Thermal probe

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. 

The typical patterns shown were generated using small probes. When large probes are used there may not be significant differences between temperature rises AT, and AT2. 

The calculation of sound intensity requires a knowledge of the acoustic absorption coefficient of the imbedding material and its heat capacity per unit volume at the temperature at which measurements are made, according to, 

This probe has been compared to a radiation pressure probe (barium titanate), and calibrated by comparison with a radiation pressure measurement. Its accuracy was estimated to better than 4%. The drawback of this device is its relative fragility and the need to employ complex electronics for accurate pulsing with square top 

Martin and Law [33] carried out accurate measurements using several probe designs made of thermistors elements 0.2 mm in diameter and 0.35 mm long with pair of leads 0.02 mm in diameter. The thermistors were either held by their connecting wires in the center of a ring (60 mm in diameter) to minimize disturbance of the sound field, or mounted at the end of a thin rod, or supported in a chamber filled with castor oil (a system similar to Fry s device), or embedded in 

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Full use can be made for the on-site and off-site calibration of traditional anemometers such as thermal probes and propeller anemometers. 

A temperature accuracy test of the column oven measured with a calibrated thermal probe is used. An acceptance criterion of 35 2°C is adopted. 

Significant time is saved in this calibration procedure by minimizing sample preparation and the number of calibrated test apparatus wherein only a balance, a 5-mL volumetric flask, a stopwatch, a thermal probe, two standards, one HPLC column, and one mobile phase are used. The use of MS Word template forms (see Figure 10) saves considerable time by eliminating any notebook entries and also improves the consistency of calibration records. Two important system parameters, dwell volume, and... 

In order to provide a steady-state signal, heat must be continuously evolved. An ideal reaction for this is the biocatalytic enzymatic reaction, which combines high substrate specificity with a high amplification factor. Thus, an enzyme-containing layer is deposited over the thermal probe, and the substrate is allowed to diffuse in. 

When the gel has polymerized, the comb is removed and wells are flushed thoroughly with deionized (RNase-free) water. At this stage, the gel can be covered with plastic wrap and stored overnight at 4 °C, if desired. The binder clamps (and tape if used) are removed and the gel is placed on the gel box. The upper and lower reservoirs are filled with 1 x THEM and prerun for 30-45 min with the water bath at the desired temperature, making sure that the current is not blocked by ice, precipitate, or air bubbles. The gel temperature can be monitored with a contact thermometer or small thermal probe inside the gel. 

The extent to which the molecules formed by recombination are in thermal equilibrium with the catalyst is of fundamental interest for the light it sheds on the nature of the interaction with the surface at the instant of reaction. It is also of practical interest, particularly in the use of thermal probes for the determination of atom concentrations, where the need to take account of factors influencing energy transfer processes has not always been recognised. Fresh interest in the phenomenon has been stimulated by the demands of space technology for information on surface heating due to recombination during re-entry into the earth s atmosphere. 

According to the FOREGS recommendations (Salminen et al., 1998), soil samples should be dried at 40 °C or less to avoid Hg vaporisation. Samples can be air-dried or dried using IR lamps controlled by a thermal probe to keep the temperature of samples below 40 °C. 

The holding times of this cycle profile are not optimized, but can be shortened. Optimal holding times are dependent on the type of temperature (external or internal thermal probes) and time control (the clock usually starts below or above the holding temperature and the exact temperature difference varies among different models of the thermal cycling machines). 

Methods giving absolute energy values thermal measurements (calorimeter, thermal probes). 

When the method used depends upon an estimation of the slope of the tangent at zero time, it is essential to provide good stirring of the liquid. In the absence of stirring the response of the thermal probe is far from simple. 

As already mentioned thermal probes of several different types have been used for acoustic power measurements. In 1954 Fry published a very detailed study on the determination of absolute sound levels and acoustic absorption coefficients [32], The probe which was used is shown in Figure 9 and is made of a thermocouple junction (copper constantan or iron constantan 0.0005 inch diameter) imbedded in a thin disk of absorbing liquid. The absorbing liquid is separated from the medium... 

Table 2. Response of a Thermal Probe Coated in Different Materials...

Figure 14. Details of thermal probe using a bare and a covered thermocouple.

Figure 15. Thermal probe response vs. distance from surface for vessels of different shape.

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

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. 

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],... 

It is difficult at this time to give a full interpretation of the results described and these problems are still under investigation. It is clear however that this technique provides interesting information on the energy distribution in a sonicated reactor, quantitatively similar and therefore complementary to those given by thermal probes. 

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 ... 

These problems are illustrated by the following example from the work of Pettier et al. [169], Sonoluminescence and thermal probes measurements were compared at 500 kHz in a cylindrical cup-horn cell. The influence of liquid height and of... 

As can be seen in Figure 33a, that with the thermal probe a maximum effect (maximum temperature rise of the probe) is detected at the center of the reactor, and near the liquid-air interface. This is also the case for sonoluminescence intensity at low power (Wx < 50 W), but at higher power two symmetrical maxima are observed one on each side the center of the reactor (Figure 33b). Again, maximum intensity is observed near the liquid-air interface for liquid heights in a range 2.5-7.5 cm. 

From a practical point of view, the most generally applicable and the easiest dosimeters to use are those based on thermal methods, especially those using thermal probes. These probes have almost no limitations since they can be used (a) in any reaction vessel below or beyond the cavitation threshold, (b) in free or... 

The two main drawbacks to chemical dosimetry are that they have low sensitivity at low power and are often strongly frequency-dependent. For accurate measurements they should also be calibrated with another method, e.g. with a thermal probe. 

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