Vapour-pressure Thermometer

Vapour-pressure Thermometer (filled-system thermometer)

-What is Vapour-pressure Thermometer  ??

a thermometer in which the variable saturated vapor pressure of a volatile liquid is used as a measure of the temperature and which thus has the advantage over some other types of thermometers of being free from errors due to bulb expansion

-Vapor pressure or equilibrium vapor pressure is defined as the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phases (solid or liquid) at a given temperature in a closed system. 

-The equilibrium vapor pressure is an indication of a liquid's evaporation rate. It relates to the tendency of particles to escape from the liquid (or a solid).

-A substance with a high vapor pressure at normal temperatures is often referred to as volatile. The pressure exhibited by vapor present above a liquid surface is known as vapor pressure.

-As the temperature of a liquid increases, the kinetic energy of its molecules also increases.

-As the kinetic energy of the molecules increases, the number of molecules transitioning into a vapor also increases, thereby increasing the vapor pressure.

Temperature Ranges of Liquid used in Vapour-preessure System

Liquid	Critical temperature	Boiling point	Typical ranges available
Argon	-122◦c	-185.7◦c	Used to measure very low temperature down to -253◦c
Methyl chloride	143◦c	-23.7◦c	30-130◦K(0◦-50◦c)
Sulphur dioxide	157◦c	-10◦c	30-120◦c
Ethyl-alcohol	243◦c	78.5◦c	200-350◦c
Toluene	321◦c	110.5◦c	150-250◦c
Ethyl-dioxide	187◦c	12.2◦c	30-100◦c
Bulane (n)	154◦c	-0.6◦c	20-80◦c
Methyl bromide	-	4.6◦c	80-180◦K
Di-ethyl ether	194◦c	34.5◦c	60-160◦c
Water	375◦c	100◦c	120-220◦c

Working Principle 

-The thermometer consists of a steel bulb, a steel tube and Bourdon tube.

-The bulb is partly filled with a volatile liquid, evacuated and sealed off. 

-With rising temperature, the average velocity of the molecules in the liquid will increase. 

-As a result, more liquid molecules will acquire enough energy to escape from the surface of the liquid and saturate the evacuated area.

-The result will be an increase in vapour pressure in the capillary tube and Bourdon tube. 

-The Bourdon tube, sensitive to the changes in pressure, will record the temperature via the pointer that is linked to the moving end of the Bourdon tube.

Advantage: 

-rugged construction.
-require low maintenance.
-low in cost
-no need electrical power (self-sufficient).
                   

Disadvantages:

 -narrow temperature range, from -50 to 300 °C;
 - need a large bulb for the sake of accuracy.
 - slow response time
 - non-uniformity of the temperature scale
 -Limited maximum temperature.