Thermometer problem definition

The starting point for the problem definition is develop a model for a liquid thermometer as shown in Fig. 1.3. This looks like a simple problem, however, it is easy to get lost in details. Important questions that could surface are how does the thermometer function exactly and what can be ignored . First the problem definition and the goal should be clearly stated. What should be modeled and how ... 

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. 

The definition of thermodynamic variables adopted here has also been used by Green and Mori, and seems to be the most natural generalization of the equilibrium definitions. It should be noted, however, that there is a problem connected with the measurement of, say, the temperature for a non-equilibrium state. One is assured by the general principles of thermodjmamics that a properly calibrated thermometer will always yield correct results in an equilibrium measurement. This is no longer true in a non-equilibrium situation. The result of a temperature measurement could depend on, say, the orientation of the thermometer with respect to the temperature gradient. However, we will simply dismiss this difficulty by taking the point of view that it concerns a matter of experimental practice. Note that there is really no difficulty in principle, since one could measure the mechanical quantities and invert Eqs. (67) and (68) to obtain the thermodynamic variables. 

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