Question

What are calibration points for absolute T scale, why we choose these points?​

Answer 1

Answer:

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The Absolute Temperature Scale:

The absolute temperature scale is based on the (p, V, T) relationships for an ideal gas as given by equation (1.7)

(1.7)pV=nRT

where T is the absolute temperature, p, V, and n are the pressure, volume, and number of moles as described, earlier and R is the gas constant.o An ideal gas thermometer compares the temperatures T1 and T2 along two different isotherms (constant temperature). From equation (1.7)

(1.8)T2=T1(pV)2(pV)1

where (pV)2 is the pV product of the ideal gas at T2 and (pV)1 is the pV product at T1. The size of the temperature increment (or degree) is obtained by dividing the temperature interval between the absolute zero of temperature (where the volume of the ideal gas becomes zero) and the temperature of the triple point of water into exactly 273.16 units. With this definition, the temperature T can be obtained by comparing (pV)T, the (pV) product at temperature T, with (pV)t.p., the (pV) product at the triple point through the relationship

(1.9)T=273.16(pV)T(pV)t.p..

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

(1.10)T=273.16limp→0(pV)T(pV)t.p..

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.