Wednesday, March 13, 2019

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.




Image result for bourdon tube vapor pressure system



























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.