winsem2012 13 cp0401 28 mar 2013 rm01 temperature transducers

7/28/2019 Winsem2012 13 Cp0401 28 Mar 2013 Rm01 Temperature Transducers

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Thermocouples

Resistance-Temperature Detectors (RTD)

Thermistors

Temperature Sensors


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In 1821, T.J.Seebeck discovered that an electric potentialoccurs when two different metals are joined into a loop and thetwo junctions are held at different temperatures.

Seebeck e.m.f

a voltage difference between the two ends ofthe conductor that depends on the temperature difference ofthe ends and a material property.

If the ends of the wire has the same temperature, no e.m.f

occurs, even if the middle of the wire is hotter or colder.

Thermocouple


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Thermocouple - Principle

Twisting or welding of 2 wires
http://en.wikipedia.org/wiki/Image:Tc-dia.jpg


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In normal operation, cold junction is

placed in an ice bath


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In normal operation, cold junction is

placed in an ice bath


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Type K : Chromel-Alumel

Type J : Iron-Constantan

Type E : Chromel-Constantan

Type N : Nicros-Nisil

Type T : Copper-Constantan

It is important to note that thermocouplesmeasure the temperature difference betweentwo points, not absolute temperature.

Thermocouples


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Thermocouples Characteristics


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where

c andk = constants of the thermocouple materials

T1= the temperature of the hot junction

T2= the temperature of the cold or reference junction

Magnitude of thermal EMF

)()( 222

121 TTkTTcE


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A thermocouple was found to have linear calibrationbetween 0oC and 400oC with emf at maximumtemperature (reference junction temperature 0oC) equalto 20.68 mV.

a) Determine the correction which must be made to the indicated e.m.f if the cold

junction temperature is 25oC.

b) If the indicated e.m.f is 8.92 mV in the thermocouple circuit, determine thetemperature of the hot junction.

Problem


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(a) Sensitivity of the thermocouple

= 20.68/(400o-0o)

= 0.0517 mV/C

Since the thermocouple is calibrated at thereference junction of 0C and is being used at25C, then the correction which must be made,E

corr

between 0C and 25C

Ecorr= 0.0517 x 25

Ecorr= 1.293 mV

Solution


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Thermocouples are most suitable for measuring

over a large temperature range, up to 1800 K.

Example:

Type K: Chromel-Alumel (-190C to 260C)

Type J: Iron-Constantan (-190C to 760C)

Type E: Chromel-Constantan(-100C to 1260C)

Thermocouple - applications


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Thermocouples are most suitable for measuring

over a large temperature range, up to 1800 K.

They are less suitable for applications wheresmaller temperature differences need to be

measured with high accuracy, for example the

range 0100C with 0.1C accuracy. For suchapplications, Thermistors and RTDs are more

suitable.

Thermocouple - applications


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Resistance temperature detectors (RTDs), alsocalled resistance thermometers, are temperaturesensors that exploit the predictable change inelectrical resistance of some materials with

changing temperature.

Temperature Metal Resistance

The resistance ideally varies linearly withtemperature.

Resistance temperature detector (RTD)


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RTD/Resistance Theromometer/PRT100 Circuit

Connection:


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Resistance Vs Temperature

Approximations


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A straight line has been drawn between the

points of the curve that represent temperature,

T1 and T2, and T0 represent the midpoint

temperature.

Resistance vs Temperature

Approximations


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Linear Approximation

R(T) = approximation of resistance at

temperature T

R(T0) = resistance at temperature T0o = fractional change in resistance per

degree of temperature at T0

T = T - T0

Resistance Vs Temperature

Approximations

21]1)[()( TTTTTRTR oo


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Linear Approximation

R2 = resistance at T2R1 = resistance at T1

Resistance Vs Temperature Linear

Approximations

)()(

1

12

12

0 TT

RR

TRo

E l


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Example


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More accurate representation of R-T curve

over some span of temperatures.

RTD quadratic approximation


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R(T) = quadratic approximation of

resistance at temperature T

R(T0) = resistance at temperature T01 = linear fractional change in resistance

with temperature2 = quadratic fractional change in

resistance with temperature

T = T - T0

RTD quadratic approximation

21

2

21 ])(1)[()( TTTTTTRTR o


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Example - Solution


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Platinum

Copper

TungstenNickel

Platinum: very repeatable, sensitive,expensive

Nickel: not quite repeatable, moresensitive, less expensive


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Sensitivity is shown by the value o

Platinum 0.004/ C

Nickel 0.005/ C

Thus, for a 100 platinum RTD, a change of

only 0.4 would be expected if the

temperature is changed by 1C

RTD - sensitivity


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Generally 0.5 to 5 seconds or more

The slowness of response is due principally to

the slowness of thermal conductivity in

bringing the device into thermal equilibrium

with its environment.

RTD response time


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Construction of a platinum resistance

thermometer


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thermometer

Wire is in a coil to achieve small size and improve thermalconductivity to decrease response time.


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thermometer

Protect from the environment


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Semiconductor resistance sensors

Unlike metals, thermistors respond negatively totemperature and their coefficient of resistance is

of the order of 10 times higher than that ofplatinum or copper.

Temperature semiconductor resistance

Symbol

Thermistors
http://en.wikipedia.org/wiki/Image:Thermistor_symbol.png


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Thermistor: resistance vs temperature


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Thermistor


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Scan example 6.3 module page 109

winsem2012 13 cp0401 28 mar 2013 rm01 temperature transducers

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