Thermometers, fiber-optic

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 ...

K. T. V. Grattan, A. W. Palmer, and Z. Zhang, Development of a high-temperature fiber-optic thermometer probe using fluorescent decay, Rev. Sci. Instrum. 62(5), 1210-1213. 

Fiber-optic thermometers can be applied up to 300°C, but are too fragile for real industrial applications. In turn, optical pyrometers and thermocouples can be used, but pyrometers measure only surface temperatures which in fact can be lower than the interior temperatures in reaction mixtures. Application of thermocouples which in case of microwaves are metallic probes, screened against microwaves, can result in arcing between the thermocouple shield and the cavity walls leading to failures in thermocouple performance. 

For both type of microwave reactors, if the reactor is not supplied with a temperature sensor or more likely accurate temperature measurment is prerequisited during an experiment, the fiber-optic temperature sensor is directly applied to the reaction mixture. In order to secure the sensor from harsh chemicals, the sensor is inserted into a capillary that in turn is inserted into the reaction mixture. In such a case, it is strongly advocated to use capillaries that are made of quartz glass and are transparent to microwave irradiation. Any capillary that is made of glass or even borosilicate glass can always slightly absorb microwave energy, in particular, while the reaction mixture does not absorb microwaves efficiently, and in turn lead to failures of fiber-optic thermometer performance. 

Domestic ovens can be inexpensively and safely modified, however this almost eliminates these disadvantages and enables independent temperature measurement and reasonable temperature control. For temperature measurement an IR thermometer or, better, a fiber-optic thermometer [75-77] has been recommended. Such a batch microwave reactor made by modification of a domestic microwave oven is depicted in Fig. 13.1 and has been described elsewhere (Refs. [51, 75-77, 141-144] and references cited therein). 

The epoxy prepolymer and the curing agent were mixed together prior to use, then the epoxy mixture (13 g) was poured into moulds (inside dimension 96 mmx 16 mmx8 mm), which were irradiated in a microwave applicator with TEqi propagation mode. The sample temperature was measured continuously by means of an infrared pyrometer that gave the surface temperature and fiber-optic thermometer that recorded the bulk temperature. Samples cured by both thermal... 

Traditional method as by spirit thermometer, mercury thermometer or thermocouple thermometer cannot be used to measure temperature profile of foods under microwave heating conditions. However, fiber optic thermometer is able to complete the task. In this chapter, MWS OSR-8 type fiber optic thermometer system made by FISO Technology (Canada) is used to obtain temperature-time profile of edible oils under different microwave power conditions. 

There is a general agreement that the application of fiber-optic thermometers is a reliable way to determine temperature under microwave conditions. However, according to some studies in which the reaction mixture was also monitored with a thermovision camera, it was shown that for the reactions in heterogeneous systems under microwave irradiation, the temperature measurement with a fiber-optic thermometer can lead to serious errors like pyrometry in particular, this is observed in those experiments that are planned without any attention being paid to temperature homogeneity of the reaction mixture. ... 

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