Thermometry

The experiments described in this paragraph are the basis of thermometry and of calorimetry, that is, of the scientific measurement of temperatures and of quantities of heat. The advance of science has brought with it considerable alterations and improvements in the methods of measurement. Some of the more important methods in use at the present day will be described in the following pages. 

The instruments used in the measurement of temperature may be divided into five classes, namely  

In the chnical chemistry laboratory, measurements of temperature are made primarily to verify that devices measure within the prescribed temperature limits. Water baths or heated cells where reactions take place are examples of such devices, as are refrigerators, whose temperatures must be measured and recorded daily to meet laboratory regulatory requirements. 

The two most popular types of thermometers in the chemistry laboratory are hquid-in-glass thermometers and thermistor probes. A thermistor made of a transition metal 

All thermometers must be verified against a certified thermometer before being placed into use. For example, the NIST SRM 934 is a mercury-in-glass thermometer with calibration points at 0 °C, 25 °C, 30 °C, and 37 °C. Some manufacturers supply hquid-in-glass thermometers that have ranges greater than the SRM thermometer and are verified to have been calibrated against the NIST thermometers. Details of the verification of the calibration of a thermometer have been described in a NCCLS standard.  

The NIST also supplies several materials that melt at a known temperature. For example, gaUium (SRM 1968) and rubidium (SRM 1969), which melt at 29.7723 °C and 39.3 °C, respectively, are particularly useful in the clmical laboratory (see Table 1-10). 

The procedure is easily reduced to a formula by which the temperature can be calculated from the measured value of the thermometric property y. Let y be the value at the ice point and y be the value at the steam point. These points are separated by 100 degrees. Then 

The right-hand side of this equation is a constant multiplying through by dt and integrating, we obtain 

More generally, suppose we choose any two fixed temperatures to which we assign the arbitrary values and t2 If yi and 2 are the values of the thermometric property at these temperatures, the thermometric equation, Eq. (6.2), becomes 

Fortunately, there is a way out of this predicament. Using the second law of thermodynamics it is possible to establish a temperature scale that is independent of the particular properties of any substance, real or hypothetical. This scale is the absolute, or the thermodynamic, temperature scale, also called the Kelvin scale after Lord Kelvin, who first demonstrated the possibility of establishing such a scale. By choosing the same size degree, and with the usual definition of the mole of substance, the Kelvin scale and the ideal gas scale become numerically identical. The fact of this identity does not destroy the more fundamental character of the Kelvin scale. We establish this identity because of the convenience of the ideal gas scale compared with other possible scales of temperature. 

Having overcome the fundamental difficulties, we use all sorts of thermometers with confidence, requiring only that if the temperatures of two bodies A and B are measured with different thermometers, the thermometers must agree that t tg or that t = tg or that t tg. The different thermometers need not agree on the numerical value of either or tg. If it is necessary, the reading of each thermometer can be translated into the temperature in kelvins then the numerical values must agree. 

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Hall R J and Boedeker L R 1984 CARS thermometry in fuel-rich combustion zones Appl. Opt. 23 1340-6... 

Most thermometry using the KTTS direcdy requites a thermodynamic instmment for interpolation. The vapor pressure of an ideal gas is a thermodynamic function, and a common device for reali2ing the KTTS is the helium gas thermometer. The transfer function of this thermometer may be chosen as the change in pressure with change in temperature at constant volume, or the change in volume with change in temperature at constant pressure. It is easier to measure pressure accurately than volume thus, constant volume gas thermometry is the usual choice (see Pressure measurement). 

The KTTS depends upon an absolute 2ero and one fixed point through which a straight line is projected. Because they are not ideally linear, practicable interpolation thermometers require additional fixed points to describe their individual characteristics. Thus a suitable number of fixed points, ie, temperatures at which pure substances in nature can exist in two- or three-phase equiUbrium, together with specification of an interpolation instmment and appropriate algorithms, define a temperature scale. The temperature values of the fixed points are assigned values based on adjustments of data obtained by thermodynamic measurements such as gas thermometry. 

The ITS-90 has its lowest point at 0.65 K and extends upward without specified limit. A number of values assigned to fixed points differ from those of the immediately previous scale, IPTS-68. In addition, the standard platinum resistance thermometer (SPRC) is specified as the interpolation standard from 13.8033 K to 961.78°C, and the interpolation standard above 961.78°C is a radiation thermometer based on Planck s radiation law. Between 0.65 and 13.8033 K interpolation of the scale rehes upon vapor pressure and constant-volume gas thermometry. The standard thermocouple, which in previous scales had a range between the upper end of the SPRT range and the lower end of the radiation thermometer range, has been deleted. 

Temperature (R) Design of treatment/recovery Resistance thermometry... 

D. P. DeW itt. Theory and Practice of Radiation Thermometry. New York John Wiley Sons, 1988. J. C. Richmond. Applications of Radiation Thermometry. ASTM Special Technical Publication. American Society for Testing and Materials, 1984. 

We have remarked that a temperature of zero on the absolute temperature scale would correspond to the absence of all motion. The kinetic energy would become zero. Very interesting phenomena occur at temperatures near 0°K (the superconductivity of many metals and the superfluidity of liquid helium are two examples). Hence, scientists are extremely interested in methods of reaching temperatures as close to absolute zero as possible. Two low temperature coolants commonly used are liquid hydrogen (which boils at 20°K) and liquid helium (which boils at 4°K). Helium, under reduced pressure, boils at even lower temperatures and provides a means of reaching temperatures near 1°K. More exotic techniques have been developed to produce still lower temperatures (as low as 0.001°K) but even thermometry becomes a severe problem at such temperatures. 

Thermochemistry, 254, 507 Thermodynamic potentials, 99 Thermo-electric circuit, 450 inversion, 451 theories, 453 Thermometers, 3, 140, 166 Thermometry, 1, 353 Thomsen-Berthelot principle, 257, 506... 

Infrared thermometry is to identify the maximum permissible temperature of tube alloys, to determine furnace heat flux, scale heat conductivity, and tube heat transfer rates. 

The ITS-90 scale is designed to give temperatures T90 that do not differ from the Kelvin Thermodynamic Scale by more than the uncertainties associated with the measurement of the fixed points on the date of adoption of ITS-90 (January 1, 1990), to extend the low-temperature range previously covered by EPT-76, and to replace the high-temperature thermocouple measurements of IPTS-68 with platinum resistance thermometry. The result is a scale that has better agreement with thermodynamic temperatures, and much better continuity, reproducibility, and accuracy than all previous international scales. 

Temperatures on ITS-90, as on earlier scales, are defined in terms of fixed points, interpolating instruments, and equations that relate the measured property of the instrument to temperature. The report on ITS-90 of the Consultative Committee on Thermometry is published in Metrologia and in the Journal of Research of the National Institute of Standards and Technology The description that follows is extracted from those publications.3 Two additional documents by CCT further describe ITS-90 Supplementary Information for the ITS-90, and Techniques for Approximating the ITS-90.4... 

At temperatures above the melting point of silver (1234.93 K), radiation thermometry is used. The equation that applies is... 

The original reports on 1TS-90 are found in B. W. Mangum. "Special Report on the International Temperature Scale of 1990. Report on the 17th Session of the Consultative committee on Thermometry." J. Res. Natl. Inst. Stand. TeehnoL. 95. 69 (1990) H. Preston-Thomas, The International Scale of 1990 (ITS-90)." Metro/ogia. 27. 3 (1990). 

This comprehensive review of theoretical models and techniques will be invaluable to theorists and experimentalists in the fields of infrared and Raman spectroscopy, nuclear magnetic resonance, electron spin resonance and flame thermometry. It will also be useful to graduate students of molecular dynamics and spectroscopy. 

Chaudhari AM, Woudenberg TM, Albin M, Goodson KE (1998) Transient liquid crystal thermometry of microfabricated PCR vessel arrays. J Microelectromech Sys 7 345-355 Cheng P, Wu WY (2006) Mesoscale and microscale phase heat transfer. In Greene G, Cho Y, Hartnett J, Bar-Cohen A (eds) Advances in heat transfer, vol 39. Elsevier, Amsterdam Choi SB, Barron RF, Warrington RQ (1991) Fluid flow and heat transfer in micro- tubes. ASME DSC 40 89-93... 

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