mechanical measurement and instrumentation mecn 4600

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LectureLecture

77Mechanical Measurement and Mechanical Measurement and

InstrumentationInstrumentation MECN 4600 MECN 4600

Professor: Dr. Omar E. Meza CastilloProfessor: Dr. Omar E. Meza [email protected]

http://www.bc.inter.edu/facultad/omeza

Department of Mechanical EngineeringDepartment of Mechanical Engineering

Inter American University of Puerto RicoInter American University of Puerto Rico

Bayamon CampusBayamon Campus

Lecture 7Lecture 7MEC

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Tentative Lecture ScheduleTentative Lecture Schedule

TopicTopic LectureLecture

Basic Principles of MeasurementsBasic Principles of MeasurementsResponse of Measuring Systems, System DynamicsResponse of Measuring Systems, System DynamicsError & Uncertainty AnalysisError & Uncertainty Analysis

1, 2 and 31, 2 and 3

Sensor & TransducersSensor & TransducersBasic Electronics, Signal ProcessingBasic Electronics, Signal Processing

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Measurement of PressureMeasurement of Pressure 55

Measurement of TemperatureMeasurement of Temperature 66

Measurement of Fluid FlowMeasurement of Fluid Flow 77

Measurement of LevelMeasurement of Level 88

Measurement of Stress-StrainMeasurement of Stress-Strain 99

Measurement of Time ConstantMeasurement of Time Constant 1010


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ThermocoupleThermocouple

Topic 6: Measurement of Topic 6: Measurement of TemperatureTemperature

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To measure temperature using a To measure temperature using a thermocouple.thermocouple.

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Course ObjectivesCourse Objectives


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TemperatureTemperature

Temperature is the most common used and Temperature is the most common used and measured variable. Actually, the definition of measured variable. Actually, the definition of temperature and scale is not well understood. temperature and scale is not well understood.

Temperature is described as the property of an Temperature is described as the property of an object that describes its hotness and coldness.object that describes its hotness and coldness.

The zeroth law of thermodynamics states that two The zeroth law of thermodynamics states that two systems in thermal equilibrium with a third systems in thermal equilibrium with a third system are in thermal equilibrium with each other.system are in thermal equilibrium with each other.

The conversion equations for the four modern The conversion equations for the four modern temperature scales are:temperature scales are:

ooC = 5/9 (C = 5/9 (ooF - 32)F - 32) ooF = 9/5 F = 9/5 ooC + 32C + 32 K = K = ooC + 273.15C + 273.15 ooR = R = ooF + 459.67F + 459.67

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Temperature can be measured via a diverse array Temperature can be measured via a diverse array of sensors. All of them infer temperature by of sensors. All of them infer temperature by sensing some change in a physical characteristic. sensing some change in a physical characteristic. Six types with which the engineer is likely to come Six types with which the engineer is likely to come into contact are: THERMOCOUPLES, RESISTIVE into contact are: THERMOCOUPLES, RESISTIVE TEMPERATURE DEVICES (RTD and THERMISTORS), TEMPERATURE DEVICES (RTD and THERMISTORS), INFRARED RADIATORS, BIMETALLIC DEVICES, INFRARED RADIATORS, BIMETALLIC DEVICES, LIQUID EXPANSION DEVICES, AND CHANGE-OF-LIQUID EXPANSION DEVICES, AND CHANGE-OF-STATE DEVICES.STATE DEVICES.

The most common method of measuring and The most common method of measuring and controlling temperature uses an electrical circuit controlling temperature uses an electrical circuit called a called a THERMOCOUPLETHERMOCOUPLE..

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Thermocouple

RTD

Thermistor


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A thermocouple is a device based upon the A thermocouple is a device based upon the findings of findings of SEEBECK (1821) SEEBECK (1821) who showed that a who showed that a small electric will flow in a circuit composed of two small electric will flow in a circuit composed of two dissimilar conductors when their junctions are dissimilar conductors when their junctions are kept at different temperatures. The electromotive kept at different temperatures. The electromotive force force (emf) (emf) produced under these conditions is produced under these conditions is known as the known as the “Seebeck emf”. “Seebeck emf”. The pair of The pair of conductors that constitute the thermoelectric conductors that constitute the thermoelectric circuit is called circuit is called THERMOCOUPLETHERMOCOUPLE..

The output of a thermocouple circuit is a voltage, The output of a thermocouple circuit is a voltage, and there is a definite relationship between this and there is a definite relationship between this voltage, and temperatures of the junctions that voltage, and temperatures of the junctions that make up the thermocouple circuit.make up the thermocouple circuit.

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99

emf

CopperCopper

ConstantanConstantan

T

Thermocouple Type TThermocouple Type T

To demonstrate the errors introduced in this To demonstrate the errors introduced in this procedure, introduced the junction of a type T procedure, introduced the junction of a type T thermocouple in boiling water (known to be at thermocouple in boiling water (known to be at 100ºC) and read the voltage across the leads. The 100ºC) and read the voltage across the leads. The reading was 3.634 mV, which corresponds to reading was 3.634 mV, which corresponds to 86.1ºC.86.1ºC.

Thermocouple CircuitThermocouple Circuit


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This temperature error arises because the This temperature error arises because the connection of the thermocouple leads to the connection of the thermocouple leads to the voltimeter constitutes two additional voltimeter constitutes two additional thermoelectric junctions that substract voltage thermoelectric junctions that substract voltage from the signal being measured.from the signal being measured.

This problem can be remedied using the This problem can be remedied using the arrangement show in this figure:arrangement show in this figure:

1010

emf

CopperCopper


T

CopperCopper

Ice bathReference junctionReference junction

Probe junctionProbe junction


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One thermocouple junction is held in an ice bath at One thermocouple junction is held in an ice bath at 0ºC. This called 0ºC. This called THE REFERENCE JUNCTIONTHE REFERENCE JUNCTION. The other . The other thermocouple junction is thermocouple junction is THE TEMPERATURE PROBETHE TEMPERATURE PROBE. If . If the probe is at 0ºC, then there is no thermoelectric the probe is at 0ºC, then there is no thermoelectric voltage across the leads because the thermoelectric voltage across the leads because the thermoelectric voltage created by each junction cancel each other voltage created by each junction cancel each other out.out.

There are three basic phenomena that can occur in a There are three basic phenomena that can occur in a thermocouple circuit: (1)Seebeck effect, (2)Peltier thermocouple circuit: (1)Seebeck effect, (2)Peltier effect, and (3)Thomson effect.effect, and (3)Thomson effect.

SEEBECK EFFECTSEEBECK EFFECT

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

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There is a fixed, reproducible relationship There is a fixed, reproducible relationship between the between the emfemf and the junction temperatures T and the junction temperatures T11 and Tand T22..

This relationship is expressed by the Seebeck This relationship is expressed by the Seebeck coefficient, defined as:coefficient, defined as:

1212

emf

Material A

T2

Material AMaterial B

T1 21

circuitopen

AB T

emf

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PELTIER EFFECTPELTIER EFFECT

1313

Peltier Coefficient

Heat Transfer

Material A Material B

emf (external)

Current I

IQ AB


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THOMPSON EFFECTTHOMPSON EFFECT

1414

Thomson Coefficient

Voltage supply

i

q1

T1 T2

q2

12 TTIQ


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Fundamental Thermocouple LawsFundamental Thermocouple Laws

1.1. Law of Homogeneous Materials.-Law of Homogeneous Materials.-

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

1515

emf = 0

AA

AA

T3T1

T2


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2.2. Law of Intermediate Materials.-Law of Intermediate Materials.-

The algebraic sum of the thermoelectric forces in The algebraic sum of the thermoelectric forces in a circuit composed of any number of dissimilar a circuit composed of any number of dissimilar materials is zero if all of the circuit is at a uniform materials is zero if all of the circuit is at a uniform temperature.temperature.

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A

B

C

C

T1

T1

emf = 0T1


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3.3. Law of Successive or Intermediate Temperatures.-Law of Successive or Intermediate Temperatures.-

If two dissimilar homogeneous materials produce If two dissimilar homogeneous materials produce thermal emfthermal emf11 when the junctions are at T when the junctions are at T11 and T and T22 and produce thermal emfand produce thermal emf22 when the junctions are when the junctions are at Tat T22 and T and T33, the emf generated when the junctions , the emf generated when the junctions are at Tare at T11 and T and T33 will be emf will be emf11 + emf + emf22..

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emf1

CopperCopper


T1

CopperCopper

T2

emf2

CopperCopper


T2

CopperCopper

T3

emf1+emf2

CopperCopper


T1

CopperCopper

T3


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Experiment SetupExperiment Setup

1818


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Type of ThermocoupleType of Thermocouple

1919

TypeType PositivePositive NegativeNegative ApplicationsApplications

EE ChromelChromel ConstantanConstantan <1000ºC<1000ºC

JJ IronIron ConstantanConstantan <760ºC<760ºC

KK ChromelChromel AlumelAlumel <1372ºC<1372ºC

SS Platinum 10% rhodiumPlatinum 10% rhodium PlatinumPlatinum <1768ºC<1768ºC

TT CopperCopper ConstantanConstantan <400ºC<400ºC


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Omar E. Meza Castillo Ph.D.Omar E. Meza Castillo Ph.D.

Laboratory 3 Laboratory 3 WebPage WebPage

2020

mechanical measurement and instrumentation mecn 4600

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