The document discusses the zeroth law of thermodynamics and temperature measurement. It can be summarized as follows: 1) The zeroth law states that if two bodies (A and B) are each in thermal equilibrium with a third body (C), then A and B are also in thermal equilibrium with each other. This establishes the transitive property of temperature. 2) Temperature is measured using thermometers that relate a physical property's change (like mercury expansion) to temperature. Common temperature scales are Celsius, Fahrenheit and Kelvin. 3) Various thermometer types are discussed, including liquid-in-glass, electrical resistance, thermocouple, and those using gas/vapor pressure changes

1. ZEROTH LAW OF
THERMODYNAMICS

2. Temperature
• Thermodynamics – science of temperature
• Temperature – is associated with ability to
differentiate the hot from cold.
• Thermal equilibrium – When two bodies at
deferent temperature are brought into
contact, some time later they attain a
common temperature – now they are in
thermal equilibrium.

3. Zeroth law of Thermodynamics
• Base for all temperature measurement.
• Thermal equilibrium is the key word for zeroth
law.
Definition: 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

4. Zeroth law of Thermodynamics
 Temperature measurement procedure
– A reference body is used, and a certain physical
characteristic of this body which changes with temperature
is selected.
– The changes in the selected characteristic may be taken
as an indication of change in temperature. The selected
characteristic is called the thermometric property,
• expansion of the mercury in the tube is used as the
thermometric property
– The reference body which is used in the determination of
temperature is called the thermometer

5. Thermodynamic Equillibirium

6. Temperature Scales
• All temperature scales are based on some
easily reproducible states such as the freezing
and boiling points of water, which are also
called the ice point and the steam point
o Celsius Temperature Scale
o Fahrenheit scale
o Kelvin scale

7. Celsius Temperature Scale
• The Celsius temperature scale employs a degree of the
same magnitude as that of the ideal gas scale, but its zero
point is shifted, so that the Celsius temperature of the
triple point of water is 0.01 degree Celsius or 0.01°C. If t
denotes the Celsius temperature,
then t = T- 273.15
• Thus the Celsius temperature ts at which steam condenses
at I atm. pressure
• ts = Ts - 273.15 = 373.15 - 273.15= 100.00°C
• Similar measurements for ice points show this temperature
on the Celsius scale to be 0.00°C. The only Celsius
temperature which is fixed by definition is that of the triple
point

8. Fahrenheit scale
• the freezing point of water is 32 degrees
Fahrenheit (°F) and the boiling point is 212 °F.
• On the Celsius scale, the freezing and boiling
points of water are 100 degrees apart. A
temperature interval of 1 °F is equal to an interval
of ​5⁄9 degrees Celsius. The Fahrenheit and Celsius
scales intersect at −40° (i.e., −40 °F = −40 °C).
• f °Fahrenheit to c °Celsius : (f − 32) °F
× 5°C/9°F = (f − 32)/1.8 °C = c °C

9. Kelvin Scale
The lower standard point on this scale is 273K and upper standard
point is 373K. There 273 divisions below lower standard point. Its 0K is
equal to -273oC. To convert celsius temperature to kelvin temperature
following formulas is used.
Temperature in Kelvin = Temperature in oC + 273
For example, 30oC = (30+273) = 303K
Conversely, (353-273) = 80oC
To convert temperature from celsius scale to kelvin scale the value is
increased numerically by 273. Therefore, the temperature on the
kelvin scale and celsius scale are related to each other as follows.
T(K) = T(oC) + 273
The celsius and the kelvin scales are used all over the world for
scientific measurements. These scales are also found to be more
convenient than the scales introduced by Fahrenheit and Reaumer
which were commonly used till recently.
The following formula is used for the conversion between different
temperature scales.
K-273/100 = C/100 = F-32/180 = R/80

10. Expansion thermometers
The expansion thermometers make use of the
differential expansion of two different
substances. Thus in liquid-in-glass
thermometers, it is the difference in expansion
of liquid and the containing glass. And in
bimetallic thermometers, the indication is due
to the difference in expansion of the two solids
(i) Liquid-in-glass thermometers
(ii) Bimetallic thermometers.

11. Liquid-in-glass thermometer
This is a very familiar type of thermometer. The mercury or
other liquid fills the glass bulb and extends into the bore of
the glass stem. Mercury is the most suitable liquid and is used
from – 38.9°C (melting point) to about 600°C. The
thermometers employed in the laboratory have the scale
engraved directly on the glass stem. A usual type of mercury-
in-glass thermometer is shown in Fig. 2.8. An expansion bulb
is usually provided at the top of the stem to allow room for
expansion of mercury, in case the thermometer is subjected
to temperature above its range. The upper limit for mercury-
in-glass thermometers is about 600°C. As the upper limit is far
above the boiling point of mercury, some inert gas i.e.,
nitrogen is introduced above the mercury to prevent boiling

13. Bimetallic thermometers
In a bimetallic thermometer differential expansion of
bimetallic strips is used to indicate the temperature. It
has the advantage over the liquid-in-glass thermometer,
that it is less fragile and is easier to read. In this type of
thermometer two flat strips of different metals are placed
side by side and are welded together. Many different
metals can be used for this purpose. Generally one is a
low expanding metal and the other is high expanding
metal. The bimetal strip is coiled in the form of a spiral or
helix. Due to rise in temperature, the curvature of the
strip changes. The differential expansion of a strip causes
the pointer to move on the dial of the thermometer.

14. Pressure thermometers
In pressure thermometers liquids, gases and vapours can all
be used. The principle on which they work is quite simple. The
fluid is confined in a closed system. In this case the pressure is
a function of the temperature, so that when the fluid is
heated, the pressure will rise. And the temperature can be
indicated by Bourdon type pressure gauge. In general, the
thermometer consists of a bulb which contains bulk of the
fluid. The bulb is placed in the region whose temperature is
required. A capillary tube connects the bulb to a Bourdon
tube, which is graduated with a temperature scale.
(i) Vapour pressure thermometers
(ii) Liquid-filled thermometers
(iii) Gas-filled thermometers

15. Vapour pressure thermometer
• A schematic diagram of a vapour pressure
thermometer is shown in Fig. 2.9. When the bulb
containing the fluid is installed in the region whose
temperature is required, some of the fluid
vapourizes, and increases the vapour pressure. This
change of pressure is indicated on the Bourdon
tube. The relation between temperature and
vapour pressure of a volatile liquid is of the
exponential form. Therefore, the scale of a vapour
pressure thermometer will not be linear.

17. Liquid-filled thermometer
• A liquid-filled thermometer is shown in Fig. 2.10. In thiscase, the
expansion of the liquid causes the pointer to move in the dial. Therefore
liquids havinghigh co-efficient of expansion should be used. In practice
many liquids e.g., mercury, alcohol,toluene and glycerine have been
successfully used. The operating pressure varies from about 3 to100 bar.
These type of thermometers could be used for a temperature upto 650°C
in which mercurycould be used as the liquid
• In actual design, the internal diameter of the capillary tube and Bourdon
tube is, mademuch smaller than that of the bulb. This is because the
capillary tube is subjected to a temperaturewhich is quite different from
that of the bulb. Therefore, to minimise the effect of variation
intemperature to which the capillary tube is subjected, the volume of the
bulb is made as large aspossible as compared with the volume of the
capillary. However, large volume of bulb tends toincrease time lag,
therefore, a compensating device is usually built into the recording or
indicatingmechanism, which compensates the variations in temperature
of the capillary and Bourdontubes

19. Gas-filled thermometers
• The temperature range for gas thermometer is
practically the same as that of liquid filled
thermometer. The gases used in the gas thermometers
are nitrogen and helium. Both these gases are
chemically inert, have good values for their co-efficient
of expansion and have low specific heats. The
construction of this type of thermometer is more or
less the same as mercury-thermometer in which
Bourdon spring is used. The errors are also
compensated likewise. The only difference in this case
is that bulb is made much larger than used in liquid-
filled

20. Electrical Resistance Thermometer
• In the resistance thermometer (Fig. 2.3) the change in
resistance of a metal wire due to its change in temperature
is the thermometric property. The wire, frequently
platinum, may be incorporated in a Wheatstone bridge
circuit. The platinum resistance thermometer measures
temperature to a high degree of accuracy and sensitivity,
which makes it suitable as a standard for the calibration of
other thermometer.
• In a restricted range, the following quadratic equation is
often used,
R = Ro(l + At + Br)
where R0 is the resistance of the platinum wire when it is
swrounded by melting ice and A and B are constants.

22. Thermocouple
• A thermocouple circuit made up from joining two wires A and B
made of dissimilar metals is shown in Fig. 2.4. Due to the Seeback
effect, a net e.m.f. is generated in the circuit which depends on the
difference in temperature between the hot and cold junctions and
is, therefore, a thermometric property of the circuit. This e.m.f. can
be measured by a microvoltmeter to a high degree of accuracy.
• The choice of metals depeods largely on the temperature range to
be investigated, and. copper-constantan, chromel-alumel and
platinum-platinumrhodium are typical combinations in use. A
thermocouple is calibrated by measuring the thennal e.m.f. at
various known tempera'tures. the reference junction being kept at
0°C. The results of such measureme.1tts on most thennocouples
can usually be represented by a cubic.equation of the form .,•
e=a+bt+cr+df

23. • where £ is the thermal e.m.f. and the constants a, b, c and
dare different for each thermocouple. The advantage of a
thermocouple is that it comes to thermal equilibrium with the
system, whose temperature is to be measured, quite rapidly,
because its mass is small.

24. Optical pyrometer
An optical pyrometer works on the principle that
matters glow above 480°C and the colour
of visible radiation is proportional to the temperature of
the glowing matter. The amount of light radiated from
the glowing matter (solid or liquid) is measured and
employed to determine the
temperature.

25. Radiation pyrometers
• A device which measures the total intensity of radiation
emitted from a body is called radiation pyrometer.
• The elements of a total radiation pyrometer are
illustrated in It collects the radiation from an object
(hot body) whose temperature is required. A mirror is
used to focus this radiation on a thermocouple. This
energy which is concentrated on the thermocouple
raises its temperature, and in turn generates an e.m.f.
This e.m.f. is then measured either by the
galvanometer or potentiometer method. Thus rise of
temperature is a function of the amount of radiation
emitted from the object.

26. • Advantages of the pyrometers
• The temperatures of moving objects can be
measured.
• A higher temperature measurement is possible
than that possible by thermocouples etc.
• The average temperatures of the extended surface
can be measured.
• The temperature of the objects which are not easily
accessible can be measured.