Ultrasonic devices probes

The procedure approved by Gosstandart may have status of state standard, for example, GOST Non-destructive Testing. Ultrasonic Testing of rails , where types of devices, probes, procedure of the instrument adjustment are specified. Such kind of procedure may be used on site. 

Two different types of ultrasonic devices are used in laboratories ultrasonic bath and ultrasonic probe. However, as a result of inhomogeneity of ultrasonic energy distribution in the whole solution and a decrease in power with time, the repeatability and reproducibility of experimental conditions for ultrasonic baths is often unsatisfactory. With ultrasonic probes the energy is focused on a small sample area, which significantly improves cavitation efficiency and, thereby, extraction effectiveness [56]. 

Except in some special cases where the users themselves have designed and produced their own ultrasonic devices, US equipment for leaching consists of commercial ultrasonic baths or probes. 

Thermal probe systems are inexpensive, easy to handle in almost all ultrasonic devices and particularly those used in sonochemistry. Field distributions and optimization of the geometry of the system can be rapidly obtained and the accuracy of the method is high enough to ensure reproducibility. Chemists who make use of ultrasonic equipment should, as a very minimum, consider this method to calibrate and optimize sonication conditions prior to carrying out sonochemical reactions. 

Since this chapter appears in a volume devoted to sonochemistry, chemical probes would appear to be the most attractive since they could allow direct comparisons with other chemical reactions. Chemical dosimeters are generally used to test the effect of an ultrasonic device on the total volume of the reactor. Local measurements can however be made with very small cells containing the dosimeter which could be moved inside the reaction vessel as with a coated thermocouple. Most of these chemical probes are derived from reactions carried out in an homogeneous medium, e.g. Weissler s solution, the Fricke dosimeter, or the oxidation of terephthalate anions. Among these the latter shows promise in that despite the fact that to date it has been much less used than Weissler s reaction it seems to have higher sensitivity and better reproducibility. 

Ideally when a chemical dosimeter is used to test or assess an ultrasonic device, care should be taken to match the system under study with the dosimeter type. The optimum conditions determined for a reactor using a chemical probe may well not be the same optimum as that required for the chemical system under investigation. Similar observations apply to the use of sonoluminescence. 

TOPFLOW will be equipped with advanced two-phase instrumentation mainly and adapted and developed in Rossendorf, such as wire-mesh sensors, needle-shaped conductivity probes with integrated thermocouple, gamma and X-ray tomography and passive ultrasonic droplet probes. Additionally, laser-doppler anemometry and a phase-doppler particle analyser are available. Two of these devices, the needle-shaped conductivity probes with integrated thermocouple and the wire-mesh sensors will be described in detail. 

As noted above this type of mechanical transducer is predominantly used for homo-genisation/emulsification. These devices differ markedly from the more usual bath and probe types in that they derive their power from the medium (by mechanical flow across the blade) rather than by the transfer of energy from an external source to the medium. The majority of the chemical effects observed on using whistle type transducers for the sonication of homogeneous reactions can be attributed mainly to the generation of very fine emulsions rather than the ultrasonic irradiation itself. 

Sonication This involves the generation of shear forces in a cell sample in the vicinity of a titanium probe (0.5 mm in diameter and 10 cm long) that vibrates at 20,000 Hz. The device contains a crystal of lead zirconate titanate that is piezoelectric, i.e., it expands and contracts when an oscillatory electric field is applied to it from an electronic oscillator. The ultrasonic pressure waves cause microcavitation in the sample, and this disrupts the cell membranes, usually in a few seconds. 

Of the four types of laboratory ultrasonic apparatus commercially available for practising chemists in general (namely, whistle reactors, ultrasonic cleaning baths, probes and cup-horn devices) analytical chemists, except for a few specialists working in (or with) ultrasound detectors, use mainly cleaning baths and probes both of which are usually operated at a fixed frequency dependent on the particular type of transducer, that is usually 20 kHz for common probe systems and 40 kHz for baths. Both types of devices are described below. 

Probe devices undoubtedly provide the most efficient method for transmitting ultrasonic... 

Industry uses special devices similar to ultrasonic baths and probes but appropriately scaled up in size and ultrasound irradiation power. The UIP16000 model from Hielscher Ultrasound Technology is by far the most powerful ultrasonic processor available worldwide the apparatus is capable of delivering a continuous power of 16000 W at efficiency above 80%. Such powerful systems have been developed in response to the demand for the ultrasonic treatment of liquids on a large scale in fact, the ultrasound power required usually increases in proportion to the amount of liquid to be treated within a certain time. 

In a disorete approach, the analytical system is confined in a vessel or container through the walls of which US energy is transmitted if an ultrasonic bath is used. The use of a US probe in this case can involve either to dip it into the vessel or into the transmitting liquid where the vessel is located. The complexity of the analytical system determines the type of vessel or container to be used, namely an open or closed, atmospheric pressure or pressurized device, a jacket-tailored device for maintaining the optimum temperature, etc. 

That soft digestion enhanced by US was first demonstrated by Kannan and Pathan [15], who soft-digested aqueous benzoic add In the experimental device shown in Fig. 3.3. The cylindrical geometry for the sample was intended to ensure uniformity in the solid surface and improved accessibility by the liquid phase. The solid was immersed in a water bath with temperature control at 31 °C. The ultrasonic source was a probe that was inserted vertically into the water bath. The device was rated at 500 W and 20 kHz frequency. Trial experiments of USASD without temperature control of the water bath... 

Another criticism of the previous methods is that their proponents have failed to state the type of US device used as they seemingly believe that ultrasonic baths are the sole available choice for this purpose. In fact, only one USALLE method using a probe appears... 

Figure 8.11. Portable Raman spectrometer coupled to an acoustic levitator. A — Dantec ultrasonic acoustic levitation device, B — Raman fIbre-optIc probe, C — control unit for the acoustic levitation device, D — quartz halogen light source and E — InPhotonIcs portable 785-nm Raman spectrometer. (Reproduced with permission of the American Chemical Society, Ref [120].)...

This section discusses the potential of sonoelectroanalysis, expansion of which is currently at a standstill owing to the few groups working on it. With few exceptions involving baths, probes are the ultrasonic sources used to assist electroanalytical processes with US. Some authors have pointed that the low, spatially variable distribution of ultrasonic intensity provided by baths is a major hindrance for using these devices with electroanalytical techniques [131]. Therefore, most of the examples described in this section involve the use of probes as US sources. 

The specific design of the various sample introduction devices or spray probes depends to a large extent on the technique applied, i.e., ESI, APCI, or other. With respect to ESI, systems have been described for conventional pure ESI, pneumatically-assisted ESI or ionspray, ultrasonically-assisted ESI, thermally-assisted ESI, and micro- and nano-ESI (Ch. 5.5). The heated-nebulizer system (Ch. 5.6.2) is used in APCI and atmospheric-pressure photoionization (APPI). 

Thermal probes can be constructed quite easily and cheaply. Their response is non-directional, and very small devices with diameters between 0.5 and 1 mm can be fashioned. They are very simple to use with almost every kind of ultrasonic equipment and measurements can be made very rapidly. Several kinds of thermal probes have been described which are basically thermocouples or thermistors used bare or embedded in an absorbing medium. Bare probes are used to measure the actual temperature of the medium, just as in a calorimeter. Coated probes will generate internal heat under the influence of the sound wave and are used to determine local power dissipation in the absence of stirring. Coated probes are often used in conjunction with a bare probe, and the temperature difference between the two probes is then proportional to the acoustic power. Great care should be taken since the response of a coated probe strongly depends on its nature and geometry, and on the medium used. 

There are many different kinds of acoustical probes including microphones [57-62], hydrophones, radiometers, and piezoelectric devices (most often small barium titanate transducers) [63-68], and the hot wire microphone (based on acousto-resistive effect) [63], Their resonance frequency is generally very different from that of the ultrasonic field under study. 

Although these acoustical probes can be made very small they will always slightly disturb the ultrasonic field. Just as in the case of coated thermal probes, the response signal depends on the nature and size of the probe, thus it is important that the microphones are carefully calibrated. They are however widely used, especially to calibrate medical ultrasonic equipment. Recently, very small and sensitive devices using PVDF membranes [68,69] or fiber optics [70] have been described. PVDF has piezoelectric properties and miniature membrane hydrophones (about 0.5 mm in diameter) are available. Fiber optic probes can even be smaller and a spatial resolution of 0.1 mm has been claimed [70],... 

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