Measuring ultrasonic power

For all of these reasons, many methods have been devised to measure ultrasonic power [ 16-23]. It is extremely important that we define exactly what type of power is to be measured, e.g. power associated with the transducer system, with physical effects generated in the liquid, or with chemical effects induced by cavitation. 

PVDF exhibits pyroelectric as well as piezoelectric properties [44], a feature that has made the material useful for infrared sensing in motion detectors and thermal cameras. This attribute has been used recently in a thermally based method to measure ultrasonic power [45]. A 52 pm thick film of PVDF 60 mm in diameter was used as the sensor, which was backed with a rubber based material that generated heat due to ultrasonic absorption. Powers up to 1 W over a frequency range of 1-3.5 MHz were measured in this preliminary study. 

Methods for measuring ultrasonic power have been reviewed [42] but, in short, there does not seem to be a simple method for the quantitative measurement of local ultrasonic intensity when cavitation is present. Pugin has developed a number of methods for the characterisation of sound fields in a variety of reactors [43]. These were used to develop profiles of the acoustic intensity for both cleaning probes and probe systems with a view to examining the reproducibility of reactions. 

Turbidity of the sonicated (30 min), control and boiled samples were measured and given in Table 9.19. As could be seen, the turbidity increased gradually, as the volume of solution or of CH3COONa increased in all solutions (control, sonicated and boiled). When 20 ml of Al3+ was sonicated for 30 min with 5 ml of CH3COONa, turbidity did not appear, but when a solution containing, 10 ml of Al3+ and 2.5 ml of CH3COONa was sonicated for the same duration, the turbidity appeared, indicating the role of ultrasonic power which decreased to about half as the volume of the solution increased. 

Ruthenium catalysts, supported on a commercial alumina (surface area 155 m have been prepared using two different precursors RUCI3 and Ru(acac)3 [172,173]. Ultrasound is used during the reduction step performed with hydrazine or formaldehyde at 70 °C. The ultrasonic power (30 W cm ) was chosen to minimise the destructive effects on the support (loss of morphological structure, change of phase). Palladium catalysts have been supported both on alumina and on active carbon [174,175]. Tab. 3.6 lists the dispersion data provided by hydrogen chemisorption measurements of a series of Pd catalysts supported on alumina. is the ratio between the surface atoms accessible to the chemisorbed probe gas (Hj) and the total number of catalytic atoms on the support. An increase in the dispersion value is observed in all the sonicated samples but the effect is more pronounced for low metal loading. 

Measurements of sound velocity at ultrasonic frequencies are usually made by an acoustic interferometer. An example of this apparatus11 is shown in Fig. 2. An optically flat piezo-quartz crystal is set into oscillation by an appropriate electrical circuit, which is coupled to an accurate means of measuring electrical power consumption. A reflector, consisting of a bronze piston with an optically flat head parallel to the oscillating face of the quartz, is moved slowly towards or away from the quartz by a micrometer screw. The electrical power consumption shows successive fluctuations as the distance between quartz and reflector varies between positions of resonance and non-resonance of the gas column. Measurement of the distance between resonance positions gives a value for A/2, and if /... 

It is often difficult to compare the sonochemical results reported from different laboratories (the reproducibility problem in sonochemistry). The sonochemical power irradiated into the reaction system can be different for different instruments. Several methods are available to estimate the amount of ultrasonic power entered into a sonochemical reaction, the most common being calorimetry. This experiment involves measurement of the initial rate of a temperature rise produced when a system is irradiated by power ultrasound. It has been shown that calorimetric methods combined with the Weissler reaction can be used to standardize the ultrasonic power of individual ultrasonic devices. ... 

Figure 3 Measured number of crystals as function of ultrasonic power input.

Scheme 1. Positions for the measurements of ultrasonic power in a typical sonication system.

On a relative scale thermal probe systems give reproducible and accurate measurements of ultrasonic power input provided that a few basic precautions are taken ... 

This is a good method for local measurements, but rather tedious for overall power. Furthermore, to calculate the ultrasonic power one needs to measure the particle velocity, and this is not a trivial task. Indeed one might assume that the particle velocity is the same in the liquid and at the tip of the probe (see Section 4.2). This is almost never exactly true, but this assumption can lead to a reasonable estimate of the dissipated ultrasonic power. 

The intensity of sonoluminescence can easily be measured with photocells [159,163] or fiber optics [169] connected to a photomultiplier in darkened surroundings. This measurement is not invasive, and has been suggested as a standard [19]. In principle the sonoluminescence intensity could be correlated to the ultrasonic power, but at this time no direct theoretical correlation has been established. It has been used to determine the areas of maximum cavitational activity in a reactor. Any empirical correlation with power would necessitate preliminary calibration with another method, e.g. with thermal probes. Some care should be exercised when using a sonoluminescence probe for the following reasons ... 

The stimulation of chemical reactions has been known for a many years [1-15] and it has been suggested that some of them might be used as standards for the measurement of the efficiency of ultrasonic systems. Unfortunately, as is the case in the use of sonoluminescence as a probe, there seems to be no theoretical correlation between chemical effects and ultrasonic power. Nevertheless it is an undeniable fact that when sonochemistry is reported in the literature it would be extremely useful if the response of the system used to a standard sonochemical reaction could be included. 

Mason et al. reported for the first time the response of the TA dosimeter with different ultrasonic sources and frequencies. They employed an ultrasonic cleaning bath (Kerry Pulsatron 55 operating at 38 kHz) with different immersed reactors (flat bottom Erlenmeyer and round bottom flask) and the Undatim Sonoreactor with 20-, 40-, or 60-kHz horns. Ultrasonic power measurements were monitored using the calorimetric method described previously. 

Both these geometric parameters altered diffusion data measured as Sherwood dimensionless number or as diffusion coefficients maxima and minima in these parameters mirrored nodes and antinodes from the ultrasound. This involved relative motions between the various components of several centimeters since the wavelength of sound at 20 kHz is of this order, depending on the medium. These workers were using electrochemistry as a probe to monitor ultrasonic power, and a fuller account of this work is given in another chapter of this volume, but the effects of geometry upon behavior of the electrochemical probe are noteworthy. 

Cavitation is substantially more difficult to induce at higher frequencies, and since these experiments were performed at the same ultrasonic power (measured calorimetrically), suggests that the observed sonoelectrochemical benefits are not cavitational in origin. However, it may be simply an effect of the shorter wavelength... 

A very precise determination of the power absorbed in a system is described by Margulis, who compared the ultrasonic conditions to Joule s effect from a calibrated thermistor.35 Other physical measurements of power (pp. 13 and 14), less commonly applied, include the use of microphones, acoustic balances, and the erosion of metallic foils by cavitation. ... 

Chan, H.L.W., Ramelan, A.H., Guy, I.L. and Price, D.C. (1989) VF-JVF copolymer hydrophone for ultrasonic power measurements. Proceedings of the IEEE Ultrasonics Symposium, Montreal, Canada, 3-6 October 1989, 617-20. 

In the case of liquid applications, the calorimetric methods should be emphasized. A simple calorimetric method consists of measuring the temperature increase during the first few moments ( =a90s) of ultrasound application (Raso et al., 1999). The procedure assumes that, in this early stage, the system behaves in a quasi-adiabatic manner, and the temperature variation is produced only by the application of ultrasound. The apphed ultrasonic power, P, can then be calculated from ... 

Fig. 8.14 Slope of the linear relationship between effective diffusivity and applied ultrasonic power for the drying of different products, versus instrumentally measured hardness.

Lin, S., Zhang, F., 2000. Measurement of ultrasonic power and electro-acoustic efficiency of high power transducers. Ultrasonics 37 549-554. 

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