Thermometer, gas

The ultimate definition of thermodynamic temperature is in terms of pV (pressure X volume) in a gas thermometer extrapolated to low pressure. The kelvin (K), the unit of thermodynamic temperature, is defined by specifying the temperature of one fixed point on the scale—the triple point... 

The fixed points in the lTS-90 are given in Tabie 11.39. Platinum resistance thermometers are recommended for use between 14 K and 1235 K (the freezing point of silver), calibrated against the fixed points. Below 14 K either the vapor pressure of helium or a constant-volume gas thermometer is to be used. Above 1235 K radiometry is to be used in conjunction with the Planck radiation law,... 

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). 

Fig. 1. Gas thermometer A, helium gas B, mercury P, plunger for adjusting mercury column and P, pressure, where in (a), bulb is surrounded by water...

The 2ero and the interval of the KTTS are defined without reference to properties of any specific substance. Real measurements with real gas thermometers are much more difficult than the example suggests, and all real gases condense before 0 K is reached. 

VP = vapor pressure point CVGT, constant volume gas thermometer point TP, triple point MP, melting point FP, freezing point. Note MP and FP at 101.325 Pa (1 atm) ambient pressure. 

Between the fixed points, temperatures on the ITS-90 are obtained by interpolation using standard instruments and assigned formulae. These standard instruments are the helium gas thermometer (3 K to 24.5 K), the platinum resistance thermometer (13.8 K to 1235 K), and the optical thermometer (above 1235 K). 

Any instrument which can be used for measuring temperatures is called a thermometer. Thermometers may be, and are, constructed which utilise any property of a body such as those mentioned above. To evade the difficulty of comparison of scales, they are usually all referred to a gas thermometer, with Centigrade scale as standard. The ice and steam-points on the latter are taken as 0° and 100° respectively. 

As unit mass we shall take 1 gram, and unit rise of temperature 1° C. on the hydrogen gas thermometer. 

T) = T. (17), so that absolute thermodynamic temperatures are equal to the gas temperatures measured with an ideal gas thermometer. 

It is not possible to construct an ideal gas thermometer. Instead, a real gas thermometer must be used under conditions where the real gas behaves as an ideal gas. This is done by extrapolating the pV product to zero pressure (where all gases behave ideally), and equation (1.9) becomes... 

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. 

The International Temperature Scale — ITS-90 For ITS-90, temperatures of a series of fixed points are measured as accurately as possible with a gas thermometer. A complete description of ITS-90 is given in the literature5 7 and... 

These fixed points are used to calibrate a different kind of thermometer that is easier to use than a gas thermometer. Over the temperature range from 13.8033 to 1234.93 °A (or K), which is the temperature interval most commonly encountered, the thermometer used for ITS-90 is a platinum resistance thermometer. In this thermometer, the resistance of a specially wound coil of platinum wire is measured and related to temperature. More specifically, temperatures are expressed in terms of W(T9o), the ratio of the resistance R(Ttriple point of water R (273.16 K), as given in equation (1.11)... 

The ability to measure temperature and temperature differences accurately and reproducibly is essential to the experimental study of thermodynamics. A thermometer constructed with an ideal gas as its working fluid yields temperatures that correspond to the fundamental thermodynamic temperature scale. However, such thermometers are extremely difficult to use, are not amenable to miniaturization, and are very expensive. Therefore, other means to measure temperatures that reproduce the ideal gas or thermodynamic temperature scale (Kelvin) have had to be developed. The international temperature scale represents a method to determine temperatures over a wide range with measuring devices that are easier to use than the ideal gas thermometer. The goal is to make temperature measurements that correspond to the thermodynamic temperature as accurately as possible. 

Temperature Tgo in the range between 3.0 and 24.5561 K is defined in terms of 3He or 4He constant volume gas thermometers (CVGT), calibrated at the triple points of Ne and H2, and at a temperature between 3.0 and 5.0 K that has been obtained from vapor pressure versus temperature relations for He. 

Two additional points near 17.0 and 20.3 K are required. These may be determined by using either ihe constant volume gas thermometer or by vapor pressure measurements of H . 

Any substance that somehow changes with alterations in its temperature can be used as the basic component in a thermometer. Gas thermometers work best at very low temperatures. Liquid thermometers are the most common type in use. They are simple, inexpensive, long-lasting, and able to measure a wide temperature span. The liquid is almost always mercury, sealed in a glass tube with nitrogen gas making up the rest of the volume of the tube. 

The experimental realization of a Carnot cycle to measure the temperature T is unusual. The coincidence of the thermodynamic temperature T with the temperature read by a gas thermometer, for example, allows the use of such thermometer to know T. As we shall see, also other laws of physics relating T with physical parameters other than heat can be used to get an absolute measure of T. 

In general, a thermometer is called primary if a theoretical reliable relation exists between a measured quantity (e.g. p in constant volume gas thermometer) and the temperature T. The realization and use of a primary thermometer are extremely difficult tasks reserved to metrological institutes. These difficulties have led to the definition of a practical temperature scale, mainly based on reference fixed points, which mimics, as well as possible, the thermodynamic temperature scale, but is easier to realize and disseminate. The main characteristics of a practical temperature scale are both a good reproducibility and a deviation from the thermodynamic temperature T which can be represented by a smooth function of T. In fact, if the deviation function is not smooth, the use of the practical scale would produce steps in the measured quantities as function of T, using the practical scale. The latter is based on ... 

A thermometer is a device by which we can measure a property of matter function of temperature. If a relation, based on fundamental laws of physics, between such property and the thermodynamic temperature is considered reliable, the thermometer does not need a calibration and is called primary thermometer. In the other cases, the thermometer needs a calibration and is called secondary. Examples of primary thermometers are gas thermometers and noise thermometers. 

Over the years, a simpler instrument called interpolating constant volume gas thermometer (ICVGT) has been developed. It differs from traditional CVGT because it is calibrated at a number of temperatures known from independent measurements. 

The problems encountered in performing accurate measurements of T with a primary CVGT can be overcome by the use of the dielectric constant gas thermometer [27-31] or by the acoustic gas thermometer [3,32,33],... 

Techniques for accurate and reproducible measurement of temperature and temperature differences are essential to all experimental studies of thermodynamic properties. Ideal gas thermometers give temperatures that correspond to the fundamental thermodynamic temperature scale. These, however, are not convenient in most applications and practical measurement of temperature is based on the definition of a temperature scale that describes the thermodynamic temperature as accurately as possible. The analytical equations describing the latest of the international temperature scales, the temperature scale of 1990 (ITS-90) [1, 2]... 

But there are no ideal (perfect) thermometers in the real world. In practice, we generally experiment a bit until we find a thermometer for which a property X is as close to being a linear function of temperature as possible, and call it a standard thermometer (or ideal thermometer ). We then calibrate other thermometers in relation to this, the standard. There are several good approximations to a standard thermometers available today the temperature-dependent (observed) variable in a gas thermometer is the volume of a gas V. Provided the pressure of the gas is quite low (say, one-hundredth of atmospheric pressure, i.e. 100 Pa) then the volume V and temperature T do indeed follow a fairly good linear relationship. 

Non-stoichiometry, which originates from various kinds of lattice defect, can be derived from the phase rule. As an introduction, let us consider a trial experiment to understand non-stoichiometry (this experiment is, in principle, analogous to the one described in Section 1.4.8). Figure 1.1 shows a reaction vessel equipped with a vacuum pump, pressure gauge for oxygen gas, pressure controller for oxygen gas, thermometer, and chemical balance. The temperature of the vessel is controlled by an outer-furnace and the vessel has a special window for in-situ X-ray diffraction. A quantity of metal powder... 

None of these is far from = 0.003663, which is therefore commonly taken as the expansion coefficient for gases especially as ihc value for hydrogen, commonly used in the standard gas thermometer, is very near it. If the pressure as well as the volume is allowed to vary, the behavior of the ideal gas must be expressed by the Boyle-Charlcs law or the ideal gas law and the behavior of a real gas by one of the other equations of stale. See also Ideal Gas Law. 

The relation between the international temperature scale and the thermodynamic temperature scale must be determined empirically with the aid of careful measurements involving gas thermometers. 

In liquid-filled thermal systems, mercury or various organic fills are used. In vapor-filled systems, a number of hydrocarbons, including ethane, ethyl chloride, ethyl ether, chlorobenzene, and propane, among others, are used. Nitrogen or other fully-dried and purified gases may be used in a gas thermometer. 

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