Optical fiber thermometer

A high temperature optical fiber thermometer has beea developed (32,33). It coasists of a sputtered iridium blackbody tip oa a single crystal sapphire laser. Such a device has beea showa to be accurate to within 0.03° C at 1000°C. 

The other limit is the problem of temperature measurements. Classical temperature sensors could be avoided in relation to power level. Hence, temperature measurements will be distorted by strong electric currents induced inside the metallic wires insuring connection of temperature sensor. The technological solution is the optical fiber thermometers [35-39]. However, measurements are limited below 250 °C. For higher values, surface temperature can be estimated by infrared camera or pyrometer [38, 40], However, due to volumic character of microwave heating, surface temperatures are often inferior to core temperatures. 

Figure 3.5 Reactor for batchwise organic synthesis. 1. Reaction vessel 2, top flange 3, cold finger 4, pressure meter 5, magnetron 6, forward/reverse power meters 7, magnetron power supply 8, magnetic stirrer 9, computer 10, optic fiber thermometer 11, load matching device 12, waveguide 13, multimodal cavity (applicator). (From Ref. 712, reproduced with permission.)...

Z. Y. Zhang, K. T. V. Grattan, and A. W. Palmer, Sensitive fibre optic thermometer using Cr LiSAF fluorescence for bio-medical sensing applications, Proc. 8th Optical Fiber Sensors Conf., Monterey, California, pp. 93-96, IEEE, New York (1992). 

Another special case is the fiber optic-coupled thermometer, which allows inaccessible targets to be measured by replacing the optic with a flexible or rigid fiber optic bundle. This, of course, limits the spectral performance, and hence the temperature range, to the higher values, but it has allowed temperature measurements to be made when previously none were possible. 

Possible future applications of up-converting phosphors include (i) three-dimensional displays 249-251 (ii) fiber optic amplifiers (referred to above) that operate at 1.55, 1.46, and 1.31 pm,, 2 1-255 (iii) up-conversion lasers 250 and (iv) remote sensing thermometers for high-temperature applications (utilizing the temperature dependence of optical properties of, for example, cubic Y203 Er3+).256-258... 

A particularly difficult problem in microwave processing is the correct measurement of the reaction temperature during the irradiation phase. Classical temperature sensors (thermometers, thermocouples) will fail since they will couple with the electromagnetic field. Temperature measurement can be achieved either by means of an immersed temperature probe (fiber-optic or gas-balloon thermometer) or on the outer surface of the reaction vessels by means of a remote IR sensor. Due to the volumetric character of microwave heating, the surface temperature of the reaction vessel will not always reflect the actual temperature inside the vessel [7]. 

Fig. 10.2 Schematic diagram of the microwave batch reactor 1. reaction vessel, 2. retaining cylinder, 3. top flange, 4. cold finger, 5. pressure meter, 6. magnetron, 7. power meters, 8. power supply, 9. stirrer, 10. fiber optic thermometer,...

The reactor has facilitated a diverse range of synthetic reactions at temperatures up to 200 °C and 1.4 Pa. The temperature measurements taken at the microwave zone exit indicate that the maximum temperature is attained, but they give insufficient information about thermal gradients within the coil. Accurate kinetic data for studied reactions are thus difficult to obtain. This problem has recently been avoided by using fiber optic thermometer. The advantage of continuous-flow reactor is the possibility to process large amounts of starting material in a small volume reactor (50 mL, flow rate 1 L hr1). A similar reactor, but of smaller volume (10 mL), has been described by Chen et al. [117]. 

In the system which uses crystalline alexandrite as the sensor material/381 a measurement reproducibility of 1 °C is achieved over a wide temperature region from 20 to 700°C. The same technique is applied to another fiber optic thermometer system which is designed for biomedical sensing applications and uses LiSrAlF6 Cr3+ as sensor material/391 The standard deviation of the measurement recorded by this system is better than 0.01°C within the 20 Cand 50°C region. 

Figure 11.15. Schematics of the optical arrangement and temperature probes for the Cr+ fluorescence lifetime-based fiber optic thermometers. F = short-pass optical filter Fa = bandpass or long-pass optical filter LD = laser diode LED = light emitting diode S = the fluorescence material used as sensing element vm = signal to modulate the output intensity of the excitation light source v/= the detected fluorescence response from the sensing element.

Figure 11.18. The ruby fluorescence lifetime-based fiber optic thermometer system. Fi short-pass optical filter Ft /f-line band pass optical filter.

The subsequent development of laser diode sources at low cost, and improved electronic detection, coupled with new probe fabrication techniques have now opened up this field to higher-temperature measurement. This has resulted in an alexandrite fluorescence lifetime based fiber optic thermometer system,(38) with a visible laser diode as the excitation source which has achieved a measurement repeatability of l°C over the region from room temperature to 700°C, using the lifetime measurement technique. 

Compared to the great diversity in proposed fiber optic sensor ideas, the types of fiber optic thermometers that are commercially available are actually quite few. Though several reports have been given by a major manufacture 68> 69) reviewing various existing commercial systems, cross-comparisons between the performances of these systems are rarely made. 

Table 11.3. Assessment of Early Commercialized Fiber Optic Thermometer Systems, by Harmed70 ...

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