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.
