Temperature measurement thermometry

For the analysis of the chemical structure of flames, laser methods will typically provide temperature measurement and concentration profiles of some readily detectable radicals. The following two examples compare selected LIF and CRDS results. Figure 2.1 presents the temperature profile in a fuel-rich (C/O = 0.6) propene-oxygen-argon flame at 50 mbar [42]. For the LIF measurements, 1% NO was added. OH-LIF thermometry would also be possible, but regarding the rather low OH concentrations in fuel-rich flames, especially at low temperatures, this approach does not capture the temperature rise in the flame front [43]. The sensitivity of the CRDS technique, however, is superior, and the OH mole fraction is sufficient to follow the entire temperature profile. Both measurements are in excellent agreement. For all flames studied here, the temperature profile has been measured by LIF and/or CRDS. 

Droplet temperature is of interest in practical spray processes since it influences the associated heat and mass transfer, chemical reactions, and phase changes such as evaporation or solidification. Various forms of Rayleigh, Raman and fluorescence spectroscopies have been developed for measurements of droplet temperature and species concentration in sprays.16471 Rainbow refractometry (thermometry), polarization ratioing thermometry, and exciplex method are some examples of the droplet temperature measurement techniques. 

Several theories have been developed to explain the rainbow phenomena, including the Lorenz-Mie theory, Airy s theory, the complex angular momentum theory that provides an approximation to the Lorenz-Mie theory, and the theory based on Huy gen s principle. Among these theories, only the Lorenz-Mie theory provides an exact solution for the scattering of electromagnetic waves by a spherical particle. The implementation of the rainbow thermometry for droplet temperature measurement necessitates two functional relationships. One relates the rainbow angle to the droplet refractive index and size, and the other describes the dependence of the refractive index on temperature of the liquid of interest. The former can be calculated on the basis of the Lorenz-Mie theory, whereas the latter may be either found in reference handbooks/literature or calibrated in laboratory. 

The ITS is an artifact scale, designed to relate temperature measurements made with practicable instruments as closely as possible to the thermodynamic scale. The scale is established and controlled by the International Committee of Weights and Measures (BIPM) through its Consultative Committee on Thermometry, which was established in 1937. The BIPM itself is established to maintain and implement the Treaty of the Meter, to which most nations of the wodd subscribe thus the ITS has not only scientific but legal status in most nations. Within nations, the Temperature Scale is maintained by national standards establishments, eg, in the United States the National Institute for Standards and Technology (NIST), in England the National Physical Laboratory (NPL), and in Germany the Physikalisch-Technische Bundesanstalt (PTB). 

Practical measurements of temperature long preceded the theory of this important concept. Thermodynamics clearly requires the temperature concept, but thermometry (the theory of temperature measurements) is so deeply intertwined with general thermodynamic theory that we must take care to avoid logical circularity. 

A temperature of 0 K is called absolute zero . It coincides with the minimum molecular activity, i.e., thermal energy of matter. The thermodynamic temperature was formerly called absolute temperature . In practice, the International Temperature Scale of 1990 (ITS-90) [i] serves as the basis for high-accuracy temperature measurements. Up to 700 K, the most accurate measurements of thermodynamic temperature are the NBS/NIST results for Constant Volume Gas Thermometry (CVGT). Above 700 K, spectral radiometry is used to measure the ratio of radiances from a reference... 

In principle, a resonance frequency or difference in frequencies can be employed to perform NMR thermometry. The accuracy of temperature measurement depends on the accuracy of the thermocouple (0.2°C) in the NMR spectrometer. Generally, a precision of better than 0.2°C has been reported with these NMR thermometry measurements. Individual calibration plots may not be required when using aqueous buffers. Proper calibration procedures should be adopted with nonaqueous buffers and in the presence of organic modifiers. 

The ITS-90 scale extends from 0.65 K to the highest temperature measurable with the Planck radiation law (—6000 K). Several defining ranges and subranges are used, and some of these overlap. Below —25 K, the measurements are based on vapor pressure or gas thermometry. Between 13.8 K and 1235 K, Tg is determined with a platinum resistance thermometer, and this is by far the most important standard thermometer used in physical chemistry. Above 1235 K, an optical pyrometer is the standard measrrremerrt instmment. The procedtrres used for different ranges are sttmmarized below. 

The observed temperature dependence of the absorption cross section of 3-pentanone and the corresponding fluorescence intensity offers the possibility for a new type of temperature measurements.This technique gives access to 2D-temperature distributions between 300 and 1000 K relevant for precombustion conditions that could hardly be assessed with other laser spectroscopic techniques developed for combustion thermometry. By calculating temperatures from the ratio of simultaneously acquired intensity distributions, the measurement is independent on local tracer concentrations. Measurements in inhomogeneously mixed environments are therefore feasible. 

Reliable temperature measurement in a microwave reaction presents a considerable challenge to the experimentalist, as the microwave field directly affects conventional instruments such as thermometers and thermocouples. Although thermocouples may be used if they are suitably shielded and earthed, there is an inevitable perturbation to the microwave field pattern. A number of other methods are available that are appropriate to use at moderate temperatures. A gas pressure thermometer, or a microwave-transparent liquid thermometer, may be used as inexpensive options, whilst thermal imaging and fluoro-optic thermometry, although expensive, provide more reliable, higher precision information. 

Coherent Anti-Stokes Raman Scattering (CARS) Thermometry is a technique for temperature measurement in high temperature environments using a third-order nonlinear optical process involving a pump and a Stokes frequency laser beam that interacts with the sample and generates a coherent anti-Stokes frequency beam. 

A discussion of the present standards of temperature measurement is preceded by a discussion of the history of thermometry. The goal of Sanctorius, the first person to perceive the potential usefulness of the thermometer in medicine, has not changed in principle since the early seventeenth century. He hoped to obtain quantitative measurements rather than subjective appraisals. For us, this should be a caution not to ignore careful temperature measurements as we study complex reactions. 

Precision thermometry based on the thermodynamic temperature scale had its beginnings with the work of P. Chappuis and of H. L. Callendar during the period from the late 1880s to the early 1900s. Chappuis transferred the hydrogen scale in the range from 0 to 100°C, provided by his constant-volume hydrogen gas thermometer, to several carefully made mercury thermometers (Chappuis, 1888). These were then used to calibrate many other mercury thermometers which in turn were to be used in many countries to put temperature measurements on the same scale. The probable uncertainty of those thermometers was stated to be 0.002°C. 

Another noncontact technique for measuring high temperatures involves Raman spectroscopy, in particular the nonlinear process known as coherent anti-Stokes Raman spectroscopy (CARS) (Radiation Thermometry, 1982). This technique is finding practical applications in measurements of temperatures of flames (in internal combustion engines, in jet engines) and of hot gases. The imprecision of such temperature measurements is generally a few percent. 

Finally, we mention several current applications somewhat outside of biochemistry in the usual sense. Thermography has slowly been coming to the fore. Many of the problems associated with the analysis of thermograms were treated at the Fifth International Symposium on Temperature (Plumb, 1972) in 1972 and new applications were discussed at the Sixth Symposium (Schooley, 1982) in 1982. Of perhaps more current interest is the greatly expanded interest in temperature measurement in hyperthermia and hypothermia. A recent New York Academy of Sciences conference has done an excellent job of reviewing this (Ann. N.Y. Acad., 1980). Cetas also wrote a general review of thermometry in this field (Cetas, 1968). Perhaps the most exciting new method in thermometry is that of optical fluorescence, which we described earlier. Catheters, whole-body scanners, etc., have been made for use with this method. At this point, 0.01°C is probably the least imprecision that can be obtained with the commercial instrument (Luxtron), with data obtained every 0.1 sec. Improvements are likely, however, as needs are made known to the company. 

Guildner, L. A. and Burns, G. W. (1979), Accurate Thermocouple Thermometry", in High Temperatures—High Pressures 11, 183 and Manual on the Use of Thermocouples in Temperature Measurement (1981), American Society for Testing and Materials, Baltimore, Maryland. 

Introduction. Historically, low-pressure, constant-volume gas thermometers were the only primary thermometers that had the accuracy required for determining the temperatures of defining fixed points. Recent advances in thermometry have resulted in the development of several types of primary thermometers capable of accurate thermodynamic temperature measurements [5-6]. A list of present-day primary thermometers capable of thermodynamic temperature measurement includes ... 

Resistive materials used in thermometry include platinum, copper, nickel, rhodium-iron, and certain semiconductors known as thermistors. Sensors made from platinum wires are called platinum resistance thermometers (PRTs) and, though expensive, are widely used. They have excellent stability and the potential for high-precision measurement. The temperature range of operation is from -260 to 1000°C. Other resistance thermometers are less expensive than PRTs and are useful in certain situations. Copper has a fairly linear resistance-temperature relationship, but its upper temperature limit is only about 150°C, and because of its low resistance, special measurements may be required. Nickel has an upper temperature limit of about 300°C, but it oxidizes easily at high temperature and is quite nonlinear. Rhodium-iron resistors are used in cryogenic temperature measurements below the range of platinum resistors [11]. Generally, these materials (except thermistors) have a positive temperature coefficient of resistance—the resistance increases with temperature. 

Thermometry can also be realized with techniques that are less popular within the combustion research community. For example, LITGS and PS, mentioned before for the detection of chemical species, can be useful for temperature measurements. The principles of operation do not differ from earlier remarks. For example, the temperature in LITGS can be extracted from the measured oscillating temporal decay of the signal fitted with the theoretical model having the temperature as free parameter. 

Temperature is undeniably the most important property for all calorimetric measurements, because it is the common denominator. Two different techniques for temperature measurements are used for pulse calorimetry contact thermometry (e.g. thermocouples) and radiation thermometry or pyrometry. Because pulse calorimetry is often used to handle and measure liquid materials, non-contact radiation thermometry is far more common in pulse-heating than contact thermometry. Other reasons for non-contact temperature measurement methods include the fast heating rates and temperature gradients (inertia of the thermocouples), difficulties mounting the contact thermometers (good thermal contact needed), and stray pick-up in the thermocouple signal because the sample is electrically self-heated. 

For more information on temperature measurements, a general review article concerning high-temperature thermometry (not necessarily limited to use in fast dynamic pulse techniques) is given in [94]. 

A knowledge of the electrolyte temperature is important in CE as temperature changes in the electrolyte influence precision, accuracy, separation efficiency, and method robustness [7,14,32], During the past two decades, a considerable amount of research has been conducted toward electrolyte temperature measurements in CE [1,14,19,21,32 2], Early methods have included using the variation of electroosmotic mobility (eieof), electrophoretic mobility (p-ep), and electrical conductivity (k) to measure temperature [38,39], More recently, techniques such as external thermocouples [21], Raman thermometry [19,40], NMR spectroscopy [32,35], thermochromic probes [41], and the variation in fluorescence response [42] have been used to measure temperatures. Most of these methods require the modiflcation of the existing instrument and/or the purchase of additional equipment. 

Temperature measurement using a single fluorescent dye is the simplest adaptation of fluorescent thermometry at the microscale. If the intensity of the illuminating light flux, /q, is assumed constant in both space and time, then the ratio of the emitted fluorescence intensity measured at a temperature T to that measured at a reference temperature for a fixed dye concentration is given by the ratio of the quantum efficiencies of the fluorescent dye at these temperatures ... 

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