Thermocouple Lecture

7/27/2019 Thermocouple Lecture

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Expt. 4 Temperature sensors and their calibration

Objectives:

Calibration of thermocouples and thermistors

Determine the dynamic response (time constants) and of the

given thermocouples

Temperature measuring devices

a) Thermocouples

When a pair of electrical conductors are joined together, a thermal

emf is generated when the junctions are at different temperatures This phenomenon is called Seebeck effect

Such a device is called a thermocouple

The resultant emf developed by the thermocouple is generally in

the millivolt range when the temp. difference between junctions is~100oC


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To calibrate: the temperature of second junction can be varied using a constant

temperature bath and the emf recorded as function of temperature difference

between the nodes

Ice bath

VoltmeterE0.002VMetal A

Metal B

Measuring the emf of a thermo couple

At temperatureT

Vary this temp. using

a constant temp bath

E=AT+1/2 BT2+1/3CT3

The output voltage, E of such simple thermocouple circuit is usually written

In the form

The constants, A, B, and C are dependent on the thermocouple material

A Classic Thermocouple


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Commercially available thermocouples usually consist of two leads

terminating in a single junction (no reference temperature, ice bath)

The leads are connected to a thermocouple signal conditioning box

containing an electrical circuit which provides a reference voltage

equal to that produced by a reference junction placed at 0 oC

This is called ice point compensation

Most thermocouple signal conditioners usually display the temperature

directly and or provide a voltage that is proportional to the thermocouple

temperature.

A similar thermocouple signal conditioner with a digital temperature displayand an analog voltage output is used in the present experiment

Thermocouple Properties


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b) Thermistors

The thermistor, a thermally sensitive resistor, is made of semi-conducting

material

Unlike metals, thermistors respond inversely to temperature. i.e. their

resistance decreases as the temperature increases.

Thermistors are usually composed of oxides of manganese, nickel, cobalt

copper and several other nonmetals

The resistance is generally an exponential function of the temperature as

oo TTRR 11ln

Where Ro is the resistance at a reference temperature, To, while is a constant,

characteristic of the material


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Thermal runaway (generally not a good thing)

Thermistor resistance varies inversely with temperature.

The voltage applied directly across a thermistor causes its

temperature to rise, and its resistance to decrease. Sufficiently high voltage may cause thermal runaway, in which

case, higher currents and temperature are induced until the thermistor

fails or power is reduced.

A

Temperature

Time

T

E

T

E

R

B

oo TTR

R 11ln

or power is reduced.

Thermal runaway is generally prevented by placing a large resistor in series


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Static Calibration of Temperature Sensors

Calibration of Thermocouple and Thermistor

RTD (Resistance Thermal Detector) is used as reference

temperature

Measure Vtc and Vtrfor different

steady reference temperatures.

Draw static calibration curves for

the thermo couple and the

thermistorWhy static calibration?

Heater

RTD display-T oC

RTDThermo

Couple

Thermistor

DAC

system

Computer-Lab

view-Measure

Voltage- Vtc and Vtr

R1

R2R3

V

V

Rthermistor


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Unsteady response of the Thermocouples are measured

DAC

system

Computer-Lab

view-Measure

Voltage- Vtc(t)

Time constant is a parameter used to define the

dynamic response of a system.

In this experiment, we measure the time constant

of various thermocouples

Heater

Th

Ice bath

Tc

Transfer quickly from

hot bath to cold bath.

The time response ofthermo couples are

measured

Different

thermocouples are

used ( size, material)

Dynamic Calibration of Temperature Sensors


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Transient/Dynamic response of a system

Any system will take a finite amount of time to respond to a

change in input

This delay in response is called the rise time or response

time

Larger the rise time, slower its system response

time

Input(ideal)

time

Output

Rise time

In many systems (especially Ist order systems, governed by Ist

order ODEs) the rise or delay of the system is exponential

SystemInput output


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Time Constant

Time constant is a determined by the physical properties of the

system. Note that 1/e =0.368

Time constant = Time taken for a system to attain 63.2% of the

difference between initial and final steady state values

Ei

Ef

E

t

atefEiEfEtEbygivenis

functionresponsetransientThe

)()()()(E(t)

tconsTimea

t

atefEiE

fEtE

tan,1

11

)()(

)()(

Time constant is a parameter used to define the dynamic response of

a system.

By definition, time constant is defined as the time needed for

E0.632

E


10/13

Time constant of a Thermocouple

(a model problem)

Consider this problem, a thermocouple bead is initially exposed to atemperature T(t=0)=23

oC is instantaneously transferred to an ice bath

(T=T) Find the temperature variation T(t) and time constant of the Thermo

Couple ( assume TC as a lumped system ie Temperature is afunction of time only, Temperature variation in spatial directions areneglected)

Ti(t0=23)

Ice bath (T=T)

T(t) can be found by using energy conservation

dt

dTmcEE outin

T(t)=?

Bi


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)(,0

,

TThAEEdt

dTmcEE

outin

outin

Deriving Time constant of a Thermocouple

)(

)(

TT

dT

hA

mcdt

dt

dTmcTThAie

d

hA

mcdtTTdefine )(

)(,

,0

TTtt

tICi

T

dE/dtEout


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iswhereeTTTT

t

i ,

The time constant

att

mchA

i

eeTT

TT

getweSolving

)(

hA

mc

a

1

mchAa


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How do you find the time constant from your

dynamic calibration data

T(Volts)

time

Ti

Tf=T

tconstime

slopethethentVsTT

TTPlot

t

TT

TTie

eTT

TThaveWe

i

i

t

i

tan

1,ln

ln

Thermocouple Lecture

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