TEMPERATURE MEASUREMENTS.pptx

1. Temperature Measurement
INSTRUMENTATION

2. INTRODUCTION
Temperature measurements are amongst the most
common and the most important measurements
made in controlling industrial processes.
Precise control of temperature is key factor in
process industry especially those which involve
chemical operations.
Consequently:
It is most important to make a comprehensive
survey of methods of measurements of
temperature and their advantages and
disadvantages.

3. Applications
Measuring temperature of
• Household oven,
• Distant Planet,
• Red hot bloom of iron being rolled,
• Parts of human body,
• Windings of electrical machines,
• Bearings of steam turbines etc.

4. Information about the operating limitations in
terms of :
• Time of response.
• Temperature range.
• Distance of operation and
• Compatibility with other control elements.
This will permit one to select the best method of
measurement.

5. Important note on temperature measurement.
• Temperature is not measured directly like
displacement, pressure or flow, but is
measured through indirect means.
Change of temperature of a substance causes a
variety of effects:
• Physical,
• Chemical,
• Electrical or
• Optical
And may be used for the measurement of
temperature through use of proper temp
sensing devices.

6. For Example:
Pressure, Volume, electrical resistance,
expansion coefficients, colour of radiation etc.,
are all related to temperature through
fundamental molecular structure.
They change with temperature and these
changes can be used for measurement of
temperature.

7. Definitions of temperature:
The degree of hotness or coldness of a body or an
environment measured on a definite scale.
A condition of a body by virtue of which heat Is
transferred to or from other bodies.
Difference between the quantities temperature
and heat.
Temperature = “degree” of heat
Heat = “quantity” of heat

8. The fundamental law used in temperature
measurement is the zeroth law of thermodynamics.
This states that
“if two bodies are in thermal equilibrium with a
third body, then they are all in thermal
equilibrium with each other”.

9. TEMPERATURE SCALES
Two temperature scales in common use are the
Fahrenheit and Celsius scales. These are based on
a specification of the number of increments
between freezing point and boiling point of water at
the standard atmospheric temperature. The Celsius
scale has 100 between these points, while the
Fahrenheit scale has 180 units. The Celsius scale is
currently more in use because of the adoption of
metric units.

10. However, the absolute temperature scale based on
the thermodynamic ideal Carnot cycle has been
correlated with the Celsius Fahrenheit scales as
follows:
K (Absolute temp. Kelvin scale) = C 0 +273.15
where C 0 is temp on Celsius scale.
R (Absolute temp. Rankine scale) = F 0+459.69
where F 0 is the temperature on the Fahrenheit
scale.

11. Classification of Temperature measuring devices
1. Nature of change produced in the temp sensing
element:
a) Those which are primarily electrical or electronic
in nature.
b) Those which do not employ electrical or
electronic methods for their working.
Electrical and non-electrical operating principles.
2. Temperature range of the instrument.

12. Nature of change produced in the temp sensing element
i) Glass Thermometers
ii) Pressure Gauge Thermometers
iii) Differential Expansion Thermometers
iv) Electrical Resistance Thermometers
v) Thermocouples
vi) Optical Pyrometers
vii) Radiation Pyrometers
viii) Fusion Pyrometers
ix) Calorimetric Pyrometers
x) Colour Temperature Charts.

13. Pressure Thermometers

14. Bimetallic Thermometer
This type of thermometer also employs the
principle of solid expansion and consists of a
'bimetal’ strip usually in the form of a cantilever
beam

15. The deflection of the free end is directly
proportional to the temperature change and
square of the length of the strip, and inversely
proportional to the thickness throughout the
linear portion of deflection.

16. The radius of arc is given by
    
   
2 2
2
2 1
3 1 1 1/
6 1
A B
n
t m m m
r
m
T T
m
n
 

 



  



t = total thickness of strip
n = ratio of moduli of elasticity = EB/EA.
m = ratio of thicknesses = tB /tA.
T2 – T1 = change in temperature
tB , tA.= thickness of metal B and A respectively.
A, B = thermal co - efficient of expansion of metals A and B
respectively.

17. In most practical application, the metals and
their dimensions are so chosen that their
moduli of elasticity and thickness are equal.
i.e. n = 1 and m = 1.
  
2 1
3
2
A B T T
r
t
 
 


18. Advantages of Bimetallic thermometers
1. Simple, robust and inexpensive.
2. Accuracy ranges from  0.5% for lab type
and  2% for process type instruments.
3. These thermometers in general can
withstand about 50% over-range in
temperature.

19. Disadvantages of Bimetallic thermometers
1. Not recommended for use at temperatures
above 4000C for continuous duty or above
5000C for intermittent duty.
2. All metals have physical limitations and are
subject to permanent warp distortion.

20. Applications
• Refineries,
• Oil burners,
• Tire vulcanizers,
• Hot solder tanks,
• Hot wire heaters,
• Tempering tanks,
• Impregnating tanks.
• Simple on – off temperature devices
(thermostats)

21. Electrical Resistance Thermometers
In resistance thermometers, the change in
resistance of various materials, which varies in
reproducible-manner with temperature forms the
basis of this important sensing technique.
The materials in actual use fall in two classes
namely, conductors (metals) and semiconductors.
In general, the resistance of the highly conducting
materials (metals) increases with increase in
temperature and the coils of such materials are
called metallic resistance thermometers.

22. Metallic Resistance Thermometers
Metals such as platinum, copper, tungsten and
nickel exhibit small increase in resistance as the
temperature rises because they have a positive
temperature coefficient of resistance.
For the measurement of lower temperatures, up to
600°C, metallic resistance thermometer are very
suitable for both laboratory and industrial
applications because of their high degree of
accuracy as well as long-term stability.

26. Metallic resistance thermometers are constructed
in many forms, but the temperature sensitive
element is usually in the form of a coil of fine wire
supported in a stress free manner. A typical
construction is shown in Fig., where the wire of
metal is wound on the grooved hollow insulating
ceramic former and covered with protective
cement.
The ends of the coils are welded to stiff copper
leads that are taken out to be connected in one of
the arms of the Wheatstone bridge circuit.

27. In some cases, this arrangement can be used
directly in the medium whose temperature is being
measured, thus giving a fast speed of response.
However, in most applications, a protective metal
sheath is used to provide rigidity and mechanical
strength.
Platinum, in spite of its low sensitivity and high
cost as compared to nickel and copper is the most
widely used material for metallic resistance
element.

28. This is because of following:
1. The temperature-resistance characteristics of
pure platinum are well defined and stable over a
wide range of temperature.
2. It has high resistance to chemical attack and
contamination ensuring long-term stability.
3. It forms the most easily reproducible type of
temperature transducer with a high degree of
accuracy.

29. In general, the resistance relationship of most
metals over a wide range of temperatures is given
by the quadratic relationship
2
0 1
R R aT bT
 
  
 
Where
R = resistance at absolute temperature T
R0 = resistance at 00C
a and b = experimentally determined constants.

30. Over a limited temperature range around 00C
(273 K),
 
0 1
t
R R T

 
where:
α = the temperature coefficient of
reistance of material ( Ω Ω)/0
C or 0
C−1
R0 = reistance at 00
C.
t = temperature relative to 00
C

31. For a change in temperature from t1 to t2,
 
 
2 1 0
2 1
2 1
2 1
0
Rearranging gives:
R R
t
R
R R t
t
R t




 
 

32. Thermo – Electric sensors
(Thermocouples)
The most common electrical method of
temperature measurement uses the thermo-
electric sensor, also known as the
thermocouple.

33. The construction of a thermocouple is quite
simple.
It consists of two wires of different metals
twisted and brazed or welded together with
each wire covered with insulation which may be
either:
1. Mineral (magnesium oxide) insulation for
normal duty, or
2. Ceramic insulation for heavy duty .

34. The basic principle of temperature measurement
using a thermo-electric sensor was discovered by
Seebeck in 1821 and is illustrated in figure.

35. When two conductors of dissimilar metals, say A
and B, are joined together to form a loop
(thermocouple) and two unequal temperaturesT1
and T2 are interposed at two junctions J1, and J2
respectively, then a voltmeter detects the
electromotive force E, or if a low resistance
ammeter is connected, a current flow I is
measured.

36. Experimentally, it has been found that the
magnitude of E depends upon the materials as well
as the temperatures T1 and T2.
Now, the overall relation between emf E and the
temperatures T1 and T2 forms the basis for electric
measurements and is called the Seebeck effect.
Thus, in practical applications, a suitable device is
incorporated to indicate the emf E or the flow of
current I.
For convenience of measurements and
standardisation of the two junctions is usually
maintained at some known temperature.

37. The measured emf E then indicates the
temperature difference relative to the reference
temperature, such as ice point which is very
commonly practice.

38. It may be noted that temperatures T1 and T2 of
junctions J1 and J2 respectively are slightly altered
if the thermo-electric current is allowed to flow in
the circuit.
Heat is generated at the cold junction and is
absorbed from the hot junction thereby heating the
cold junction slightly and cooling the hot junction
slightly. This phenomenon is termed Peltier effect.
If the thermocouple voltage is measured by means
of potentiometer, no current flows and Peltier
heating and cooling are not present.

39. Further, these heating and cooling effects are
proportional to the current and are fortunately quite
negligible in a thermocouple circuit which is
practically a millivolt range circuit.
In addition, the junction emf may be slightly altered
if a temperature gradient exists along either or both
the materials. This is known as Thomson effect.
Again, the Thomson effect may be neglected in
practical thermo-electric circuits and potentiometric
voltage measurements are not
susceptible to this error as there
is no current flow in the circuit.

40. Laws of thermo - electricity
Law of Intermediate Temperatures
This states that the emf generated in a thermocouple
with junctions at temperatures T1 and T3 is equal to
the sum of the emf's generated by similar
thermocouples, one acting between temperatures T1
and T2 and the other between T2 and T3 when T2 lies
between T1 and T3 (See figure).

41. This law is useful in practice because it helps in
giving a suitable correction in case reference junction
temperature (which is usually an ice bath at 0°C)
other than 00C is employed.

42. For example, if a thermocouple is calibrated for a
reference junction temperature of 00C and used
with junction temperature of say 200C, then the
correction required for the observation would be
the emf produced by the thermocouple between
0°C and 20°C.

43. Law of Intermediate Metals
The basic thermocouple loop consists of two
dissimilar metals A and B [see figure]. If a third
wire is introduced then three junctions are formed
as shown. The emf generated remains unaltered if
the two new junction B-C and C-A are at the same
temperature.

45. It may be noted that extension wires are needed
when the measuring instrument is to placed at a
considerable distance from the reference junction.
Maximum accuracy is obtained when the leads are
of the same material as the thermocouple
element.
However this approach is not economical while
using expensive thermocouple materials. Further, a
small inaccuracy is still possible if the binding post
of the instrument is made of say Copper and the
two binding posts are at different temperatures.

46. Therefore, it is preferable to employ the system
shown in Fig(b) to keep the copper-iron and
copper-constantan junction in the thermos flask
at 00C and provide binding posts of copper. This
ensures maximum accuracy in the thermocouple
operation.

47. The applications of this law are :
(i) The law makes it possible to use extension wires of a
metal different from the metals used for thermocouple. For
example, the extension wires used for Platinum/Platinum
Rhodium thermocouple may made of copper which is an
inexpensive material.
(ii) The law enables a measuring instrument to be
introduced into the circuit without affecting emf generated
by the thermocouple.
(iii) Another important consequence of this law is that the
wires forming the junction can be soldered or brazed
together without altering the performance of the junction.
(iv) The thermal emf of any two homogeneous metals with
respect to another is the algebraic sum of their individual
emfs with respect to a third homogeneous metal.

49. Symbols used for thermocouples
Material used for
the thermocouple
Symbol Working
Temperature range
Copper - Constantan T 3 – 673
Chromel -
Constantan
E 3 - 1273
Iron - Constantan J 63 - 1473
Chromel - Alumel K 3 - 1643
Platinum – Platinum
(10% Rhodium)
S 223-2033
Platinum 13%,
Rhodium - Platinum
R 223-2033
nickel-chromium-
silicon/nickel-silicon
N −270 to 1300

50. Thermocouple Materials
The choice of materials for thermocouples is
governed by the following factors:
1. Ability to withstand the temperature at which
they are used.
2. Immunity from contamination/oxidation, etc.
which ensures maintenance of the precise
thermo-electric properties with continuous
use. and
3. Linearity characteristics.

51. It may be noted that the relationship between
thermo-electric emf and the difference between
hot and cold junction temperatures is
approximately of the parabolic form:
E=aT+bT2

53. A typical thermo-electric response of a copper-iron
thermocouple is shown in Fig. It has a nearly linear range
between OA which is a usable range of the thermocouple.
As is clear from Fig, as the temperature difference
increases the thermo emf increases and reaches a
maximum at 285°C and after that it starts to fall.
This feature of neutral temperature is interesting but has a
disadvantage in practical measurements. In the region
neutral temperature the thermocouple is extremely
insensitive to the change in temperature and there is even
a possibility of ambiguity in the temperature reading.

54. Thermocouples can be broadly classified in two
categories:
1. Base-metal thermocouples, and
2. Rare-metal thermocouples.
Base-metal thermocouples use the combination of pure
metals and alloys of iron, and nickel and are used for
temperatures up to 1450 K. These are most commonly
used in practice as they are more sensitive, cheaper and
have nearly linear characteristics. Their chief limitation
is the lower operating range because of their low
melting point and vulnerability to oxidation

55. On the other hand, rare-metal thermocouples use a
combination of pure metals and alloys of platinum
for temperatures up to 200 K and tungsten,
rhodium and molybdenum for temperatures up to
2900 K.

56. For special purposes where high sensitivity is
needed, thermocouples may be attached in series.
The output is then the numerical sum of the
voltages expected from each of the single couples
This is commonly known as thermopile.

58. When connected in parallel, a group of
thermocouples will give a reading that is the
numerical average of the individual ones provided,
the resistance of each individual thermocouple
being the same.

59. A copper-constantan thermocouple was found to
have linear calibration between 00C and 400°C
with emf at maximum temperature (reference
junction temperature 0°C) equal to 20.68 mV.
(a) Determine the correction which must be made
to the indicated emf if the junction temperature
is 25°C.
(b)If the indicated emf is 8.92 mV in the
thermocouple, determine the temperature of
the hot junction.

60.  
0
20.68
) Sensitivity of the thermocou
0
ple =
.0517
4
m
00 0
V/ C
a


Since the thermocouple is calibrated at the
reference junction of 00C and is being used at
250C then the correction which must be made
should be the thermo emf say Ecorr between 00C
and 250C, that is:
0.0517 25
1.293 mV
corr
E  


61. (b) Indicated emf between the hot junction and
reference junction at 25°C = 8.92 mV
Difference of temperature between hot and cold
junctions
Since the reference junction temperature is equal to
25°C, Hot junction temperature
= 172.53+25
= 197.53°C
0
8.92
0.0157
172.53 C



62. RADIATION METHODS
(PYROMETRY)
All the temperature measuring devices discussed so
far require the thermometer to be brought into
physical contact with the body whose temperature is
to be measured.
This means that the thermometer must be capable of
withstanding this temperature, which in the case
very hot bodies presents real problems since the
thermometer may melt at the high tempeture.
Secondly, for bodies that are moving, a non-
contacting device for measuring temperature is most
convenient.

63. Thirdly, if the distribution of temperature over the
surface of an object is required, a non-contacting
device can readily 'scan' the surface.

64. For temperatures above 650°C, the heat radiations
emitted from the body are of sufficient intensity to
be used for measuring the temperature.
Instruments that employ radiation principles fall into
two general classes:
(a)Total radiation pyrometer, and
(b)Selective partial radiation pyrometers.
The first is sensitive to all the radiation that enters
the instrument and the second only to radiation of a
particular wavelength.

65. Selective Radiation Pyrometers
Disappearing Filament Optical Pyrometer
The principle of this instrument is based on
Planck's law which states that the energy levels in
the radiations from a hot body are distributed in the
different wavelengths. As temperature increases,
the emissive power shifts to shorter wavelengths.
The Planck's distribution equation is:
2
5
1
/
1
c T
c
W
e 




66. 1
2
0
3
2
where
= 3.740 X 10-12 W-cm
= 1.4385 cm- C
= wavelength in cm
= absolute temperature in K
=energy level associated with
wavelength at temperature T in W
c
c
T
W
/cm


67. The classical form of this optical pyrometer is the
Disappearing Filament optical (or the
monochromatic brightness radiation pyrometer).
It is most accurate of all pyrometers;
however, it is limited to temperatures greater
than about 7000C requires visual brightness match
by a human operator.
This instrument is used to realise the International
Practical Temperature Scale above 10630C.

75. It is obvious from Planck's distribution equation that
for a given wavelength, the radiant intensity
(brightness) varies with the temperature.
In the disappearing filament instrument shown in the
Fig., an image of the target is superimposed on the
heated filament.
The tungsten lamp, which is very stable, is previously
calibrated so that when the current through the
filament is known the brightness temperature of the
filament is also known.
A red filter that passes only a narrow band of
wavelengths around 0.65 m, is placed between the
observer eye and the tungsten lamp and the target
image.

76. The observer controls the lamp current until the
filament disappears in the superimposed target
image.
The temperature calibration is made in terms of
the lamp heating current. Because of the manual
null balancing principle, the optical pyrometer is
not usable for continuous recording or automatic
control applications.
However, it is more accurate and less subject to
large errors than the total radiation pyrometer.

77. The accuracy of such pyrometers is usually ± 5%
in the range of 850°C to 1200C. Further when
used in the extended range of 11000C to 19500C,
its accuracy is better than ± l00C.