Fibre optic techniques

K. T. V. Grattan, Fibre optic techniques for temperature sensing, in Fibre Optic Chemical Sensors and Biosensors, Vol. II (0. S. Wolfbeis, ed.), pp. 151-192, CRC Press, London (1991). 

These include such instruments as opacity monitors, turbidimeters, colorimeters, refractometers and spectrophotometers. A selection of these is described—particularly where the instrument has a more general application as an on-line process analyser and/or to illustrate a general principle of operation. It is likely that development of fibre-optic techniques (Section 6.12.4) will extend the use of this type of sensor in the future(56). 

The light emitted from the excitation source is dispersed and recorded by multiple photodetectors placed behind suitably located exit slits (Fig. 6.65). Fibre-optical techniques can also be used to collect light at a suitable location in the spectrometer focal plane for transmission to a battery of PMT.s. Calibration can be provided using standard solutions or standard electrodes. Using a computer-controlled data collection system the concentrations of the selected elements can be printed out shortly after the introduction of the sample. A complete system frequently incorporates means for automatic sample exchange. 

In a completely different application, Shimoyama et al. [530] have reported a 3D fluorescence method using quartz fibre optics for the non-destructive determination of colorants on woodblock prints. Cleve et al. [531] have developed an on-line fluorescence spectroscopic method to measure sizing effects and optical brightening agents in polyamide woven fabrics. According to Allen [532] the in situ capability of fluorescence is very viable as a monitoring probe via a fibre optic technique. 

ABSTRACT. Characteristics and fluid dynamics of gas phase recirculation in a novel Riser Simulator Reactor have been investigated using constant temperature hot wire anemometry. In situ concentration and velocity measurements enabled to evaluate the mixing time and the inner recirculation ratio of the gas phase. In addition, fibre optic techniques allowed to characterize the degree of fluidization of the catalyst particles and the effect of gas phase density changes. By combining the anemometry and the fibre optic techniques, mixing patterns in the Riser Simulator have been evaluated. The importance of the study can be realized in the context of the potential use of the Riser Simulator for gas-solid reaction kinetics. 

To further evaluate the fluid-solid mixing characteristics taking place in the catalyst bed a fibre optic probe technique was used. The fibre optic probe was inserted from the side of the reactor through the basket where the probe contacts with catalyst particles. A schematic of the system is shown in Figure 1. It has to be noted that two models, cold and hot model, were constructed to apply the fibre optic technique (Kraemer, 1990). The cold model was made of plexiglass material and had the same internal dimensions as the hot model or the Riser Simulator Reactor made of inconel. 

The fibre optic technique was previously developed for measuring hydrodynamic parameters in a gas-solid fluidized bed (Huskey, 1990) and in three-phase fluidized beds (Lee, 1986 Chabot,... 

From the fibre optic technique and pressure drop measurement it was proven that the FCC catalyst was fluidized at the impeller rotational speed of above 6300 RPM and somewhere above 10,000 RPM pneumatic transport could occur. The condition that favours the pneumatic transport was not desired because the catalyst could eventually become stuck at the upper grid. Therefore 7875 RPM, which corresponds to 75% of the motor speed on the speed controller, was taken as the normal operational speed of the impeller and falls in the well fluidized regime (see Figure 8 and 9). 

The continuous determination of compounds, which may adversely affect ecosystems and/or human health, is a major regulative and legislative goal of environmental protection nowadays. Considering the costs and efforts related to this task corroborates a clear demand for portable, real-time, in-situ, field applicable and cost-effective monitoring techniques. Due to their inherent properties, vibrational spectroscopic sensors, in particular fibre-optic sensors show a high potential to contribute to these applications. 

This chapter reviews the development of optical gas sensors, starting with an initial emphasis on optical-fibre remoted techniques and finishing with a particular focus on our own group s work on highly selective methods using correlation spectroscopy. This latter section includes extensive theoretical modelling of a correlation spectroscopy method, and compares theory with practice for a CO2 sensor. 

Gas detection methods may be split into two groups, (i) direct methods, which monitor a physical parameter of the target gas, and (ii) indirect methods, which use a chemical reaction or indicator to show the concentration of the gas being sensed. This division of methods may be further split into optical sensing techniques and non-optical techniques. This review will predominantly focus on direct optical-spectroscopy fibre sensing techniques. 

There is increasing interest in the use of specific sensor or biosensor detection systems with the FIA technique (Galensa, 1998). Tsafack et al. (2000) described an electrochemiluminescence-based fibre optic biosensor for choline with flow-injection analysis and Su et al. (1998) reported a flow-injection determination of sulphite in wines and fruit juices using a bulk acoustic wave impedance sensor coupled to a membrane separation technique. Prodromidis et al. (1997) also coupled a biosensor with an FIA system for analysis of citric acid in juices, fruits and sports beverages and Okawa et al. (1998) reported a procedure for the simultaneous determination of ascorbic acid and glucose in soft drinks with an electrochemical filter/biosensor FIA system. 

C.W. Brown, Fibre optics in molecular spectroscopy, in Modern Techniques in Applied Molecular Spectroscopy, F.M. Mirabella (ed), John Wiley Sons, New York, 1998. 

Brown, C.W., Fibre Optics in Molecular Spectroscopy. In Mirabella, F.M. (ed.), Modern Techniques in Applied Molecular Spectroscopy, John Wiley 8t Sons New York, 1998 Chapter 10. pp. 377-402. Chalmers, J.M. and Griffiths, P.R. (eds), Handbook of Vibrational Spectroscopy John Wiley 8c Sons New York, 2002. 

An interesting modification of this technique is the fibre-optic dynamic anemometer (FODA)143. A length of fibre-optic cable carries the laser beam to the interior of the dispersion. Back-scattered light, with its Doppler frequency shift, is returned to the detector along with reflected light and, again, the resulting beat frequency pattern is analysed. Since only a very small volume around... 

Optoelectronic components for remote control and monitoring and for data transmission are being installed in hazardous areas more and more. In particular, the optical-fibre transmission technique enables numerous modem applications. Apart from the great advantages of this technology, the risk of an ignition caused by optical radiation at a power level sufficiently high is to be taken into account (see Section 1.2.7). 

A block-scheme of this apparatus is presented in Fig. 2.12, A and B. Compared to that given in Fig. 2.11, an adequate improvement is the use of fibre optic cable which conducts the light from Hg-Xe arc lamp to the reflected light microscope. The thickness is determined by the microinterferometric technique of Scheludko-Exerowa (see Section 2.1). 

The advantages of NIR diffuse reflectance techniques which rapidly develop are due to the direct analysis of solids without any necessity for special sample preparation (Weyer, 1985 Stark et al., 1986 Murray and Cowe, 1992). In NIR absorption and reflectance techniques fibre optics may be used which allow analysis remote from the spectrometer. 

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