Presentation (Temperature )

• Temperature can be defined as
a) The condition of a body which
determines the transfer of heat to or
from other bodies.
b) The degree of hotness or coldness
as referenced to a specific scale of
temperature measurement.

HEAT
Energy that flows to a body and causes it to increase its
temperature, melt, boil, expand, or undergo other
changes is called heat.
Unit of heat is BTU (British thermal unit). The amount of
thermal energy required to raise the temperature of 1 lb
of water 1 degree F at atmospheric pressure.
1 BTU = 1055 joules.
Calorie
The amount of heat needed to raise the temperature of
one gram of water 1 deg C (starting at 15 deg C) at
atmospheric pressure. 1 Cal = 4.184 joule.

Specific heat
It is defined as the ratio of heat required to raise the
temperature of a certain weight of a substance 1o
F to
that required to raise the temperature of the same
weight of water 1o
F (measured under constant
pressure).
Vaporization
The change of physical state from liquid to gas is called
vaporization.
Condensation
The change of physical state from gas to liquid is called
condensation.

Latent heat of vaporization
The amount of heat necessary to change a substance
at the boiling point from liquid to gas is called latent heat
of vaporization.
Fusion
The change of physical state from liquid to solid is
called fusion ( or freezing).
Melting
The change of physical state from solid to liquid is
called melting.
Latent heat of fusion:
The amount of heat that must be removed as a
substance changes from a liquid to a solid , or added as
the solid becomes liquid is referred to as the latent heat
of fusion.

Temperature scales
Celsius: Melting point of ice is 0 and boiling point is 100.
o
C = (o
F – 32) / 1.8.
Fahrenheit: Melting point of ice is 32 and boiling point is 212.
o
F=(1.8 * o
C ) + 32.
Kelvin It is absolute scale.
K= o
C +273.15.
Rankine
o
R = o
F +459.67
Absolute zero
A hypothetical temperature at which a substance would have no
thermal energy. T (abs.) = -273.15 o
C
Triple point
The temperature at which gaseous, liquid, and solid states of a
substance exist simultaneously.

All temperature measurements in industry are of two
basic groups.
A). Non Electric type
Bimetallic, Liquid in glass, Filled thermal system.
B). Electric type
Thermocouple, RTD, Thermistor, Pyrometer (Radiation & Optical)
Temperature is indicated by the changes it causes in
certain instruments.
• Changes in density ( liquid in glass thermometers)
• Changes in length or volume (bimetallic or filled system)
• Voltage generated at junction of dissimilar metals
(thermocouple)
• Resonant frequency of crystals (quartz thermometers)
• Changes in electrical resistance (Resistance
thermometers)

BIMETALLIC THERMOMETERS
Coefficient of thermal expansion
• It is the change in length or volume of a substance per
degree of temperature change.
• Bimetallic thermometers use the difference in thermal
expansion of two different metals to indicate
temperature. Two straight metal strips are placed side by
side and welded together lengthwise. Bimetallic strip
bends towards the side that has the lower rate of thermal
expansion.
• If one end of strip is fixed , the distance the other end
bends is directly proportional to the square of the length
of the metal strip, as well as to the total change in
temperature, while inversely proportional to the thickness
of the metal. Invar (alloy of iron and nickel) is used as
low expansion material while brass (alloy of copper and
zinc) is used as high expansion material.
• Bimetallic thermometers are available in ranges from -75 o
C to 540
o
C with accuracy of ±1%.

LIQUID IN GLASS THERMOMETERS
• Its operation is based on the principle that liquids
expand as temperature increases. It consists of a
small bore glass tube with a thin wall glass tube at
its lower end. Liquid filled is usually mercury or an
organic compound.

FILLED THERMAL SYSTEMS
• The basic components of a
filled system thermometer are
• Thermometer bulb
• Capillary tube
• Bourdon tube
• The entire system is filled with
filling fluid. A change in
temperature causes the fluid to
expand or contract, in turn
causing the bourdon tube to
move.

1. Liquid filled system
A bulb is inserted into the substance to be measured. Filling liquid inside the
bulb (commonly toluene or xylene) is heated or cooled until the temperature of
the filling liquid matches the temperature of the measured substance. Mercury
is also used as filling fluid due to rapid response to temperature changes.
2. Gas filled systems
It operates on the principle that the pressure of a confined gas varies directly
with its absolute temperature.
P=kT, where k=constant, and T=absolute temperature.
Nitrogen gas is normally used as filled gas.
3. Vapor pressure systems
Liquid in vapor pressure system vaporizes during operation. Bulb is partially
filled with liquid while capillary and bourdon are filled with vapor. Liquids used
are methyl chloride, sulfur dioxide, ether alcohol, and toluene.
A gas remains gaseous under pressure at normal room temperature, but a
vapor under pressure at normal room temperature returns to its liquid or solid
state.

Thermometer bulbs
• It is usually made of stainless
steel due to good heat transfer
properties.
• Plain bulb: Not used where
rapid response is important.
• Averaging bulb: provides
rapid response to temperature
changes.
• Capillary bulb:
• It is wound in helix, increasing
surface area and improving
response time.

Capillary tube
A capillary should contain the smallest possible volume of filling fluid. Change
of temperature in liquid in capillary can cause movement of bourdon tube.
Bourdon tubes
Helical bourdon tube is commonly used. Bourdon tube uncoils due to
temperature rise in system. An attached linkage is used to indicate system
temperature.
A transducer is used to convert position of bourdon tube to an electric or
pneumatic signal for remote transmission.
Advantages
Filled system thermometers are used with accuracy of ±0.5%. Their
advantages are lesser maintenance, no electric power requirement, satisfactory
time response and accuracy.
Disadvantage
The disadvantage is that the entire system usually must be replaced in case of
failure.

RESISTANCE TEMPERATURE DETECTOR
• It works on the principle that electrical resistance of any
material increases with temperature increase and vice
versa. A piece of nickel or platinum wire can be used to
measure resistance for different temperatures.
• RTD resistance elements are constructed of platinum,
copper or nickel. The metal should have a high
coefficient of resistance (change in resistance that
occurs with a change in temperature).
• A platinum RTD is normally used to measure precise
temperature from -259 to 631 o
C.
• Pt 100 means temperature is 0 o
C at 100 ohms
resistance.

Measurement of unknown resistance
• The electrical circuit used for
temperature measurement is a
Wheatstone bridge. The bridge
converts the RTD’s change in
resistance to a voltage output.
This circuit uses four separate
electrical resistors, one of
which is the RTD.
• The bridge is initially balanced,
with voltage output equal to
zero, because all four resistors
are equal. If the resistance of
the RTD changes, due to a
temperature change, the
bridge becomes unbalanced,
resulting in a voltage output
other than zero.

• In two conductors RTD, these conductors
become part of resistance being measured, so
ambient temperature variation will result in error
of net temperature.

• Three conductor RTD cable is mostly used to minimize the effect of ambient temperature
variations on cable.
• In three lead circuit, L1 and L2 are in opposite arms of the bridge. The change in L1 is equal to
L2. Because L3 is in series with the input voltage, the bridge output voltage is unaffected.
• Four conductor RTD is used to reduce temperature effects on cable. This circuit is used in a
system with long lead wires whose temperature varies greatly during measurement.

• Resistance elements are usually
long spring like wires enclosed in a
metal sheath. The platinum element
is surrounded by a porcelain
insulating material that prevents a
short circuit from developing
between the wire and the sheath.
The sheath is made of Inconel, an
alloy of nickel, iron, and chromium.
This material has excellent corrosion
resistance and can be used in
extremely harsh environments for
long periods of time without
deteriorating.
• The thermowell protects the RTD
from any contamination or corrosion
caused by the gases or liquids being
measured.
• A heavy metal head, made of cast
iron or aluminum, covers the
terminal block containing the
electrical connections between the
RTD and Wheatstone bridge.
• The main advantage of RTD is
stability , linearity and accuracy.

THERMISTOR
• Its name is derived from thermally sensitive resistor. Resistance of a
thermistor varies as a function of temperature. Thermistor is an
electrical device made of solid semiconductor with high temperature
coefficient of resistivity.
• It is usually made of complex metal oxides (manganese-nickel,
manganese – nickel –iron).
• In negative coefficient thermistor resistance decreases as
temperature increases.
• A positive coefficient thermistor (resistance increases as temp.
increases) is connected in series with a control relay to react to
temperature limits or overload conditions. As temperature increases
the resistance of thermistor increases. This causes current in the
relay coil to decrease, and relay trips out. It is used to protect
industrial motors.
• Thermistor is very sensitive so it often does not require wheat stone
bridge.
• They are used in range of -260 to 315 o
C.

THERMOCOUPLE
• It works on the principle that if two wires made of different metals are joined
at one end and joined ends are at different temperatures than the open
ends, a small voltage is produced across the open ends. Joined ends are
called hot junction while open ends are called cold junction.
• To keep the wires apart, porcelain insulator is used for higher temperature.
• For accurate temperature measurement, reference junction temperature
must remain constant. Indicating or recording instruments use internal and
automatic cold junction compensation. Either a thermistor directly measures
mV in junction box and is compensated for net temperature output or a
thermocouple added to actual thermocouple can give net temperature.

Two main classes of thermocouples are
Noble metal thermocouples: platinum or gold is used as one wire. Highly resistant to
corrosion, low electric resistivity, good repeatability. Type S,R and B are all noble metal
thermocouples.
Base metal thermocouples: type J, T, E, and K are all base metal thermocouples.
Thermal emf developed is dependant on metals of thermocouple. Common types of
thermocouples are as follows.
A). COPPER-CONSTANTAN (T TYPE)
Copper element used as positive conductor and constantan element for negative
conductor. Constantan contains 55% copper and 45% Nickel. This thermocouple is used
in Oxidizing or reducing atmosphere.
Range: -300 o
F to +600 o
F
B). IRON – CONSTANTAN (J TYPE)
Iron element is used as positive conductor and constantan element for negative
conductor. This thermocouple is used in Oxidizing or reducing atmosphere.
Range: -100 o
F to +1500 o
F
C). CHROMEL- ALUMEL (K TYPE)
Chromium alloy is used as positive conductor and Alumel element for negative
conductor. This thermocouple is used in Oxidizing atmosphere.
Range: -300 o
F to +1600 o
F

D). CHROMEL-CONSTANTAN (E TYPE)
• This thermocouple is used in Oxidizing
atmosphere.
• Chromel = 90% Nickel & Chromium = 9%
• Constantan = 44% Nickel & Cupper = 55%
• Range: +32 o
F to +1600 o
F
E). PLATINUM RHODIUM/PLATINUM
(R ,B, S TYPE)
• This thermocouple is used in Oxidizing
atmosphere.
• In R type thermo couple, pt = 87% & Rh =
13%
• Range: Up to +2700 o
F
• In S type thermo couple, pt = 90% & Rh =
10%
• Range: 0 o
F to +2100 o
F
• In B type thermo couple, pt = 70% & Rh =
30%
• Range: 1472 o
F to +3092 o
F

• An extension wire made of the same material as the thermocouple
is referred to as thermocouple wire, but a wire made of different
material (with characteristics similar to the thermocouple’s) is
referred to as compensating lead wire. thermocouple wire can be
used for type T and J because material is not expensive.
Compensating lead wire is normally made of copper or
copper/nickel alloy.

SPECIAL PURPOSE THERMOCOUPLES
HOT BLAST
• It has fast response. It is used to measure temperature of preheated air to furnace.
GASKET THERMOCOUPLE
• It is mounted on studs or bolts to measure skin temperature of process lines, shell vessels or other process
machinery.
TUBE WALL THERMOCOUPLE
• It is used to measure furnace tube temperature. It is installed by welding pad to tube or other surface.
SPECIAL FEATURES OF THERMOCOUPLES
• 1). THERMOPILE
• Thermocouples connected in series to measure net voltage of thermocouples.
• 2). DIFFERENTIAL TEMPERATURE
• Two thermocouples can be used for measuring differential temperature between two points.
Connections are made in such a way that emf’s developed oppose each other. If temperature of
both thermocouples is equal , net emf is zero. It can be used to measure differential temperature
of top and bottom of steam line.
• 3). AVERAGE TEMPERATURE
• To measure average temperature across a vessel or duct, thermocouples may be used in parallel
connection. Net voltage produced at instrument is average voltage developed by thermocouples.
• For accurate measurement, resistance of all thermocouples and extension wires should be same.
Due to variation in temperature of thermocouple and length of extension wire, a swamping
resistor is used (1.5 Kilo ohm normally).

Pyrometry
• It is a method of measuring temperature without physical contact between the transducer and the
heated material. Its principle is based on relationship between temperature of a hot body and the
electromagnetic radiation it emits.
• Electromagnetic wave: A single burst of energy, like the one emitted by the solid, is referred to
as an electromagnetic wave.
• Electromagnetic radiation: A series of electromagnetic waves is referred to as electromagnetic
radiation.
• Pyrometry is based on the principle that intensity of electromagnetic radiation emitted by a body
depends partly on body’s emissivity or emittance and also depends on temperature of a body.
• Emissivity: the ability of a surface to send out radiant energy.
• Emittance: amount of energy emitted per unit area by a radiating surface.
• The radiation is not visible because it is in the infrared range of the electromagnetic spectrum,
and therefore is lower in frequency than visible light.
• Emittance varies inversely with reflection, so good emitters are poor reflectors but good
absorbers. Black body is an ideal emitter/absorber.
• Emittance = Test object’s total radiation / black body’s total radiation.
• Two bodies at same temperatures but with different values of emittance, radiate different
amounts of heat.
• Most bodies emit and absorb radiation at specific wavelengths. The wave length causes an
object to have a characteristic color if it is heated.

Types of pyrometers
Narrowband pyrometer (optical pyrometer)
• It operates on the principle of the relationship between the intensity
of emitted radiation and the temperature of the body. To measure
the temperature of a body the intensity (brightness) of radiation
emitted by body is matched to brightness of a reference object
whose temperature is known. It works in portion of electromagnetic
spectrum that lies in visible light range (0.35 to 0.75 µ).
Broadband pyrometers (radiation pyrometers)
It is based on relationship between total emitted radiation and
temperature. It works in all range of electromagnetic spectrum but
likely in visible and infrared light range. As the total energy radiated
by a hot body enters the pyrometer, it is focused by the lens onto
the detector (thermopile, photocell, thermistor or RTD). Suppose the
detector measures temperature 80 o
C for 1200 o
C temperature of
target, then increase in targets temperature causes increase in
detectors temp.

Types of narrow band pyrometers
Manual optical pyrometer
Its use is limited to the wavelengths to which the human eye can respond.
Reference filament is heated by an electric current, which can be varied by a
control knob. Brightness of filament is matched to that of target object. When
they are perfectly matched, filament disappears and can’t be seen. Control knob
is scaled to read directly in degrees Celsius or Fahrenheit. Filter limits (allows)
the portion of electromagnetic spectrum visible to human eye. Its advantage is
flexibility and portability. It cannot measure temperature below 800 o
C because
level of radiation emitted is too low.

Automatic optical pyrometer
• It compares the intensity of energy radiated by a target to that emitted by a
controlled reference (filament). The modulator passes the radiation, first from target
then from the reference through to the filter. Filter passes radiation of selected
wavelength to detector. Detector (photomultiplier tube or solid state photo
detectors) senses radiations alternately and generates an electrical signal
proportional to the intensity of the radiation. Controller receives signal from detector
and varies intensity of filament radiation until both intensities matched. The current
through filament is measure of target’s temperature.

Primary and secondary standards
• There are three standard industrial temperature-measuring instruments.
• platinum resistance thermometer
• platinum rhodium-platinum thermocouple
• Infrared optical pyrometer.
• These instruments are periodically calibrated from National Institute of
Standards and Technology (NIST). NIST used to be called the National
Bureau of Standards (NBS).
Primary calibration standards
• The international practical temperature scale (IPTS) is based on 11
temperatures at which specific substances change their state. These
temperature points are primary fixed points. These points were chosen
because the specific conditions and corresponding temperatures can be
easily duplicated.
• Secondary fixed points provide additional data points in the middle portion of
the IPTS. These points are often used for testing and calibration of
temperature measuring instruments because they are more conveniently
obtained.

Primary fixed points for the IPTS
SUBSTANCE (STATE) TEMPERATURE (oc)
Hydrogen (triple point) (-) 259.34
Hydrogen /liquid/gas
equilibrium at 25/76
standard atmospheres)
(-) 256.108
Hydrogen (boiling point) (-) 252.87
Neon (boiling point) (-) 246.048
Oxygen (triple point) (-) 218.789
Oxygen (boiling point) (-) 182.962
Water (triple point) (+) 0.01
Water (boiling point) (+) 100
Zinc (freezing point) (+) 419.58
Silver (freezing point) (+) 961.93
Gold (freezing point) (+) 1064.43

Secondary fixed points for the IPTS
SUBSTANCE (STATE) TEMPERATURE (oc)
Carbon dioxide (boiling point) (-) 78.476
Mercury (freezing point) (-) 38.862
Water (freezing point) (+)0
Phenoxybenzene (diphenyl ether) (triple point) (+) 26.87
Benzoic acid (triple point) (+) 122.37
Indium (freezing point) (+) 156.634
Tin (freezing point) (+) 231.968
cadmium (freezing point) (+) 321.108
Lead (freezing point) (+) 327.502
Mercury (boiling point) (+) 356.66
Antimony (freezing point) (+) 630.74
Aluminum (freezing point) (+) 660.37

Primary standard instruments
A primary standard is an instrument that meets specifications based on legal
international definitions of a fundamental unit of measurement. a Pt100 is the standard
for the temperature range of -259.34 to 630.74 o
C (the triple point of hydrogen to the
freezing point of antimony). A platinum/10% rhodium-platinum thermocouple (Type S) is
the standard from 630.74 to 1064.43 o
C (the freezing point of antimony to the freezing
point of gold). An optical pyrometer is the standard above 1064 o
C.
Secondary standard instruments
These are calibrated against primary standards and are nearly as accurate. These are
less expensive and easy to use than primary standards.
Working standard instruments
These are calibrated from secondary standards and are used for on-the-job testing and
calibration in the field.

Presentation (Temperature )

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