27 December 2017

Gas thermometer

To measure temperature, a reference body is used, and a certain physical characteristic of this body which changes with temperature is selected. 

The change in the selected characteristics is an indication of the change in temperature and selected characteristics are the thermometric property and reference body which is used is called thermometer.

All thermometers are working examples of the zeroth law of thermodynamics. 

Significance :

They used to calibrate other thermometers. 

Working principle and construction :

Constant pressure gas thermometer :

A small amount of gas is enclosed in bulb B which is in communication via the capillary tube C with one limb of the mercury manometer M. The other limb of the mercury manometer is open to the atmosphere and can be moved vertically to adjust the mercury levels so that the mercury just touches lip L of the capillary. The pressure in the bulb is used as a thermometric property and it is given by :

p = p0 + ρM Zg
p0 = atmospheric pressure 
ρM = density of mercury

Gas thermometer

When the bulb is brought in contact with the system whose temperature is to be measured, the bulb, in course of time, comes in thermal equilibrium with the system.

The gas in the bulb expands, on being heated, pushing the mercury downward. The flexible limb of the manometer is then adjusted so that the mercury again touches the lip L. The difference in mercury level Z is recorded and the pressure p of the gas in the bulb is estimated. Since the volume of the trapped gas is constant, from the ideal gas equation, 

ΔT = V / R * Δp

The temperature increase is proportional to the pressure increase. 

Constant volume gas thermometer :

In a constant-pressure gas thermometer, the mercury levels have to be adjusted to keep Z constant, and the volume of gas V, which could vary with the temperature of the system, becomes the thermometric property. 
ΔT = V / R * ΔV

The temperature increase is proportional to the observed volume increase.

Constant volume gas thermometer is mostly use, since it is simpler in construction and easier to operate.