# Lecture07

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```					Inter - Bayamon

Mechanical Measurement and
Lecture        Instrumentation

7
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Professor: Dr. Omar E. Meza Castillo
omeza@bayamon.inter.edu
Department of Mechanical Engineering
MECN 4600

Inter American University of Puerto Rico
Bayamon Campus
Inter - Bayamon
Tentative Lecture Schedule

Topic                                             Lecture
Basic Principles of Measurements                 1, 2 and 3
Response of Measuring Systems, System Dynamics
Error & Uncertainty Analysis
Sensor & Transducers                                 4
Basic Electronics, Signal Processing
Measurement of Pressure                              5
Measurement of Temperature                           6
Measurement of Fluid Flow                            7
Measurement of Level                                 8
Measurement of Stress-Strain                         9
Measurement of Time Constant                        10
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Topic 6: Measurement of
Temperature
Thermocouple
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Course Objectives

o To measure        temperature   using    a
thermocouple.
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Temperature

o Temperature is the most common used and
measured variable. Actually, the definition of
temperature and scale is not well understood.
o Temperature is described as the property of an
object that describes its hotness and coldness.
o The zeroth law of thermodynamics states that two
systems in thermal equilibrium with a third system
are in thermal equilibrium with each other.
o The conversion equations for the four modern
temperature scales are:

n oC = 5/9 (oF - 32)   o
F = 9/5 oC + 32
n K = oC + 273.15      o
R = oF + 459.67
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Temperature

o Temperature can be measured via a diverse array
of sensors. All of them infer temperature by
sensing some change in a physical characteristic.
Six types with which the engineer is likely to come
into contact are: THERMOCOUPLES, RESISTIVE
TEMPERATURE DEVICES (RTD and THERMISTORS),
LIQUID EXPANSION DEVICES, AND CHANGE-OF-
STATE DEVICES.
o The most common method of measuring and
controlling temperature uses an electrical circuit
called a THERMOCOUPLE.
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Temperature

Thermocouple

RTD
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Thermistor
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Thermocouple

o A thermocouple is a device based upon the
findings of SEEBECK (1821) who showed that a
small electric will flow in a circuit composed of two
dissimilar conductors when their junctions are
kept at different temperatures. The electromotive
force (emf) produced under these conditions is
known as the “Seebeck emf”. The pair of
conductors that constitute the thermoelectric
circuit is called THERMOCOUPLE.
o The output of a thermocouple circuit is a voltage,
and there is a definite relationship between this
voltage, and temperatures of the junctions that
make up the thermocouple circuit.
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Thermocouple

Thermocouple Circuit
Copper

emf                              T

Thermocouple Type T

Constantan

o To demonstrate the errors introduced in this
procedure, introduced the junction of a type T
thermocouple in boiling water (known to be at
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reading was 3.634 mV, which corresponds to
86.1ºC.

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Thermocouple

o This temperature error arises because the
connection of the thermocouple leads to the
thermoelectric junctions that substract voltage
from the signal being measured.
o This problem can be remedied using the
arrangement show in this figure:
Copper
Probe junction

emf                                     T

Copper          Constantan
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Reference junction
Ice bath

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Thermocouple

o One thermocouple junction is held in an ice bath at
0ºC. This called THE REFERENCE JUNCTION. The
other thermocouple junction is THE TEMPERATURE
PROBE. If the probe is at 0ºC, then there is no
thermoelectric voltage across the leads because
the thermoelectric voltage created by each
junction cancel each other out.
o There are three basic phenomena that can occur in
a thermocouple circuit: (1)Seebeck effect,
(2)Peltier effect, and (3)Thomson effect.

o SEEBECK EFFECT
Thomas Johann Seebeck (1770-1831).The Seebeck
effect refers to the generation of a voltage
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potential, or emf, in an open thermocouple circuit
caused by a difference in temperature between
junctions in the circuit.
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Thermocouple

Material A

T1   1                                    2   T2

Material B          Material A
emf

o There is a fixed, reproducible relationship between
the emf and the junction temperatures T1 and T2.
o This relationship is expressed by the Seebeck
coefficient, defined as:
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Thermocouple

o PELTIER EFFECT

Peltier Coefficient

Heat Transfer

Material A                         Material B

Current I
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emf (external)

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Thermocouple

o THOMPSON EFFECT

Thomson Coefficient

q2

q1
T1                                T2

i

Voltage supply
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Fundamental Thermocouple Laws

1. Law of Homogeneous Materials.-
A thermoelectric current cannot be sustained in a
circuit of a single homogeneous material by the
application of heat alone, regardless of how it
might vary in cross section.

A                A

T3
T1

emf = 0
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A                    A
T2

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Fundamental Thermocouple Laws

2. 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.
T1

A                      C

emf = 0
T1

B                         C
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T1

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Fundamental Thermocouple Laws

3. Law of Successive or Intermediate Temperatures.-
If two dissimilar homogeneous materials produce
thermal emf1 when the junctions are at T1 and T2
and produce thermal emf2 when the junctions are
at T2 and T3, the emf generated when the junctions
are at T1 and T3 will be emf1 + emf2.

Copper                          Copper                            Copper

T1                         emf1 T2                                T1
emf2                      emf1+emf2

Constantan      Copper          Constantan      Copper            Constantan      Copper
T2                               T3
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T3

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Experiment Setup
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Type of Thermocouple

Type         Positive           Negative    Applications
E           Chromel           Constantan     <1000ºC
J             Iron            Constantan     <760ºC
K           Chromel            Alumel        <1372ºC
S     Platinum 10% rhodium     Platinum      <1768ºC
T           Copper            Constantan     <400ºC
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19                           Lecture 7
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Laboratory 3 à WebPage
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Omar E. Meza Castillo Ph.D.

20                         Lecture 7

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