at hydrostatic pressures up to 3.0 × 10 6 Pa. The pulse transmission technique, using two specially designed transducers across the samples, has been used for the measurements of the acoustic velocity. The measurements have been made on backing materials of epoxies loaded with suitable inclusions. For high impedance backing materials, the inclusions used are metal powders of tungsten, iron and aluminium respectively. For low impedance backing, inclusions such as wood dust have been used. The observed variation with pressure in the acoustical velocity in the various backing materials have been discussed in terms of the related information from the literature.

Primary measurement of total ultrasonic power with improved accuracy in rf voltage measurement

Out of the various existing ultrasonic power measurement techniques, the radiation force balance method using microbalance is most widely used in low power (below 1 W) regime. The major source of uncertainty associated with this technique is the error in ac voltage measurement applied to the transducer for the generation of ultrasonic waves. The sources that deteriorate the ac voltage measurement accuracy include cable length and impedance mismatch. We introduce a new differential peak to peak measurement approach to reduce the ac voltage measurement error. The method holds the average peak amplitude of each polarity. Ultralow offset difference amplifier is used to measure peak to peak voltage. The method is insensitive to the variations in the dc offset of the source. The functionality of this method has been tested and compared with the conventional rf voltage measurement method. The output of this proposed technique is dc, which can be measured with an error of less than 0.1%.

Measurement of total ultrasonic power using thermal expansion and change in buoyancy of an absorbing target.

The Radiation Force Balance (RFB) technique is well established and most widely used for the measurement of total ultrasonic power radiated by ultrasonic transducer. The technique is used as a primary standard for calibration of ultrasonic transducers with relatively fair uncertainty in the low power (below 1 W) regime. In this technique, uncertainty comparatively increases in the range of few watts wherein the effects such as thermal heating of the target, cavitations, and acoustic streaming dominate. In addition, error in the measurement of ultrasonic power is also caused due to movement of absorber at relatively high radiated force which occurs at high power level. In this article a new technique is proposed which does not measure the balance output during transducer energized state as done in RFB. It utilizes the change in buoyancy of the absorbing target due to local thermal heating. The linear thermal expansion of the target changes the apparent mass in water due to buoyancy change. This forms the basis for the measurement of ultrasonic power particularly in watts range. The proposed method comparatively reduces uncertainty caused by various ultrasonic effects that occur at high power such as overshoot due to momentum of target at higher radiated force. The functionality of the technique has been tested and compared with the existing internationally recommended RFB technique.

Performance characteristics of ultrasonic NDT probes at high hydrostatic pressure for inspection of offshore installations

Abstract In the present work an attempt has been made to study the performance characteristics of commercially available contact-type NDT probes, at pressures up to 2.8 × 106 Pa (equivalent to 280 m depth). The parameters studied are pulse-echo sensitivity, its frequency response and the electrical impedance of a normal beam probe. Pulse-echo sensitivity is observed to decrease at high pressures to about 60% of its value at the normal pressure. The observed results have been explained in terms of the effect of the pressure on the backing material of the probe and hence the effect on the probe performance. From the frequency response of the pulse-echo sensitivity it is observed that the probe bandwidth decreases at high hydrostatic pressures.

INVESTIGATIONS ON EVALUATION OF TRANSDUCER PERFORMANCE AT HIGH ELECTRIC DRIVES

Investigations have been reported in the present work on evaluation of performance of transducers at frequencies of 300 KHz and 800 KHz at high electric drives. Methods which have been used for such assessment are study of admittance, motional loops and measurement of acoustic output as the frequency is varied around resonance using hydrophone and radiation pressure technique. It is felt that the parameters such as variation of conductance, shift in resonant frequency, extent of frequency asymmetry and variation in mechanical Quality factor can be fairly indicative of transducer performance at high drives. The study would be of help in sorting out methods for design evaluation of transducer particularly for oceanographic and medical applications.

Performance characteristics of a remote temperature sensing device based on acoustic pinger tags

Abstract A pinger device of frequency in the vicinity of 56 kHz and source level of the order of 166 dB, at 1 μPa m , has been developed for sensing ambient temperatures in the range 5–30 °C under water. The device uses a thermistor for sensing the temperature which changes the pulse repetition rate of the pinger output. The acoustical performance characteristics of the pinger unit and the temperature sensing characteristics of the device are reported. The variation in the uncertainty of the measurement over the temperature range has been estimated. The device is likely to find application in the remote sensing of temperature in seawater such as for the study of fish behaviour in response to changes in ambient temperature.

Study of motional admittance of piezoelectric transducers with different pressure release backings at hydrostatic pressure of 300 psi

Abstract Pressure release materials are used in underwater transducers to enable them to withstand high hydrostatic pressure and to decouple them from the housing. Although we have information on the type of pressure release materials for use at various depths, methods for their evaluation have not been reported. In this study, an attempt has been made to study the effect of various pressure release materials on the performance of piezoelectric transducers at hydrostatic pressure of 300 psi through motional admittance measurements.

A DIAPHRAGM TYPE STRAIN GAUGE ULTRASONIC POWER METER

Measurement of acoustic power output of ultrasonic sources is important for characterization work. A compact, portable and easy to operate ultrasonic power meter has been developed using radiation pressure method. Radiation pressure on the specially designed conical diaphragm, which forms a part of the top surface of an enclosed chamber, is balanced by the extra pressure pumped in the chamber and is monitored by the semiconductor strain gauge mounted on a spring under the diaphragm. Present method has all the inherent advantages of a null method and is suitable for measurement of acoustic power in the wide range of milliwatts to several watts.