Choice of Thermometer

The type of thermometer used to interpolate between the reference points depends on the temperature interval. 

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Thus, the best stable isotope thermometers will have large fractionations, involve relatively common minerals, and occur in rocks where equilibration can be evaluated. The ideal case is a rock where all minerals crystallized at a specific temperature (diagenetic, metamorphic or igneous), and then cooled rapidly in a closed system. In other situations, knowledge of the exchange kinetics of the minerals in question can be important in the choice of thermometer. 

The measurement of temperature is obviously of prime importance in all cryoscopic methods. The precise measurement of temperature and the use of thermometers is well documented. The choice of thermometer will depend on the precision required and thecryoscopic method adopted, which will be largely dictated by the solvent system under investigation. 

Among nonmetallic resistance thermometers, an important class is that of thermistors, or temperature-sensitive semiconductkig ceramics (5). The variety of available sizes, shapes, and performance characteristics is very large. One manufacturer Hsts ki the catalog a choice of characteristics ranging from 100 Q at 25°C to 1 M Q at 25°C. 

Gas thermometers that employ equation (1.10) can be constructed to measure either pressure while holding the volume constant (the most common procedure) or volume while holding the pressure constant. The (pV) product can be extrapolated to zero p. but this is an involved procedure. More often, an equation of state or experimental gas imperfection data are used to correct to ideal behavior. Helium is the usual choice of gas for a gas thermometer, since gas imperfection is small, although other gases such as hydrogen have also been used. In any event, measurement of absolute temperature with a gas thermometer is a difficult procedure. Instead, temperatures are usually referred to a secondary scale known as the International Temperature Scale or ITS-90. 

For conductimetric incremental titrations, large rugged analogue conductivity meters have also become available, e.g., the Metrohm 518 conductometer (in connection with the Model 536 potentiograph its yields a rapid recording of the curve together with end-point indication) and the Philips PW 9505 analogue conductivity meter (in addition to a recorder output and an output for electrode re-platinization, there is a choice of manual or, by use of a Pt 100 resistance thermometer, automatic temperature compensation). 

The choice of a primary thermometer such as the 3He melting pressure thermometer to define the PLTS 2000 witnesses the great difficulties encountered in the measurement of very low temperatures. For example, at the beginning of 1980s, it was realized that differences up to 40% existed in the data of 3He specific heat obtained... 

At relatively high temperatures thermocouple thermometers are most commonly used to measure temperature. The thermoelectric power of three frequently used thermocouples is compared in Figure 10.2. The choice of thermocouple depends on the temperature range, the chemistry of the problem in question, sensitivity requirements and resistance towards thermal cycling. The temperature range and typical uncertainty of some of the most commonly used thermocouple thermometers are given in Table 10.2. 

The choice of sensor material determines range, sensitivity, and stability. By considering the latter factors, it is found that inorganic insulating compounds, such as most lamp phosphors and many solid state laser materials, are the most suitable materials for thermometric applications. Indeed, these materials are most commonly used in the existing commercial fluorescence thermometer schemes. 

Of course, if not accompanied by a full understanding of the working principles of these devices, the status of their development, and their potential, the results of this test would be misleading when deciding on the choice of the right type of sensor for further development. It has been shown by later development, as discussed, that the lifetime-based scheme is much to be favored in terms of performance and cost. This was the reason that a major manufacturer, Luxtron, substituted the lifetime-based thermometer for the early, intensity-based one in its commercial production/5 ... 

However, it is clear that slight variations in vessel shape, etched markings, or external pressure can lead to disagreements as to which thermometer gives the true temperature. Moreover, the reference points chosen to standardize the readings between different thermometers could be subject to disagreements (see Sidebar 2.4), as could the choice of thermometric fluid (e.g., Hg vs. water, each of which has different values of aP in different temperature ranges). Under these circumstances, the choice of the true temperature scale may become subject to non-scientific influences. We therefore seek a universal standard that avoids such arbitrary choices. 

All thermometers, regardless of fluid, provide the same reading at zero and at 100 if they are calibrated by the method described, but at other points the readings do not usually correspond, because fluids vary in their expansion characteristics. Thus an arbitrary choice of fluid is required, and the temperature scale of the SI system, with its kelvin unit, symbol K, is based on the ideal gas as thennometric fluid. Since the definition of the Kelvin scale depends on the properties of gases, its detailed discussion is delayed until Chap. 3. We note, however, that as an absolute scale, it depends on the concept of a lower limit of temperature. 

The simphcity of the relationship between the thermodynamic scale and the gas thermometer scale is due principally to the simple properties of rarefied gases, and also to the fortunate choice of mercury as thermometric substance by Celsius and Reaumur before the discovery of the gas laws. The coefficient of expansion of mercury happens to be almost exactly proportional to the coefficient of expansion of rarefied gases. All our thermodynamical relationships would have been very much more comphcated had water or alcohol, for example, or the resistance of a metal, been used for the definition of the practical scale of temperature. Their strict validity, however, would not have been affected. 

There are many different types of thermometers available for use in the biomedical temperature range and they provide a wide choice of stability, sensitivity, and ease of use. 

Each type of thermometer discussed above has its particular advantages and disadvantages and the selection of a thermometer for a given application must be based on the requirements of that application. Among the considerations that influence the choice of the thermometer are accuracy, sensitivity, reproducibility, size, temperature range, speed of response, durability, and cost. 

One of the possible choices of a thermometer is a pure gas or liquid at constant pressure. In this case, the volume V of the fluid is the thermometric property, and Eqs. (1-1) and (1-2) become... 

The Fahrenheit scale was devised by Gabriel Daniel Fahrenheit (1686-1736), a natural philosopher who was bom in Danzig and settled in Holland. He invented the mercury thermometer in 1714 before then alcohol had been used as the liquid in thermometers. As the zero point on his scale he took the temperature produced by mixing equal quantities of snow and ammonium chloride. His choice of 212 for the boiling point of water was made in order that the temperature of his body should be 100°F The normal temperature of the human body is 98.6°F perhaps Fahrenheit had a slight fever while he was calibrating his thermometer. 

The importance of choosing the thermometer or method most applicable to a specific situation is often not obvious. In some unusual circumstances, only one type of thermometer may be used. However, in most cases, this limitation does not exist, and the choice should be carefully considered with respect to such characteristics as accuracy, reproducibility, sensitivity, stability, simplicity. Joule heating effect, heat conduction, heat capacity, con-... 

In general, to say that object A is hotter than object B is to say Ta > T. In this case, A will spontaneously transfer energy via heat to B. Likewise if B is hotter than A, Ta < Tb, and ener will transfer spontaneously from B to A. When there is no tendency to transfer ener via heat in either direction, A and B must have equal hotness and Ta = Tb- A logical extension of this concept says that if two bodies are at equal hotness to a third body, they must be at the same temperature themselves. This principle forms the basis for thermometry, where a judicious choice of the third body allows us to measure temperature. Any substance with a measurable property that changes as its temperature changes can then serve as a thermometer. For example, in the commonly used mercury in glass thermometer, the change in the volume of mercury is correlated to temperature. For more accurate measurements, the pressure exerted by a gas or the electric potential of junction between two different metals can be used. 

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). 

Temperature The level of the temperature measurement (4 K, 20 K, 77 K, or higher) is the first issue to be considered. The second issue is the range needed (e.g., a few degrees around 90 K or 1 to 400 K). If the temperature level is that of air separation or liquefact-ing of natural gas (LNG), then the favorite choice is the platinum resistance thermometer (PRT). Platinum, as with all pure metals, has an electrical resistance that goes to zero as the absolute temperature decreases to zero. Accordingly, the lower useful limit of platinum is about 20 K, or liquid hydrogen temperatures. Below 20 K, semiconductor thermometers (germanium-, carbon-, or silicon-based) are preferred. Semiconductors have just the opposite resistance-temperature dependence of metals—their resistance increases as the temperature is lowered, as fewer valence electrons can be promoted into the conduction band at lower temperatures. Thus, semiconductors are usually chosen for temperatures from about 1 to 20 K. 

Since most of the properties of materials depend on temperature, there are a lot of possible choices for a thermometer. Some thermometric properties tike Mossbauer effect or osmotic pressure, of historical interest, but no longer in use, are reported in ref. [[1], pp. 200-206], Hereafter, some thermometric properties useful at low temperature are described (see Table 9.1). Due to the enormous amount of papers on the subject, the bibliography cannot be complete. References before 1980 are reported in ref. [2],... 

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