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					Inter - Bayamon




                            Mechanical Measurement and
                  Lecture        Instrumentation



          7
                                    MECN 4600


                               Professor: Dr. Omar E. Meza Castillo
                                    omeza@bayamon.inter.edu
                             http://www.bc.inter.edu/facultad/omeza
                              Department of Mechanical Engineering
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                             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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                                                  2                    Lecture 7
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                  Topic 6: Measurement of
                             Temperature
                     Thermocouple
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                          3           Lecture 7
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                  Course Objectives



                  o To measure        temperature   using    a
                    thermocouple.
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                                        4                   Lecture 7
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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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                                            5                    Lecture 7
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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),
                    INFRARED RADIATORS, BIMETALLIC DEVICES,
                    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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                                            6                     Lecture 7
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                  Temperature




                    Thermocouple

                                   RTD
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                                         Thermistor
                                     7            Lecture 7
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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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                                             8                     Lecture 7
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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
                    100ºC) and read the voltage across the leads. The
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                    reading was 3.634 mV, which corresponds to
                    86.1ºC.

                                              9                               Lecture 7
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                  Thermocouple

                  o This temperature error arises because the
                    connection of the thermocouple leads to the
                    voltimeter    constitutes    two   additional
                    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


                                                  10                                 Lecture 7
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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.
                                           11                     Lecture 7
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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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                                                        12                     Lecture 7
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                  Thermocouple

                  o PELTIER EFFECT



                                                               Peltier Coefficient




                                                             Heat Transfer

                             Material A                         Material B




                               Current I
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                                           emf (external)




                                                        13                           Lecture 7
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                  Thermocouple

                  o THOMPSON EFFECT



                                       Thomson Coefficient



                                                        q2

                                 q1
                           T1                                T2




                                            i




                                      Voltage supply
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                                            14                    Lecture 7
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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




                                            15                     Lecture 7
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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



                                                16                          Lecture 7
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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




                                                               17                                         Lecture 7
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                  Experiment Setup
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                                     18   Lecture 7
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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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