Thermometry, gas

A fundamental attribute of temperature is that for any body in a state of equilibrium the temperature may be expressed by a number on a temperature scale, defined without particular reference to that body. The applicability of a universal temperature scale to all physical bodies at equilibrium is a consequence of an empirical law (sometimes called the zeroth law of thermodynamics ), which states that if a body is in thermal equilibrium separately with each of two other bodies these two will be also in thermal equilibrium with each other. 

Practical difficulties arise in making very precise determinations of temperature on the thermodynamic scale the precision of the more refined thermometric techniques considerably exceeds the accuracy with which the experimental thermometer scale may be related to the thermodynamic scale. For this reason, a scale known as the International Temperature Scale has been devised, with several fixed points and with interpolation formulas based on practical thermometers (e.g., the platinum resistance thermometer between 13.803 K and 1234.93 K). This scale is intended to correspond as closely as possible to the thermodynamic scale but to permit more precision in the measurement of temperatures. Further details about this scale are given in Chapter XVII. 

The establishment of the International Temperature Scale has required that the thermodynamic temperatures of the fixed points be determined with as much accuracy as possible. For this purpose a device was needed that measures essentially the thermodynamic temperature and does not depend on any particular thermometric substance. On the other 

Gas thermometry measurements must of course be made with a reaf gas at ordinary pressures. However, it is possibfe to estimate the deviations from perfect-gas behavior and to convert measured pEvafues to perfect-gas pEvafues. For this purpose a viriaf equation of state for a reaf gas is often used  

The establishment of the International Temperature Scale has been accomplished largely with the aid of measurements made with the helium gas thermometer. The most precise gas thermometry method is the constant-volume method, in which a definite quantity of the gas is confined in a bulb of constant volume Eat the temperature T to be determined and the pressure p of the gas is measured. A problem is encountered however in measuring the pressure a way must be found to communicate between the bulb and the pressure gauge. This is usually accomplished by connecting the bulb to the room-temperature part of the system by a slender tube and allowing a portion of the gas to occupy a relatively small, constant dead-space volume at room temperature. Thus, it is important that the gas volume in the pressure manometer be as small as possible. 

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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. 

The three modern types of gas thermometry - constant volume gas thermometry (CVGT), acoustic gas thermometry (AGT) and dielectric constant gas thermometry (DCGT) - are presently considered primary . They are based on simple relations between the properties of an ideal gas and temperature T. However, the departure from the ideal behaviour must be carefully considered in view of the desired level of accuracy. This is done by measuring the thermometric property as a function of density. Then the ideal... 

As AGT, DCGT avoids the troublesome density determination of the conventional gas thermometry. In addition, the pressure sensing tubes can be of any convenient size, and the thermometric gas can be moved in or out the bulb without the need to allow for the amount of the gas involved. 

A detailed discussion of the problems encountered in vapour pressure measurements at low temperature is given in ref. [46,47], where also the use of an in situ manometer is described. Vapour pressure gas thermometry with other liquids besides He is discussed in ref. [43, p. 49],... 

In the past, except for the low-temperature range, the uncertainties of noise thermometry were not comparable to those of the gas thermometry due to the non-ideal performance of detection electronics. Up to now, the most successful technique is the switched input digital correlator proposed by Brixy et al. in 1992 [89], In this method, the noise voltage is fed via two separate pairs of leads to two identical amplifiers whose output signals are multiplied together, squared and time averaged (see Fig. 9.10). 

B. Fellmuth et al. Dielectric-Constant Gas Thermometry and Determination of the Boltzmann Constant TEMP-MEKO 2004, Cavtat-Dubrovnik Croatia, ISBN number 953-6313-73-1... 

Gas thermometry 1 3-950 Pressure P and volume V Ideal gas law plus correction, PV a fegT plus correaions ... 

There is also a paper on the use of platinum in high temperature gas thermometry during the 19th century112 and there are brief discussions of some aspects of the early developments of microcalorimetry in France,113 and the thermal dissociation of water.114... 

I. E. Cottington, High temperature gas thermometry and the platinum metals Some aspects of 19th-century developments , Plat. Met. Rev., 1987,31,196-207. 

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... 

What property of helium makes it particularly suitable for gas thermometry over the temperature range covered by this experiment Is the correction for gas imperfection in this experiment of significant magnitude in relation to the experimental uncertainty If not by how much must the precision of the pressure measurements be improved before gas imperfection corrections become significant How does this depend on the choice of gas to be used ... 

V = vapor pressure, G = gas thermometry, T = triple point (gas, liquid, and solid in equilibrium), MIF = melting/freezing at 1 atm. 

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. 

In the preceding discussion we identified temperature levelsby the kelvin scale, established with ideal-gas thermometry. This does not preclude taking advantage of the opportunity provided by the Camot engine to establish a thermodynamic tempQxaXnxe scale that is traly independent... 

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