Microsoft Word - Class 6_Temperature Measurements Using Thermocouples

8/3/2019 Microsoft Word - Class 6_Temperature Measurements Using Thermocouples

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University of Puerto RicoUniversity of Puerto RicoUniversity of Puerto RicoUniversity of Puerto Rico

Mayagüez CampusMayagüez CampusMayagüez CampusMayagüez Campus –––– College of EngineeringCollege of EngineeringCollege of EngineeringCollege of Engineering

Mechanical Engineering DepartmentMechanical Engineering DepartmentMechanical Engineering DepartmentMechanical Engineering Department

Temperature MeasurementsTemperature MeasurementsTemperature MeasurementsTemperature Measurements

using Tusing Tusing Tusing THERMOCOUPLESHERMOCOUPLESHERMOCOUPLESHERMOCOUPLES

Machine Science laboratory IMachine Science laboratory IMachine Science laboratory IMachine Science laboratory I

INME 4031INME 4031INME 4031INME 4031



Introduction

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: thermocouplesthermocouplesthermocouplesthermocouples, ResistiveResistiveResistiveResistive

Temperature DevicesTemperature DevicesTemperature DevicesTemperature Devices (RTD and Thermistor), infraredinfraredinfraredinfrared

radiradiradiradiatorsatorsatorsators, bbbbimetallic dimetallic dimetallic dimetallic devicesevicesevicesevices, lllliquid expansion devicesiquid expansion devicesiquid expansion devicesiquid expansion devices,

and changechangechangechange----of of of of----state devicesstate devicesstate devicesstate devices.

The most common method of measuring and controlling

temperature uses an electrical circuit called a THERMOCOUPLETHERMOCOUPLETHERMOCOUPLETHERMOCOUPLE.

Thermocouple

A thermocouple is a device based upon the findings of SEEBECKSEEBECKSEEBECKSEEBECK

(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 thermocouplethermocouplethermocouplethermocouple. 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.

F i g u r e 1 .

F i g u r e 1 . F i g u r e 1 .

F i g u r e 1 . T y p e T t h e r m o c o u p l e .

To demonstrate the errors introduced in this procedure,

introduced the junction of a type T thermocouple in boiling

water (known to be at C °100 ) and read the voltage across the

leads. The reading was mV 634.3 , which corresponds to C °2.86 .



This temperature error arises because the connection of the

thermocouple leads to the voltmeter constitutes two additional

thermoelectric junctions that subtract voltage from the signal

being measured.

This problem can be remedied using the arrangement show in

this figure:

F i g u r e 2 .

F i g u r e 2 . F i g u r e 2 .

F i g u r e 2 . T y p e T t h e r m o c o u p l e .

One thermocouple junction is held in an ice bath at C °0 . This

called the referereferereferereference junctionnce junctionnce junctionnce junction. The other junction is the

temperature probetemperature probetemperature probetemperature probe. If the probe is at C °0 , then there is no

thermoelectric voltage created by each junction cancel eachother out.

There are three basic phenomena that can occur in a

thermocouple circuit: (1) the Seebeck effectthe Seebeck effectthe Seebeck effectthe Seebeck effect, (2) the Peltierthe Peltierthe Peltierthe Peltier

effecteffecteffecteffect, and (3) the Thompson effectthe Thompson effectthe Thompson effectthe Thompson effect.

Seebeck effect

Thomas Johann Seebeck (1770-1831)

The Seebeck effectSeebeck effectSeebeck effectSeebeck effect refers to the generation of a voltage

potential, or emf, in an open thermocouple circuit caused by a

difference in temperature between junctions in the circuit.

There is a fixed, reproducible relationship between junctions in

the circuit. There is a fixed, reproducible relationship between

the emf and the junction temperatures1

T and2

T . This



relationship is expressed by the Seebeck coefficient, ABα ,

defined as

( )

t opencircui

AB

T

emf

∂

∂=α

F i g u r e 3 .

F i g u r e 3 . F i g u r e 3 .

F i g u r e 3 . S e e b e c k e f f e c t .

w h e r e A a n d B r e f e r t o t h e t w o m a t e r i a l s t h a t c o m p r i s e t h e t h e r m o c o u p l e .

Peltier effect

The Peltier heat is the quantity of heat in addition to the

quantity R I 2 that must be removed from the junction to

maintain the junction at a constant temperature. This amount of

energy is proportional to the current flowing through the

junction; the proportionality constant is the Peltier

coefficient ABπ , and the heat transfer required to maintain a

constant temperature is

I Q ABπ π =

caused by the Peltier effect alone. This behavior was discovered

by Jean Charles Athanase Peltier (1785-1845) during experiments

with Seebeck’s thermocouple. He observed that passing acurrent through a thermocouple circuit having two junctions, as

in Figure 4, raised the temperature at one junction, while

lowering the temperature at the other junction.



F i g u r e 4 .

F i g u r e 4 . F i g u r e 4 .

F i g u r e 4 . P e l t i e r e f f e c t .

Thomson effect

Consider the conductor shown in Figure 5, which is subjected to

a longitudinal temperature gradient and also subject to a

potential difference, such that there is a flow of current and

heat in the conductor.

( )12T T I Q −⋅=σ

σ

where σ is the Thomson coefficient.

F i g u r e 5

F i g u r e 5 F i g u r e 5

F i g u r e 5 .

..

. T h o m s o n e f f e c t .



Fundamental thermocouple laws

Law of Homogeneous MaterialsLaw of Homogeneous MaterialsLaw of Homogeneous MaterialsLaw 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.

Law of Intermediate MaterialsLaw of Intermediate MaterialsLaw of Intermediate MaterialsLaw 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.

Law of Successive or Intermediate TemperaturesLaw of Successive or Intermediate TemperaturesLaw of Successive or Intermediate TemperaturesLaw of Successive or Intermediate Temperatures

If two dissimilar homogeneous materials produce thermal1

emf

when the junctions are at 1T and 2T and produce thermal 2emf when the junctions are at

2T and

3T , the emf generated when

the junctions are at1T and

3T will be

21emf emf + .

Example 1Example 1Example 1Example 1

The thermocouple circuit shown in Figure 6 is used to

measure the temperature1

T . The thermocouple junction

labeled 2 is at a temperature of C °0 , maintained by an ice-point bath. The voltage output is measured using a

potentiometer, and found to be mV 669.9 . What is1T ?

KNOWN A thermocouple circuit having one junction at C °0

and a second junction at an unknown temperature. The

circuit produces an emf of mV 669.9 .

FIND The temperature1

T .

ASSUMPTION Thermocouple follows NIST standard.

SOLUTION From a standard thermocouple table referenced to

C °0 , the temperature corresponding to mV 669.9 is

C °180 .



F i g u r e 6 .

F i g u r e 6 . F i g u r e 6 .

F i g u r e 6 . S c h e m a t i c i l l u s t r a t i o n o f a t h e r m o c o u p l e .

The measured voltage,12

V , is a function of the temperature

difference,21

T T − . The overall phenomenon is termed the

“Seebeck effect.”

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Microsoft Word - Class 6_Temperature Measurements Using Thermocouples