The Reflection-Type Probes

FIGURE 4-7 Comparison of probes having different particle-to-probe diameter ratios (Matsuno, 1983). 

Type I probe has a diameter larger than that of the particle, the output signals are all generated by back scattered light from the particles, and the integrated values of the output signals can be correlated with the concentration of particles by any calibration method, from which the instantaneous concentration can be obtained by output signals analysis. These probes have been widely used in concentration measurement of particles. 

The cross beam probe can limit the measured volume, which is very important for gas-solid two-phase flow with bubbles. 

FIGURE 4-8 Comparison of measurement volume between parallel and crossed optic fiber probe (Reh and Li, 1990). 

FIGURE 4-9 Opto-electronic measuring system of Hartge et al. (1988). 1-Probe, 2-0ptical fiber, 3-Photo-diode, 4-Beam splitter, 

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Figure 11.17 Temperature probes for contact measurement, (a) The reflection-type probe, (b) the transmission-type probe.

Type (b) is called the reflection-type probe. The probe has only one tip and the effective volume of measurement depends on the diameter, numerical aperture, overlap region of the capture angles and the optic sensitivity of the photoelectric converter. The output signals of the probe are produced by the back scattering of incident light by particles, and are thus dependent on the chromaticness and reflectivity of the particles. 

Both the transmission-type probe and the reflection-type probe, need be calibrated for their measuring range in local solids concentration. The calibration of optic fiber probes is known to be a difficult problem. Calibration methods fall into two categories the first is to calibrate a probe against agitated or fluidized liquid—solid systems the second is to use particle free-fall in gas—solid systems or the traditional pressure drop method for fluidized solids the third is in a flow system with particle density deduced from mass flux of particles and measurement where phase velocities were nearly equal. 

Since dispersion of solids in a fluid-particle system is in a random state of movement, and the signals of light output from both the transmission-type and the reflection-type probes are dependent on diameter, morphology, chromaticness, distance and refractivity of the particles, the signals produced by particles in random movement can only be described by random data analysis. 

The application of the reflection-type optic fiber probes to measurement of local concentration of solids was developed on the basis of the optic fiber displacement sensor. Salins (1975) and Krohn (1986) summarized the arrangement of different optic fiber probes and their response curves, as illustrated in Figure 4-6. It shows that when the displacement sensor detects... 

Types of optical fiber probes are presented in Table 5, and some examples of probes are shown in Fig. 16. In the case of the reflection-type optical flber probe, the volume from which the probe can detect the in form a -tion depends on flber conflguration. The intensity distribution of reflected light as a function of the distance between a fiber tip and a flat surface is shown in Fig. 17. The size of the core of the optical fiber should be selected depending on the maximum diameter of particles being investigated. If individual particle passages are to be detected, the fiber core diameter should be less than or equal to the particle diameter. On the other hand, if only the passage of solids or the solids concentration is to be detected, the fiber diameter of a... 

Fiber-optical probes of the reflection type (backscattering) has been used to measure local solid volume fraction, local bubble velocity and bubble chord length (vertical bubble dimension) in fluidized bed reactors operating in different flow regimes. 

A few ATR probes are commercially available. In the near-IR ATR probes are mostly used as easy-to-use sticking probes for liquids and solids. As the aim is primarily to identify a material, not to measure low concentrations, probes with typically one or two reflections (Figure 5-d) are used. In the mid-IR, similar layouts can be found, using e.g. zinc selenide, germanium or silicon crystals as sensing elements. More sensitive and generally better suited for industrial process control DiComp -type probes (Figure 5-e). The actual ATR element is in this case a thin diamond disc supported by a suitably shaped ZnSe crystal. ATR probes of that type are available off the shelf with between one and nine reflections. If more... 

Short path length flow cells may be used in lower volume, or lower flow situations. The extremely small gap between the optics of fibers limits them to these types of applications. It is possible to make flow cells with sample gaps as small as 25 um, so that they may be used with highly absorbing species. With even higher absorbing species, attenuated total internal reflection (ATR) probes may be appropriate (see below). 

Attenuated total internal reflection (ATR) probes offer several advantages over other probe types. ATR is a phenomenon that relies on a difference in the index of refraction of a crystal and that of the solution with which it is in contact to prevent light from escaping the crystal. Only the evanescent wave of the light interacts with the solution layer at the crystal face. The result is an optical pathlength of only a few microns. Typical designs make use of faceted crystals or hemispheres (see Figure 6.1). The most common ATR material in the UV-vis is sapphire. In rare cases, fused silica may be used. ATR allows spectra to be taken of neat samples with optical density (OD) of 500-1000... 

The use of dinitrosyl-Fe(i) and -Mn(ii) as relaxation probes in the above type of study has been discussed. (216) A correlation is found between the proton hyperfine splittings of HR and the J(H-H) values of the parent hydrocarbons HRH, which reflects a similarity in the origins of these two parameters. (217)... 

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