Optical Measurement of Temperature

The laws of radiation enable us to measure temperature in four different ways, viz. 

The practical application of the optical method of measuring temperature is made possible by the fact that the radiation from most solid bodies at high temperatures corresponds very closely to that from a perfect black body. In many metallurgical processes the temperature of a mass of material in a furnace has to be measured, and here the blackness of the radiation is still more perfect, as the furnace acts to a certain extent like a Prevost chamber. 

In calculating the temperature for a constant wave length. Wanner used Wien s radiation law in the form 

For small values of XT, namely up to about XT = 3000, this equation gives values almost identical with those calculated from Planck s law. This condition is fulfilled for visible wave lengths at not too high temperatures. 

Lummer and Pringsheim determined the temperature of a black body by methods (1), (3), and (4), and obtained the following very concordant values  

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The transport of thermal energy can be broken down into one or more of three mechanisms conduction--heat transfer via atomic vibrations in solids or kinetic interaction amongst atoms in gases1 convection - - heat rapidly removed from a surface by a mobile fluid or gas and radiation—heat transferred through a vacuum by electromagnetic waves. The discussion will begin with brief explanations of each. These concepts are important background in the optical measurement of temperature (optical pyrometry) and in experimental measurement of the thermally conductive behavior of materials. 

Kno] Knotek, O., Lugscheider, E., Reimann, H., Sasse, H.G., High-Temperature Differential Thermal Analysis with Optical Measurement of Temperature (in German), Metal, 35(2), 130-132 (1981) (Phase Diagram, Experimental, 9)... 

J.S. McCormack Remote optical measurements of temperature using fluorescent materials. Electr. Lett. 17, 630 (1981)... 

Optical metiiods, in both bulb and beam expermrents, have been employed to detemiine tlie relative populations of individual internal quantum states of products of chemical reactions. Most connnonly, such methods employ a transition to an excited electronic, rather than vibrational, level of tlie molecule. Molecular electronic transitions occur in the visible and ultraviolet, and detection of emission in these spectral regions can be accomplished much more sensitively than in the infrared, where vibrational transitions occur. In addition to their use in the study of collisional reaction dynamics, laser spectroscopic methods have been widely applied for the measurement of temperature and species concentrations in many different kinds of reaction media, including combustion media [31] and atmospheric chemistry [32]. 

Another early fiber optic refractive "sensor" was the one for measurement of temperature and salinity variations of sea water31. The sensing region consisted of a partly uncovered light guide. It detects salinity variations in water of known temperature, and temperature variations in water of known salinity with an accuracy of +/- 2 g/L and 1 °C, respectively, at NaCl concentrations of 300 g/L. 

Ivanov V.N., Ivanov S.V., Kel baHkhanov B.F., Klimova L.G., Trubnikov B.N., Chemyi V.V., Elisashvili D.T., Measurement of temperature and salinity variations of water with a fiber-optical sensor, Fizika Atmosfery i Okeana 1985 21 555. 

The measurement of temperature is necessary for the calibration of most probes like blood oxygen, pH, ions, voltage, and carbon dioxide sensors. The use of optical methods to invasively measure physiological temperature has the advantage of electrical isolation, when compared to traditional approaches like the use of thermocouplers. 

D. Optical measurements of inception and temperature Boedecker, L. R., and Dobbs, G. M., Proc. Combust. Inst. 

FIG. 4 Apparatus for measuring the temperature dependence of contact angles, based on the optical measurement of the capillary rise of the liquid at a vertical plate. 

Fixed points for the calibration of the optical pyrometers are the silver, gold, and copper points and higher temperature secondary fixed points such as those given in Table 2. Calibration at other temperatures can be accomplished by use of a rotating sector or a filter of accurately known transmission factor between the fixed-point source and the pyrometer, in order to simulate a source of lower temperature in accordance with the Planck equation. Such sectors or filters are used also to permit the optical pyrometer to be used for the measurement of temperatures much above 2000 K. 

We have developed several new measurement techniques ideally suited to such conditions. The first of these techniques is a High Pressure Sampling Mass Spectrometric method for the spatial and temporal analysis of flames containing inorganic additives (6, 7). The second method, known as Transpiration Mass Spectrometry (TMS) (8), allows for the analysis of bulk heterogeneous systems over a wide range of temperature, pressure and controlled gas composition. In addition, the now classical technique of Knudsen Effusion Mass Spectrometry (KMS) has been modified to allow external control of ambient gases in the reaction cell (9). Supplementary to these methods are the application, in our laboratory, of classical and novel optical spectroscopic methods for in situ measurement of temperature, flow and certain simple species concentration profiles (7). In combination, these measurement tools allow for a detailed fundamental examination of the vaporization and transport mechanisms of coal mineral components in a coal conversion or combustion environment. 

Henrici et al. [504] carried out a shock tube study of the CO + F2O reaction in mixtures heavily diluted with argon at higher temperatures. They obtained data on overall CO2, O2 and COF2 production from single pulse experiments, and they also made time-resolved optical measurements of the rate of formation of CO2 and depletion of F2O by studying the emission at 4.3 pm and the absorption at 2200 A, respectively. The major path for the decomposition of F2O was assumed to be by reactions (xcii)—(xciv)... 

Although the optical pyrometer is essential for the measurement of temperatures above 1,500°C. its usefulness is by no means confined to the high temperature range. The thermocouple cannot be adapted to many processes at low temperatures for example, the measurement of the temperature of steel rails as they pass through the rolls, ingots and forgings in the open and small sources such as a heated wire or lamp filament. The temperatures used in the above processes may be accurately measured by the optical pyrometer. The temperature of a microscopic sample of any material can be measured by a modified form of the Leeds Northrup pyrometer. Also in many processes a thermocouple is not so convenient to use as an optical pyrometer, especially when the temperature is not required often enough to warrant a permanent installation of thermocouples. 

Actually it is at about the same temperature as the less bright surrounding wall. On account of reflection a corresponding bright patch appears on the opposite wall although this wall may be free from coke. It is evident that the measurement of temperature of a portion of a non-uniformly heated furnace by means of an optical pyrometer is difficult unless the precautions suggested above are taken. As soon as the furnace attains temperature uniformity and equilibrium the optical pyrometer gives the true temperature very easily and readily. 

Transparency or turbidity is often measured on site with a coupled electrode for the measurement of temperature and dissolved oxygen (more and more measured by optical sensor or optrode). For Rhode Island Volunteers, complementary determinations (pH, alkalinity, TP, chl-a, Ca, Mg, Na and Cl) are carried out in a laboratory and results are statistically similar to those collected by professionals (Herron, 2006). 

In addition to reaction chambers and delivery systems, a number of supervising and sensor systems are of utmost importance for control and safety reasons. Sensors in automated workstations include measurement of temperature (thermocouple, thermistor, semiconductor), pressure, liquid flow and gas or liquid level. To monitor the presence or absence of vessels or devices, systems like capacitance, inductivity, ultrasonic monitors, magnetic sensors or optical sensors (reflective, beam interruption, color) can be integrated in automated workstations. 

Benzoic acid dimer was the first carboxylic dimer to be investigated in detail spectroscopically. The first studies involved the measurements of temperature dependent relaxation times via NMR. Due to the presence of two inherent chromo-phores it can also be investigated by means of optical spectroscopy. 

Unlike intrusive thermocouple probes, optical techniques of temperature measurement do not disturb the combustion process (see Chapter 12). Sodium line-reversal is an optical technique first used for measuring flame temperature by Kurlbaum [3] as early as 1902. Although this technique has been improved through the years, line-reversal methods use line of sight optics... 

The CARS measurement system consists of two high powered pulsed lasers, optics and detection equipment, and specialized software for a system dedicated to the acquisition and analysis of CARS spectra for the time resolved point measurements of temperature in a large scale flame. 

Klimant, L, Kuhl, M., Glud, R.N. and Holst, G., 1997. Optical Measurement of Oxygen and Temperature in Microscale Strategies and Biological Applications. Sensors and Actuators B 38 29-37. 

Examples of flame temperature for various oxidizers and shellac (as fuel) with 10% sodium oxalate (which is necessary to optically measure flame temperature) [Shimizu]. 

Some years ago D. Stuerga designed a microwave reactor, called the RAMO (reac-teur autoclave microonde), which is not a commercial device. The microwave applicator and the reactor are original. The resonant frequency of the cavity can be controlled by varying the position of a plunger. The effective cavity power can be increased by three orders of magnitude. The autoclave is made of polymeric materials, which are microwave transparent, chemically inert, and sufficiently strong to accommodate the pressures induced. The reactants are placed in a Teflon flask inserted within a polyetherimide flask. A fiber-optic thermometry system, a pressure transducer, and a manometer enable simultaneous measurement of temperature and pressure within the reactor. The system is controlled by pressure. The reactor is shown in Fig. 2.32. 

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