THERMOELECTRIC
TEMPERATURE
MEASUREMENT
Presented By:
Abdul Mateen BSME01153094
Saeed Ahmad BSME01153036
Abdul Qadeer BSME01153031
Mohammad Usman BSME01153102
Thermoelectric Temperature
Measurement
Thermocouple
Electric Effects
Fundamental Thermocouple Laws
Thermocouple Standards
Thermocouple Voltage Measurement
Examples
Thermopiles
Thermopiles in Series combination
Thermopiles in Parallel combination
Thermocouple
Thermocouple is a sensor which
measures temperature.
Principle: thermoelectric
It consists of two dissimilar metals which
are joined together at one end and
separated at the other end. when there is
a temperature difference between these
two junctions , a measureable current
flows. These two junctions form
thermocouple.
• This device converts temperature
difference into voltage called seeback
voltage.
• The voltage magnitude depends upon
material and temperature.
Thermocouple Theory
A thermocouple, shown in Figure,
consists of two wires of dissimilar
metals joined together at one end,
called the measurement ("hot")
junction.
The other end, where the wires are
not joined, is connected to the signal
conditioning circuitry traces, typically
made of copper.
This junction between the
thermocouple metals and the copper
traces is called the reference ("cold")
junction.*
Thermoelectric effects
1: Seeback Effect
2 : Peltier Effect
3: Thomson Effect
Seebeck Effect
In 1821 in Germany, Thomas Johann
Seebeck discovered Seebeck effect.
He discovered that when “Two dissimilar
metals joined together at different
temperatures they produce electromotive
force or voltage called Seebeck Voltage”
Seebeck coefficient aAB
Application:
Thermoelectric Generator
Power plant industries
Automobiles
Space probes
Rechareable devices
Peltier Effect:
In 1824 in France, Jean Charles Athanase
Peltier experimented Seeback’s bismuth-
antimony thermocouple.
He simply found out that “When current
flows across the junction of two metals, it
gives rise to an absorption and liberation
of heat, depending upon the current flow”
Peltier Coefficient
The amount of heat absorbed or evolved
at a junction due to peltier effect, when 1
C of charge passes across the junction is
called peltier coefficient
H = πIt
Application:
Computer processors
Refregerators
automobiles
Thomson Effect:
In addition to the Seebeck effect and the
Peltier effect, there is a third
phenomenon that occurs in
thermoelectric circuits.
It was presented by William Thomson in
1851.
The absorption or evolution of heat when
steady current is passed through an
unequally heated conductor is called
Thomson’s effect.
Thomson coefficient, δ, as:
Qσ = σ I (T1 – T2 )
For a thermocouple circuit,
all three of these effects may
be present and may
contribute to the overall emf
of the circuit
Fundamental
Thermocouple Laws
Fundamental Thermocouple
Laws
The three fundamental empirical laws
behind the accurate measurement of
temperature by thermoelectric means are
the:
i. Law of homogeneous materials
ii. Law of intermediate materials
iii. Law of intermediate temperatures
Law of Homogenous Material:
A thermoelectric current cannot be
sustained or established in a circuit of a
single homogeneous material by the
application of heat alone, regardless of
how it might vary in cross section
Law of Intermediate Materials
The algebraic sum of the thermoelectric
forces in a circuit composed of any number
of dissimilar materials is zero if all of the
circuit is at a uniform temperature.
For Example:
Law of Successive or
Intermediate Temperature
If two dissimilar homogeneous materials that form a
thermocouple circuit produce emf1 when the junctions
are at T1 and T2
and produce emf2 when the junctions are at T2 and T3,
the emf generated when the junctions are at T1 and T3
will be emf1 + emf2.
This law allows a thermocouple calibrated for one reference
temperature, say T2, to be used at another reference
temperature, such as T3, to determine temperature T1.
Thermocouple Standards
STANDARD THERMOCOUPLE VOLTAGE
The National Institute of Standards and Technology (NIST) is a
measurement standards laboratory.
It provides specifications for the materials and construction of
standard thermocouple circuits for temperature measurement .
Many material combinations for thermocouples are identified
by a thermocouple type and denoted by a letter.
Table 8.4 shows the letter designations and the polarity of
common thermocouples,
Standard Thermocouple
Voltage
Thermocouple Voltage
measurement
Thermocouple Voltage measurement
Best method for the measurement of thermocouple voltages
is a device that minimizes current flow.
These methods are:
a. Potentiometer
A potentiometer is a three-terminal resistor
with a sliding or rotating contact that forms an
adjustable voltage divider used for measuring
electric potential (voltage).
b. Digital Voltmeters
A digital voltmeter (DVM) measures an unknown input voltage
by converting the voltage to a digital
value and then displays the
voltage in numeric form.
c. Data Acquisition Cards:
Data acquisition is the process of sampling
signals that measure real world physical
conditions and converting the resulting samples
into digital numeric values that can be
manipulated by a computer.
d. High Impedance Voltmeters:
An ideal voltmeter has infinite input impedance,
meaning that it draws zero current from the
circuit under test. This way, there will be no
“impact” on the circuit as the voltage is being
measured.
Examples
Example 8.7
Example 8.8
THERMOPILES
THERMOPILES
A thermopile is an electronic device that
converts thermal energy into electrical
energy. It is composed of several
thermocouples connected usually in series
or, less commonly, in parallel.
Thermopile are extremely reliable, low
maintenance devices are oftenly used in
remote locations for power generations
APPLICATION
Infrared thermometers
Heat flux sensor
Pyrheliometers
Gas burner safety controls.
Radioisotope thermoelectric generator
THERMOPILES IN SERIES
Thermocouples usually have voltages developed
on the order of millivolts.
To increase the voltage to a usable level, you
have to multiply the voltage. In series, voltage
sources add. So a string of ten thermocouples in
series will give you ten times the voltage, 100 in
series will give 100 times the voltage as well as
100 times
the sensitivity to temperature.
THERMOPILES IN
PARALLEL
• These Thermocouples have great wire lengths.
• The main reason for connecting thermocouples,
or thermopiles, in parallel is for mathematical
averaging.
• This type of arrangement will boost the current
but not the voltage.
References
• Theory and design for mechanical
measurements by Figliola and Beasley
• Principles and methods temperature
measurements
• YouTube
• Wikipedia
• Precision digital+
