Presentation on Temperature Measurement.

1. Temperature
• Temperature is a measure of the average kinetic energy of the
particles that make up a body
( The greater the kinetic energy of the particles is, the temperature of the body
will be )
• Temperature is the ability of one body to transfer thermal energy
to another body
( If two bodies are in thermal equilibrium and no thermal energy is exchanged ,
the bodies are at the same temperature )

2. Temperature
• Molecular motion creates heat known as
thermal energy
• Thermal movement from hot to cold is called
thermodynamics
• Absolute Zero ( no molecular motion ) means
no heat is produced

3. The word temper was used in the seventeenth century to describe the
quality of steel. It seems, after the invention of crude from of thermometer,
the word temperature was coined to describe the degree of hotness or
coolness of a material body.
For a standard to be established it is necessary
1. To have reference temperature for determining the size of the
unit
2. To suggest an appropriate interpolation technique for measuring
temperature difference
The relation between isotherm temperature T and p or v defined
theoretically as Pv = nRT
And the relation between heat Q and T defined as

4. Temperature Scales
A scale representing quantitatively the temperature of a body is arbitrary.
Temperature scales are determined by choosing a particular physical property of a
body, such as the thermal expansion of a gas at constant volume or the change of
electrical resistance of a wire with temperature; designating two thermal equilibrium
points by numbers; and subdividing the interval into a number of equal parts.
However,
since such scales depend on different physical properties, the temperature
intervals need not be identical, even though the numbers on the scales agree.

5. An ideal temperature scale is independent of the substance used in
defining it. Such a scale was suggested by Lord Kelvin and is called the
thermodynamic scale. This scale is based on the availability of energy
and is arranged so that the ratio of the values of any two temperatures is
equal to the ratio of the heat taken in to the heat rejected by a reversible
heat engine working with a heat source at the higher temperature and a
heat sink at the lower temperature.
Suppose we call the temperature at which heat is taken in by the first
engine some fixed number and then number all the engines at
progressively lower temperatures. The engine in the series that
discharges no heat would end the series. This point is called absolute
zero.
On the Kelvin thermodynamic temperature scale the equilibrium
temperature of ice and water at standard pressure is called the ice point
and is 273.16°K. The equilibrium temperature of water and steam at
standard pressure is called the steam point and is 373.16°K.

6. Industrially important :Fahrenheit, centigrade, Kelvin, Rankine, and Reaumur.
The Fahrenheit scale, abbreviated °F, was introduced about 1665 and is used in
most English-speaking countries. This scale assigns OF to the lowest temperature of
a certain salt-and-ice mixture, 32°F to the ice point, and 212°F to the steam point.
The centigrade scale, abbreviated DC, was introduced about 1740 and is commonly
used in European countries, where it is called the Celsius scale. It is also commonly
used in technical scientific literature. This scale assigns O°C to the ice point and
100°C to the steam point.
The Kelvin scale, abbreviated °K, is also called the centigrade absolute scale. It is
much used i.n technical literature. - This scale assigns 273.16°K to the ice point and
373.16°K to the steam point.
The Rankine scale, abbreviated °R' and also called the Fahrenheit absolute scale, is
commonly used in engineering literature. It assigns 491.69°R' to the ice point and
671.69°R' to the. steam point.
The Reaumur scale, abbreviated °R, was introduced about 1731 and is used in a
few European countries. It assigns OCR to the ice point and 800R to the steam
point. This scale is often used in the alcohol industries

7. • Celsius / Fahrenheit units
( are used in the common every day scales )
• Kelvin / Rankine are used when working with Absolute
Temperature Scale
( These are typically used in engineering and research calculations )

8. (°F) = 9/5*(°C) +32
(°C) = 5/9*[(°F) –32]
(°F) = (°R) – 459.67
(°C) = (K) – 273.15

9. Conversion from one scale to another

10. Temperature Measurement
Like the other process measurements temperature measuring
devices are divided into 3 general categories
• Indicators
• Sensors / transducers , switches
• Transmitters
The design and construction of these devices is based on how
different materials react when subjected to heat and what type
measurement is required ie indication only , point measurement,
analog output . ect

11. CLASSIFICATION OF TEMPERATURE MEASURING DEVICES
There are many basis for classification of temperature
measuring instruments. One of the classification on the basis of
1. Nature of change produced in the temperature sensing
element or the phenomenon used for production of a change
due to temperature.
a)Those which are primarily electrical or electronic in
nature and
b)Those which do not employ electrical and electronic
methods for their working.
2. Electrical and non-electrical operating principles.
3. Temperature range of the instrument.

12. The classification given by ASME code is :
• Glass Thermometers. These thermometers work on the principle of
expansion of liquids like mercury. Alcohol Pentane and other organic
liquids filled in glass casing.
•Pressure Gauge Thermometers . These instruments produce a
pressure output on account of vapors or liquids which work as actuating
fluids. They may be further classified as vapor pressure type and liquid or
gas pressure type.
•Differential Expansion Thermometers. The output of these
instruments is on account of differential expansion of two dissimilar
metals produced by the temperature.
•Electrical Resistance Thermometers. The temperature is indicated by
change in resistance of a conductor in these thermometers.
•Thermocouples. In these instruments the temperature is indicated by
production of an e.m.f. when the junction of two dissimilar metals are kept
at different temperatures.

13. •Optical Pyrometers. The temperature is determined by these instruments by
matching the luminosity of the radiation of the hot body with that of a calibrated
source. These instruments utilize the visible spectrum of electromagnetic
radiation.
• Radiation Pyrometers. In these instruments. the temperature is estimated by
absorbing radiation of all wavelengths upon a small body and determining the
temperature of the source from the temperature attained by the absorber.
•Fusion Pyrometers : The temperature in this case is determined by observing
which of a series of materials with graduated fusion materials melt or soften when
subjected to the temperature under measurement.
•Calorimetric Pyrometers. The temperature is determined by measuring quantity
of heat removed when the temperature of a body of known thermal capacity is
brought down from an unknown to a known level.
•Color Temperature Charts. The wave lengths (which represent different colors)
of the radiation emitted form the hot body depends upon the temperature of the
body. Charts are available which relate the -color of radiation with temperature.
The temperature of the hot body can be estimated from the color of its radiation
and referring to the color temperature charts.

16. Liquid expansion devices
This type of temperature measurement is based on
the theory that when a liquid is heated its volume
or pressure will change in proportion to the
applied temperature
There are 2 types
1. Liquid in glass Thermometers
2. Filled Thermal Systems

17. Liquid in Glass thermometers
liquid- in – glass of bulb thermometers are a common
type of temperature indicator in use today

18. Bulb:
The reservoir for containing most of the thermometric liquid
Stem :
The glass having a capillary bore along which the liquid moves with
changes in temperature
Main Scale
An engrave , etched or otherwise permanently attached scale with
well- defined , narrow graduation lines against which the height of
the liquid in the capillary is measured. There may be a colored
backing material for better visibility of the lines . The main scale is
graduated in fractions or multiples of degree Celsius. If its range
incorporates the reference temperature, it is the only scale

19. The Desirable properties for a liquid used in a glass thermometer.
1. The temperature-dimensional relationship should be linear, thereby facilitating
the use of a linear scale for the instrument.
2. The liquid should have as large a co-efficient of expansion as possible so that
the expansions are larger thereby making it possible to use large capillary bores,
and hence provide easier reading.
Greater expansions for the same temperature give higher sensitivities. For these
reasons use of alcohol is better than that of mercury.

20. 3. the liquid should accommodate a reasonable temperature range without change of
temperature.
Mercury is the most common liquid utilized at intermediate and high temperatures.
However, its freezing point - 38.87°C limits its lower range. The upper limit is the
region of 540°C and requires the' use of special glasses and an inert gas to fill the
capillary space above the mercury.
For low temperatures
alcohol is usable to - 75°C tolean to - 90°C,
pentane to - 200°C,
mixture of propane and propylene giving the lower limit of - 215°C.
4. The liquid should be clearly visible when drawn into thread. Mercury is inherently
good in this regards, whereas alcohol is usable only if dye is added.
5. The liquid, preferably, should not stick to the capillary walls. When rapid temperature
drops occur, any film remaining on the wall of the tube will cause a reading that is
too low. In this respect, mercury is much better than alcohol.

21. The sensitivity of the thermometer depends up on the portion on the immersion
of the thermometer .
1. total immersion type 2. partial immersion type.
Correction = 0.00016 n (cal - a ) °C
where n = number of scale degrees equivalent to emergent stem length °C,
cal = air temperature at calibration; °C
a = actual air temperature at use; ( reading of auxiliary thermometer) °

22. Advantages
• Easy portability
independence of auxiliary equipment
low cost
• Compatibility with most of the equipment
• Moderate ruggedness and
• Wide range ( 70K – 1000⁰C but its frequent use is
with in the range -40°C to 250°C

23. Disadvantages
• A large sensing element
• Impossibility for continuous or automatic
read out
• Long time constant
• Awkward dimensions , and hysteresis
( except for special type )
• Breakage ( Mercury contamination )

24. Industrial grade thermometers

25. PRESSURE GAUGE THERMOMETERS
( filled Systems )
The measuring principle is based on the volumetric temperature expansion of the gas
filling in the measuring element.
These thermometers essentially consist of a bulb, containing a liquid, gas, or vapor,
which is immersed in environments. The bulb is connected by means of capillary tube to
some pressure measuring device, such as a bourdon tube

26. LIQUID FILLED SYSTEMS
They utilize the volumetric expansion of a liquid caused by temperature
changes to operate a receiving element for indication of temperature. The
thermal system is solidly filled with liquid at a high pressure and care is
taken to eliminate all entrapped air
The relationship between final and initial volumes is given by :
Vt = Vo ( l +  + 2 + 3 ) ..
.
where Vt and Vo are respectively the final and initial volumes of liquid in
m e is the change in temperature in °C, and  ,  ,  are co-
efficients of volumetric expansion
Vt = Vo ( l + A  )
where A = mean co-efficient of volumetric expansion, per °C.

27. Filled Systems

28. Pressure bulb.
The size of bulb depends upon the type of fluid used, the temperature
span of system and the length of capillary tube used with it.
The materials used for systems not using mercury as filler liquid are
copper, stainless steel and Monel.
For mercury filled bulbs, stainless steel is used as mercury has the
tendency to amalgamate with copper and its alloys.
A thermal well may be used along with the bulb in applications where
temperature of fluids under pressure is to be measured, where extra
protection against corrosion is required, or where there is need for extra
mechanical protection.
The wells are made from materials like copper, brass, cast iron, steel,
stainless steel, nickel or Monel.

29. Capillary tube.
The capillary tube is made of copper or steel for systems using
fluids other than mercury.
When mercury is used as the transmitting fluid, stainless steel
capillary tubing is used
Receiving element.

31. Liquid Freezing
Point °C
Boiling point
°C
A X 103  X 103  X 103  X 103
Acetone -95 56 1.487 1.324 3081 -0.88
Ethyl Alcohol -115 78 1.120 1.012 2.20
Ethyl ether -115 35 1.656 1.513 2.36 4.00
Mercury -39 357 0.1819 0.1818 0.0078
Pentane -130 36 1.608 1.465 3.093 1.61
Toluene -92 110 1.224
Table . Characteristics of liquids used in Liquid filled systems
Mercury is the most widely used liquid because of the wide temperature range
between its freezing and boiling points. However, when higher sensitivity is
required, organic liquids like toluene and ethyl alcohol are used as their co-
efficient of volumetric expansion are approximately six times that of mercury.

32. GAS FILLED SYSTEMS
The gas filled systems work fundamentally with Charle's Law
P v = RT
where P = absolute pressure, v = specific volume, R = gas constant and T = absolute
temperature.
The volume of bulb capillary and tubing is substantially constant, and
therefore,
P = (R/ v ) T
Thus the relationship between pressure and temperature is ideally linear. This relationship
is true within a moderate range of -125°C to
300°C.
Therefore, for a gas filled system, it follows from Eqn.
The volume of gas required in the bulb is determined by the gas expansion and by the
temperature range of the instrument as

33. LIQUID VAPOUR FILLED SYSTEMS
The liquid vapor filled systems operate from the vapor pressure of a liquid that
partially fills the system..
The vapor pressure solely depends on the free surface of liquid and therefore a
liquid vapor filled system indicates only the temperature existing at the free
surface, wherever it may be in the system.

34. Fluid Boiling point °C Critical Temperature °C Range °C
Argon -185.7 -122 Down to -
250°C
Diethyl ether 34.5 193.8 60-160
Ether alcohol 78.5 234 90-170
Methyl Chloride 11.1 143 0-50
Toluene 110.5 320.6 15—250
Water 100 374 120-220
Table Characteristics of fluids for liquid vapor systems
The most commonly used fluids are methyl chloride, sulphur dioxide, ether,
toluene, butane, propane and hexane. The relationship between vapor pressure
and temperature is non-linear. The temperature is roughly a logarithmic function of
the pressure as given by following relationship
The range of liquid vapor filled systems depends entirely on the fill medium.
Temperatures in the range of -10°C to 300°C may be measured with sufficient
accuracy and the scale can be extended somewhat with special fill media.

35. Static Errors in the filled Systems
1. Ambient Temperature Effect
2. Head Effect
3. Barometric Effect
4. Immersion Effect
5. Radiation effects

36. Case compensation.
Full Compensation
Ambient Temperature Effect

38. Head Effect: In this application the bulb is
installed above the receiving element

39. Speed of Response of filled Systems
(i)thermal capacitance
(ii)thermal conductivity,
(iii)surface area per unit mass.

40. Dip effect in Mercury
Thermometers
Effect of thermal well on
Thermometer response