temperature measurement theories of operation rev1

This course has been presented at KAR Cement group
TEMPERATURE
MEASUREMENT

Outline….
# Introduction
# Temperature
# Heat
# Scale
# Glass-Thermometer
# Bi-metallic Thermometer
# RTD
# Thermocouple
# Thermistor
# IC Sensor
# How to choose

INTRODUCTION
 The accurate measurement of temperature is vital across
abroad spectrum of human activities,
 Including industrial processes (e.g. making steel)
Manufacturing;
 Health and safety.
 In fact, in almost every sector, temperature is one of the
key parameters to be measured.
 Different people will have different perceptions of what is
hot and what is cold.
.

 Temperature ?
 scalar quantity
 Degree of hotness or coldness
 Molecular K.E. = Temperature
 Heat ?
 Form of energy.
 Measured in calories or
BTU’S[British Thermal Units].

How is heat transferred?
Convection
Radiation
Conduction
 Metal coffee cup

CONVECTION, RADIATION and CONDUCTION
• CONVECTION--
Thermal energy is transferred from hot places to cold places by
convection. Convection occurs when warmer areas of a liquid or gas rise to
cooler areas in the liquid or gas. Cooler liquid or gas then takes the place
of the warmer areas which have risen higher. This results in a continous
circulation pattern. Water boiling in a pan is a good example of these
convection currents. Another good example of convection is in the
atmosphere. The earth's surface is warmed by the sun, the warm air rises
and cool air moves in.
• RADIATION--
Radiation is a method of heat transfer that does not rely upon any contact
between the heat source and the heated object as is the case with
conduction and convection. Heat can be transmitted through empty space
by thermal radiation often called infrared radiation. This is a
type electromagnetic radiation . No mass is exchanged and no medium is
required in the process of radiation. Examples of radiation is the heat from
the sun, or heat released from the filament of a light bulb.

CONVECTION, RADIATION and CONDUCTION
• CONDUCTION--
• Conduction is the transfer of heat between substances that
are in direct contact with each other. The better the
conductor, the more rapidly heat will be transferred. Metal
is a good conduction of heat. Conduction occurs when a
substance is heated, particles will gain more energy, and
vibrate more. These molecules then bump into nearby
particles and transfer some of their energy to them. This
then continues and passes the energy from the hot end
down to the colder end of the substance.

Scale
 Temperature - measure of the thermal energy.
 Measured in degrees [°]using scales.
1. Fahrenheit.[°F]
2. Celsius or centigrade. [°C]
3. Kelvin .[°K]
(0°C × 9/5) + 32 = 32°F

1.Liquid – in – Glass Thermometer
 The volume of mercury changes
slightly with temperature.
 The space above the mercury
may be filled with nitrogen or it
may be at less than atmospheric
pressure, a partial vacuum

 Temperature Indicators (TI) or Temperature Gauges (TG)
Principles :
 Expansion/Contraction - change in temperature.
 Different metals -- different co-efficient of temperatures.
The rate of volumetric change depends on this co-efficient of
temperature.
2.Bimetallic Thermometer

3.Resistance Temperature Detector (RTD)
 Resistance thermometer
 PRINCIPLE :
TEMPATURE = RESISTANCE
 Positive temperature coefficient

RTD Types
 classified according to the different sensing
elements used –
Platinum
Nickel
Copper

Resistance Temperature Detector

Resistance Temperature Detector
 In RTD devices; Copper, Nickel and Platinum are widely used metals. These
three metals are having different resistance variations with respective to the
temperature variations. That is called resistance-temperature characteristics.
Platinum has the temperature range of 650°C, and then the Copper and Nickel
have 120°C and 300°C respectively. The figure-1 shows the resistance-
temperature characteristics curve of the three different metals. For Platinum, its
resistance changes by approximately 0.4 ohms per degree Celsius of
temperature. The purity of the platinum is checked by measuring R100 / R0.
Because, whatever the materials actually we are using for making the RTD that
should be pure. If it will not pure, it will deviate from the conventional resistance-
temperature graph. So, α and β values will change depending upon the metals.

Resistance Temperature Detector (RTD)

Equation of RTD’s
PT100 Table

4.Thermocouples
What is a Thermocouple?
A thermocouple circuit is formed when two dissimilar
metals are joined at both ends and there is a difference in
temperature between the two ends. This difference in
temperature creates a small current and is called the
Seebeck effect after Thomas Seebeck who discovered
this phenomenon in 1821.

4.Thermocouples
T
X
Ta
V
Ta
A
B
 SEEBECK EFFECT

Typical Thermocouple
Configuration
Reference temperature
(slush bath)
Unknown
temperature
Metal B
MetalA MetalA
Voltmeter
Solid
Liquid

Thermocouple Types
See beck
Coif: uV/CMetalsType
J Fe-Con 50
K Ni-Cr 40
T Cu-Con 38
S Pt./Rh-Pt. 10
E Ni/Cr-Con 59
N Ni/Cr/Si-Ni/Si 39
Type K Table

5.Thermistors
 Thermally sensitive resistors
 Highly sensitive and very reproducible resistance vs. temperature.
 Limited range
 Typically used over a small temperature range (due to non-linear
characteristics)
 Thermistors do not do well at high temperatures and show
instability with time
 Manufactured from oxides of nickel, magnesium, iron,
cobalt,manganese,titatinum and other metals.
 NTC Thermistor

Simply speaking, both NTC and PTC belong to thermistor and
which act as protecting the circuits.

- NTC: Negative temperature coefficient thermistor, the higher the temperature,
the lower the resistance.
- PTC: Positive temperature coefficient thermistor, the higher the temperature, the
higher the resistance.
- NTC handles the abnormality so that the circuit can be conducted normally,
which is mainly used for temperature compensation, overcurrent protection,
overheat protection, self-controlled heating, motor starting, color TV degaussing,
etc.
- While PTC recognizes the abnormality and cuts off the circuit, which is also
mainly used for temperature compensation, overcurrent protection, overheat
protection, self-controlled heating, motor starting, and color TV degaussing.

6. I.C. Sensor
IC Temperature Sensors
Chips with temperature sensors built into the integrated
circuit may be a better alternative. IC temperature sensors
employ the principle that a bipolar junction transistor’s (BJT)
base emitter voltage to collector current varies with
temperature
As the temperature sensor is built into the integrated circuit,
manufacturers can do all the design for us. Signal
conditioning, linearization, and analog to digital conversion
can all be done on chip. One such chip, the LM75 from
National Semiconductor
Classic Bandgap Temperature Sensor

6. I.C. Sensor
 V&I signal output
 Output = Temp.
 Very linear
 Accurate @
room ambient
 Limited range
 Cheap
The AD590 is a two-terminal integrated circuit temperature transducer
that produces an output current proportional to absolute temperature.

More temperature measurement possibilities
Thyorister
Thermowell
Infrared
Thermometer
pyrometer

How to Choose a Temperature Control
Device or System ?
• Things to take into account
– Standards
– Cost
– Accuracy
– Stability over time (esp. for high
• temperatures)
– Sensitivity
– Size
– Contact/non-contact
– Temperature range
– Fluid

Examples
• Measurement
• Photochemical process
• control:
• Flower petal:
• Molten glass:
• Induction furnace:
• 100 degree Heat aging
• Sensor
• RTD (most accurate)
• Thermistor
• (lowest thermal mass)
• Optical pyrometer
• (hi temp, no contact)
• RTD (if <800C); or T/C
(Beware magnetic I noise)
• Any of the 4 sensors

 Reference :
 http://www.omega.com/temperature/z/zsection.asp
 http://www.instrumentationtoday/temperature/asp
 http://www.instrumentationtools/temperaturesensors/.jsp

temperature measurement theories of operation rev1

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