Temperature - engineering thermodynamics

1. ZEROTH LAW OF
THERMODYNAMICS
AND THE CONCEPT OF
TEMPERATURE
ARUN KAPPEN

2. ZEROTH LAW OF
THERMODYNAMICS
• “When a body A is in thermal equilibrium with a body B, and
also separately with a body C, then B and C will be in
thermal equilibrium with each other”
• This is the statement of zeroth law
• Formulated by R. H. Fowler in 1931
• It is the basis of temperature measurement

3. A
B C

4. THERMAL EQUILIBRIUM and
TEMPERATURE
• Thermal equilibrium is said t o exist
bet ween t wo syst ems, if bot h t he
syst ems have t he same t emperat ure and
t here is no heat t ransf er bet ween t he
syst em
• I n microscopic approach, t emperat ure
can be def ined as t he average kinet ic
energy of molecules
• I n macroscopic approach, it is def ined
as propert y of a syst em and by

5. • Temperat ure is def ined as t he degree
of hot ness or coldness of a body
• Based on our physiological sensat ions,
we express t he level of t emperat ure
qualit at ively wit h words like f reezing
cold, cold, warm, ext remely hot et c.
• But , we cannot assign numerical values
f or t emperat ure based on t hese
sensat ions alone, since it may be
misleading

6. THERMOMETRY
• The branch of thermodynamics concerned
with the measurement of temperature and
the design and use of thermometers
• In order to measure temperature, a
reference body is used, a certain physical
characteristic of this body which changes
with temperature is selected

7. • The changes in the selected
characteristic may be taken as an
indication of changes in temperature
• The selected characteristic is called
the thermometric property
• The reference body which is used in
the determination of temperature is
called the thermometer

8. tRIPLE PoINt oF WAtER
• Triple point of wat er is used as t he
arbit rary f ixed point of t hermomet ry
since 1954
• I t means t he st at e at which wat er co-
exist s in t hree phases viz., solid, liquid
and gaseous phases
• The t emperat ure corresponding t o
t riple point is 0.010
C and 611 Pa part ial
vapour pressure of wat er

10. TYPES OF THERMOMETERS
• Constant volume gas thermometer
• Constant pressure gas thermometer
• Electrical resistance thermometer
• Thermocouple
• Mercury in glass thermometer
• Pyrometer

11. CONSTANT VOLUME GAS
THERMOMETER

12. • In a constant volume gas thermometer, pressure is the
thermometric property
• As the name suggests, volume is kept constant
• P1/ T1 = P2/ T2
• P1and T1must be known values [like triple point of water], P2
can be measured. Now, T2 can be found out from the above
relation

15. CONSTANT PRESSURE GAS
THERMOMETER
• The apparatus used is the same as that
for constant volume gas thermometer
• Only difference is that, here we are
making volume as the thermometric
property
• Pressure is kept constant by maintaining a
constant value for the height “h”
• Volume is varied according to pressure

16. ELECTRICAL RESISTANCE
THERMOMETER
• The working principle of an elect rical
resist ance t hermomet er is t he change in
resist ance of a met allic wire
proport ional t o t he change in
t emperat ure
• Usually used mat erial is a plat inum wire
• Change is resist ance is f ound out by
using a wheat st ones bridge circuit and
t he new t emperat ure is calculat ed wit h
t he calibrat ion

18. THERMOCOUPLE
• A t hermocouple works on t he principle
of Seebeck ef f ect
• A t hermocouple is made wit h t wo
dissimilar met allic wires A and B
f orming t wo j unct ions P and Q
• When t hese t wo j unct ions are at
dif f erent t emperat ures, a current will
f low and an emf will be generat ed
• This emf will be measured by using a

19. • The commonly used mat erials f or
t hermocouple are
• copper – const ant an
• Chromel – alumel
• Plat inum - rhodium

21. MERCURY IN GLASSTHERMOMETER
• Change in length of mercury column inside an evacuated glass tube
is the thermometric property
• The length of mercury column increases with an increase in
temperature linearly
• Easy to fabricate and easy to use
• But, mercury can’t be used under -300
C, it being the freezing point
of mercury
• Similary, alcohol can’t be used above 850
C, because it’s the boiling
point of alcohol

23. PYROMETER
• Measures t he t emperat ure of an obj ect
by measuring t he t hermal radiat ion
emit t ed by t he obj ect
• Thermal radiat ion emit t ed is
proport ional t o t he t emperat ure
• Usually used f or measuring ext remely
high t emperat ures
• A pyromet er will have an opt ical syst em
and a det ect or
• Pyromet ers are also called non-cont act

24. THERMODYNAMIC TEMPERATURE
SCALES

27. IDEAL GAS TEMPERATURE SCALE
• The empirical scale, thus developed
based on the constant volume gas
thermometer is commonly referred to as
the ideal gas temperature scale