Therm Sensor

8/8/2019 Therm Sensor

1/53

1

373 100 672 212

273 0 492 32

0 -273 0 -460

C = 5/9 (F - 32 )

F = 9/5 (C) + 32

K = 273 +C

R = 460 + F

Kelvin & Rankine areKelvin & Rankine are absolute scalesabsolute scales

BOILING POINTOF WATER

ICE POINT

ABSOLUTE

ZEROkELVIN CELSIUS RANKINE FAHRENHEIT

Temperature terminologyTemperature terminologyTemperature Measurement ScalesTemperature Measurement Scales


2/53

2

Temperature MeasurementTemperature MeasurementTechnologyTechnology

METALS change in VOLUME in response to change in TEMPERATURE & DISSIMILARMETAL STRIPS having different COEFFICIENT of VOLUME CHANGE.

Example: Bimetallic Thermometer

Thermocouple (discussed later)

Bimetallic Thermometer

The degree of deflection of 2 dissimilar metals is proportional tothe change in temperature.

One end of the spiral (wounded from a long strip of material) isimmersed in the process fluid and the other end attached to apointer.


3/53

3

Example: Vapour Pressure Thermometer

A bulb connected to a small bore capillary which is

connected to an indicating device.Indicating device consist of a spiral bourdon gaugeattached to a pointer.

The bulb is filled with a volatile liquid and the entire

mechanism is gas tight and filled with gas or liquidunder pressure.

Basically the system converts pressure at constantvolume to a mechanical movement.


Expansion & Contraction of FILLED THERMAL FLUIDS


4/53

4

Example: Quartz Crystal Thermometers

Quartz crystal hermetically sealed in a stainlesssteel cylinder, similar to a thermocouple or RTDsheath but , larger.

Quartz crystal converts temperature into afrequency.

They provide good accuracy and response time withexcellent stability.

Hence, this technology is expensive.


Change in RESONANT FREQUENCY of crystal in response to change in TEMPERATURE


5/53

5

Example: Radiation Pyrometry

Infers temperature by collecting thermal radiation fromprocess and focusing it on a photon detector sensor.

The sensor produces and output signal as radiant energystriking it releases electrical charges.


Collection of THERMAL RADIATION from an object subjected to HEAT


6/53

6

Example: Thermistors

RTD (discussed later)

ThermistorsSemi-conductors made from specific mixtures of pure oxides ofnickel, manganese, copper, cobalt, and other metals sintered atvery high temperature.

Used with Wheatstone Bridge which amplifies small change in

resistance - in a simple circuit with a battery and a micro-ammeter. Stability -

Linearity -

Slope of Output -


Change in RESISTANCE with response to change in TEMPERATURE

Moderate

Poor (Logarithmic)Negative


7/53

7

Temperature SensorsTemperature SensorsRTDsRTDs

What is an RTD ?

RResistanceTTemperature DDetector

Platinumresistance changeswith temperature

Rosemounts

Series 78, 88

Rosemounts

Series 68, 58

Series 65

Two common types of RTD elements:

Wire-wound sensing elementThin-film sensing element

Operation depends on inherent characteristic of metal(Platinum usually): electrical resistance to current

flow changes when a metal undergoes a change in

temperature.

If we can measure the resistance in the metal, we knowthe temperature!


8/53

8


How does a RTD works?

Resistance changes are Repeatable The resistance changes of the platinum wiring can beapproximated by an ideal curve -- the IEC 751

0

50

100

150

200

250

300

350

-200 0 200 400 600 800

Resistan

ce(O

hms)

Temperature (oC)

oC Ohms0 100.0010 103.9020 107.7930 111.67

International Resistancevs. Temperature Chart:

IEC 751

IEC

751

IEC 751 Constants are :- A = 0.0039083, B = - 5.775 x 10 -7,If t>=0C, C=0, If t


9/53

9

C -100 0 100 200 300 400 500 600 700

F -148 32 212 392 572 762 932 1112 1292

Temperature

R

elativ

eResistanc

e(R

T

/R

0)

0

1

2

3

4

5

6

Platinum

Balco

Nickel

Thermistor

Most linear

Most Repeatable

Most Stable

Positive Slope

Platinum vs other RTD materials



10/53

10

Sensing Element(i.e. wire-wound, thin film)

Red

Red

White

Red

WhiteWhite

Black

GreenGreen

White

Why use a 2-, 3-, or 4- wire RTD?

2-wire: Lowest cost -- rarely used due to high error from leadwire resistance

3-wire: Good balance of cost and performance. Good lead wirecompensation.

4-wire: Theoretically the best lead wire compensation method

(fully compensates); the most accurate solution. Highest cost.

4-wire RTD

Typically use copper wires forextension from the sensor



11/53

11

2-wire or 4-wire RTD ? If the sensing element is at 20C,

What would be the temperaturemeasured at the end of theextension wire using a 2-wire assembly

What would be the temperaturemeasured at the end of theextension wire using a 4-wire assembly

Red

White

2-wire RTD6 metres of copper extension

wire, lead resistance =

0.06 ohms/metre

(1 ohm = 2.5 deg C approx)Sensing Element

(I.e. wire-wound, thin film)

Temperature SensorsRTDs

Error for a 2 wire assembly

0.06 x 6 x 2 = 0.72 ohms or 1.8Deg C

This means that the temperature

measured at the end of the cable

would be 21.8 Deg C

Error for a 4 wire assembly

As the lead resistances can be

accounted for the temperature

measured at the end of the cable

would be 20.0 Deg C

S


12/53

12

Supports Hot Backup capability

Dual element adds only $5 over single elementRTD

Reduce the risk of a temperature point failure

Supports Differential Temperature Measurement

Dual Element RTDs available

Red

Red

White

Black

RedRed

Green

BlueBlue

White

Dual Element:Two 3-wire RTDs



13/53

13



14/53

14


T t ST t S


15/53

15

IEC751

Curve

The IEC 751 standard curve (programmed into all our

transmitters) describes an IDEAL Resistance vs Temperature

relationship for Pt100 = 0.00385 RTDs.

TEMPERATURE (oC)

RESIST

ANCE(O

HMS)

Class B Tolerance

Standard IEC 751 Curve

Class B Tolerance

Standard IEC 751 Curve

Class B Tolerance

0.8oC at -100oC

0.3oC at 0oC

0.8oC at 100oC

1.3oC at 200oC

1.8oC at 300oC

2.3oC at 400oC

(Sensor Interchangeability Error)

The goal is to find out what the real RTDcurve looks like, and reprogram thetransmitter to use the real curve!

Every RTD is slightly

different - theyre not ideal!

Every RTD is slightly

different - theyre not ideal!


T t ST t S


16/53

16

Accuracy

Temperature Resistance Grade A Grade A Grade B Grade B

C Ohms C Ohms C Ohms

-200 18.52 0.55 0.24 1.3 0.56

-100 60.26 0.35 0.14 0.8 0.32

0 100.00 0.15 0.06 0.3 0.12

100 138.51 0.35 0.13 0.8 0.30

200 175.85 0.55 0.2 1.3 0.48

300 212.05 0.75 0.27 1.8 0.64

400 247.09 0.95 0.33 2.3 0.79500 280.98 1.15 0.38 2.8 0.93

600 313.71 1.35 0.43 3.3 1.06


EN 60751 Tolerances

Pt 100, = 0.00385

ST t S


17/53

17

Your customer is operating a process at 100C

and is using a Platinum RTD...

What is the maximum error that will beintroduced into the temperature measurement

from Sensor Interchangeability?

+/-0.35 deg C for Class A,

+/-0.8 deg C for Class B

Fortunately, Sensor Interchangeability Error can

be reduced or eliminated by Sensor Matching!

Quiz: - Find the Interchangeability Error


T t ST t S


18/53

18

oC Ohms0.0 99.9971.0 100.382.0 100.773.0 101.16

Customer ReceivesRTD-specific Resistancevs. Temperature Chart:Data generated

(RTD characterized)

Temperature Bath- One temperature

- Multiple temperatures


What is RTD Calibration?

The real RTD curve is found by characterizing anRTD over a specific temperature range or point. Temperature Range Characterization

Calibration certificate provided with sensor

Temperature Point Characterization

Calibration certificate provided with sensor

T t ST t S


19/53

19

!Transmitter reading does NOTequal process temperature.

212F212F

ProcessTemperature

138.8 138.8

RTDResistance:

Transmitter

Input:

R vs. T Curve ofREAL RTDREAL RTD

If we could tell the transmitter the shape of the Real RTD curve,

we could eliminate the interchangeability error!

The curve programmed intoevery xmtr is the IEC 751 - theIEC 751 - the

Ideal RTD curveIdeal RTD curve

With a Real RTD, the Resistance vs. Temperature

relationship of the sensor is NOT the same curve thatis programmed into the transmitter

The Transmitter

Translates 138.8

into 213.4F213.4FUsing the IEC 751

!Transmitter curve does NOTmatch RTD curve.

Outcome ??Outcome ??


T t ST t S


20/53

20

Pt100 a385 Temp vs Resistance

real sensorcurve

standardIEC 751 curve

sensor matchedcurve in tx

Res

ista

nce

Temperature

A fourth order equation can be programmed into SmartTransmitters to follow non-ideal sensor curvature; simply enterfour constants using 275.

Transmitter reading equals process temperatureTransmitter curve is perfectly matched to ideal RTD curve

Outcome ??Outcome ??

Ro = 99.9717

= 0.00385367 = 0.172491 = 1.61027

TagTag


Sensor Matching - eliminates sensor interchangeability error

T t STemperature Sensors


21/53

21


Rt = Ro + Ro [t- (0.01t-1)(0.01t)- (0.01t-1)(0.01t)3

]Rt = Resistance at Temperature t (C)

Ro = Sensor-Specific Constant (Resistance at t = 0C)

= Sensor-Specific Constant = Sensor-Specific Constant

= Sensor-Specific Constant (If t >=0C, then = 0)

IEC75

1Cu

rve

Temperature (oC)

Resistan

ce(

)

Class B

Tolerance

The transmitter does not usethe IEC 751 standard curve.

Instead, the Callendar-VanDusen constants can be used inthe equation below to create

the true sensor curve. Or, the actual IEC 751constants A,B, and C can beused in the IEC 751 equation ifknown.

The transmitter does not usethe IEC 751 standard curve. Instead, the Callendar-Van

Dusen constants can be used inthe equation below to create

the true sensor curve. Or, the actual IEC 751constants A,B, and C can beused in the IEC 751 equation ifknown.

Sensor Matching - Mapping the Real RTD Curve

4th Order

Callendar-Van

Dusen Equation

Temperat re SensorsTemperature Sensors


22/53

22

0

50

100150

200

250

300

350

400

-200 0 200 400 600 800

A 1-point trim shifts the idealcurve up or down based onthe single characterized point

Temperature (C)

Resistance

(

)

A 2-point trim shifts the ideal curveup or down AND changes the slopebased on the two characterized points

Temperature (C)

Resis

tance(

)

0

50

100

150

200

250

300

350

400

-200 0 200 400 600 800

One Point Trim

Use with X9

(or X8)

Two Point Trim

Use with X8


Sensor Trimming

Data from the resistance vs. temp. chart can be used toreduce sensor interchangeability error

Use one or two points to trim the sensor to a transmitter

Temperature SensorsTemperature Sensors


23/53

23

ProcessProcess

TemperatureTemperature

Hot junction

Two dissimilar metals joined at a Hot junction

Cold junction+

-MV

The wires are connected to an instrument (voltmeter) thatmeasures the potential created by the temperaturedifference between the two ends.

DT

The junction of two dissimilar metalscreates a small voltage output

proportional to temperature!

What is a Thermocouple ?

Temperature SensorsTemperature SensorsThermocouplesThermocouples

In 1831, Seebeck

discovered that an

electric current

flows in a closed

circuit of two

dissimilar metalswhen one of the two

junction is heated

with respect to the

other.



24/53

24

How does a Thermocouple work ?

The measured voltage is proportional to the temperaturetemperature

differencedifference between the hot and cold junction! (T2 - T1) = T.

+

-

MVHeat

Hot junction Cold junction

oC Millivolts

0 0.00010 0.59120 1.19230 1.801

Thermoelectric Voltagevs. Temperature Chart:

TYPE E THERMOCOUPLE

T

-20

0

20

40

60

80

-500 0 500 1000

Volta

ge(mV)

Temperature (oC)

IEC

584

MeasurementMeasurement

JunctionJunction

TT22

ReferenceReference

JunctionJunction

TT11



25/53

25



26/53

26



27/53

27



28/53

28




29/53

29

Grounded

improved thermal conductivity

quickest response times

susceptible to electrical noise

Ungrounded

slightly slower response time

not susceptible to electrical noiseSingle

GroundedDual

Grounded

Single

Ungrounded


Hot-Junction Configurations



30/53

30

Unisolated junctions at the same temperature

both junctions will typically fail at the same time

Isolated

junctions may/may not be at the same temperature

increased reliability for each junction

failure of one junction does not affect the other


Hot-Junction Configurations

Dual

Ungrounded,Un-isolated

Dual

Ungrounded,

Isolated



31/53

31

ICEBATH

T1 = 0C

Why is Cold Junction Compensation needed?

Reference JunctionReference Junction must be kept constant.

Measure

Reference

Iron

Constantan

+

_

Volt

Meter

T2 = 100 + 10C 5.812 mV

2 Methods used to accomplished this :Place Reference Junction in Ice BathIce Bath

NOT Practical !

C -100 -0 +0 100

MILLIVOLTS

0 -4.632 0.000 0.000 5.268

2 -4.550 -0.995 1.019 5.376

6 -4.876 -0.301 0.303 5.594

10 -5.036 -0.501 0.507 5.812

14 -5.194 -0.699 0.711 6.031

T = 110CT = 110C




32/53

32

Measure

ReferenceJunction

Iron

Constantan

+

_

Transmitter

T2 = 110C

4.186 mV

What is Cold Junction Compensation

Electronic Circuitrypassing current through a ThermistorThermistor

Common Practise !

T = 80CT = 80C ConnectionHead

Extension

Wires

Example:

Ambient Temp =

30C

C -100 -0 +0 100

MILLIVOLTS

0 -4.632 0.000 0.000 5.268

10 -5.036 -0.501 0.507 5.812

30 -5.801 -1.481 1.536 6.907

60 -6.821 -2.892 3.115 8.560

80 -7.402 -3.785 4.186 9.667

+1.536 mV

= 5.722 mV 110C




33/53

33

3Type J

Iron / Constantan White, Red

0 to 760 C

Least Expensive

Types of Thermocouple

3 Type K

Chromel / Alumel Yellow, Red

0 to 1150 C

Most Linear

3 Type T Copper /

Constantan Blue, Red

-180 to 371 C Highly resistant to

corrosion from

moisture

+ -

+ -

+ -




34/53

34

3 Type B Pt, 6% Rh / Pt, 30% Rh

38 to 1800 C

3 Type S Pt, 10% Rh / Pt

-50 to 1540oC

3 Type R Pt, 13% Rh / Pt

-50 to 1540 C

High temperature range Industrial/ laboratory standards LOWEMF output!

(Not very sensitive)

Expensive!


Other Types



35/53

35

3 Temperature range

3 Cost

Why use one type over another ?

0

10

20

30

40

50

60

70

80

0 250 500 750 1000 1250

Type E

Type KType J

Type T

3

Signal level3 Linearity of the range

Millivolts

Temperature (C)

Type JType R


T t ST t S


36/53

36




37/53

37

Correct!

Wrong!

All thermocouple lead wire extensions MUST be

with the same type of wire!

Another HotJunction iscreatednot good!

Cannot use copper wire for extensions! T/C wire is moreexpensive to run and much harder to install!




38/53

38

Better Accuracy & Repeatability RTD signal less susceptible to noise Better linearity RTD can be matched to transmitter

(Interchangeability error eliminated)

CJC error inherent with T/Cs; RTDs lead wireresistance errors can be eliminated

Why choose RTD over Thermocouple ?

Better Stability T/C drift is erratic and unpredictable; RTDs drift

predictably

T/Cs cannot be re-calibrated

Greater Flexibility Special extension wires not needed Dont need to be careful with cold junctions

Temperature SensorsTemperature SensorsComparisonComparison



39/53

39

Applications for Higher Temperatures

Above 1100F

Lower Element Cost

Cost is the same when considering temperature

point performance requirementsFaster response time

Insignificant compared to response time for T-Welland process

Perceived as more rugged

Rosemount construction techniques produceextremely rugged RTD

Why choose thermocouple over RTD ?

Temperature SensorsTemperature SensorsComparisonComparison

Temperature Sensors


40/53

40

RANGE OFFER

-200 to 500 C RTD

500 to 1100 C Thermocouple Type K

>1100 C Special Thermocouple R, S or B

Temperature SensorsComparison

Temperature transmitterTemperature transmitter


41/53

41

Converts a noise susceptible signal to a standard, more robust 4-20 mAsignal

Provides local indication of temperature measurement

Smart transmitterprovides remote communication & diagnostics

improved accuracy & stability reduced plant inventory

Temperature transmitterTemperature transmitterWhat does a Transmitter do &What does a Transmitter do &Why use Transmitter?Why use Transmitter?

Resistance Signal

=

4-20 mA Signal

==(Range: 0-200C)

Control

System

Copper Wire

(RTD only)

Smart Transmitters

also relay a digitalsignal100 C100 C

100 C100 C

IEC 751IEC 751

Ranged: 0 - 200CRanged: 0 - 200C

138.5 12 mA12 mA

Transmitter converts temperature sensors signal from resistance

or voltage into a common digital or analog 4-20 mA control signal


42/53

42SEMICONDUCTOR TEMPERATURE SENSORSEMICONDUCTOR TEMPERATURE SENSOR

LM35

Precision Centigrade Temperature Sensors


43/53


An approach has been developed where the difference in the

base-emitter voltage of two transistors operated at different currentdensities is used as a measure of temperature. It can be shown thatwhen two transistors, Q1 and Q2, are operated at different emittercurrent densities, the difference in their base-emitter voltages, .VBE,is

where k is Boltzmans constant, q is the charge on an electron, T isabsolute temperature in degrees Kelvin and JE1 and JE2 are theemitter current densities of Q1 and Q2 respectively.

SEMICONDUCTOR TEMPERATURE SENSORSEMICONDUCTOR TEMPERATURE SENSOR


44/53

44cold junction compensatorcold junction compensator


45/53




46/53

46

Marshalling

IS (Exi)Barriers

I/O Terminations

I/O Interface

PLCGW

PLC

Controller

Junction Box

2

1

Turbine

The alternative to using a transmitter

(2) Trunk: Length of

Bundled cable fromJunction Box toMarshalling Panel

(1) Spur: Length of

T/C wire run fromprocess to Junction Box

8 Temp. Measurement Points

Example

ppWire Direct vs. TransmitterWire Direct vs. Transmitter



47/53

47

Sensor

Thermowell

Transmitter Process

Process

Transmitter

Thermowell

Sensor75.4 C

ppFactors Affecting Response TimeFactors Affecting Response Time



48/53

48

x Time response depends on element

(complexity of calculation) 2-wire RTD 440 - 760 ms

3 & 4-wire RTD 520 - 920 ms

Thermocouples 300 - 750 ms

x

Transmitter update time (output)every 1/2 second

Process

Transmitter

75.4 C




49/53

49

x Velocity of the material

x

Thermal conductivity of the materialx Density and viscosity of the material

x Process time constants can be from secondsto hours:

Process

75.4 C


Water @ 3 fps t = 1 min

Air at 50 fps, 40-80o

C = 11 minutesOil agitated in a bath: t = 13 minutes

Oil not agitated: t = >45 minutes



50/53

50

Thermowells and process material/conditions havethe greatest effect on temperature point responsetime


Sensor < 7 to 10 sec

Sensor in Thermowell 60 to 120 sec

Transmitter .5 to .9 secProcess Seconds to Hours

Di ib d T S iDi t ib t d T t S i


51/53

51Distributed Temperature SensingDistributed Temperature Sensing

Distributed sensing takes advantage of the fact that the reflection

characteristics of laser light travelling down an optical fibre vary with thetemperature and strain along its length.

A distributed sensing system is made up of two basic components:

The sensor. This consists of an optical fibre usually a standardtelecoms fibre which is normally housed inside a protective sheath to

form a cable. The cable is then carefully placed to make the requiredmeasurements.

The detector system. This includes a laser which fires light pulses downthe optical fibre, and a detector which measures the reflections fromeach light pulse. By analysing these reflections it is possible to

determine temperature and strain at all points along the fibre. With thehelp of more powerful lasers and more sensitive detection systems,measurements can be made using fibres up to 30km long. But in atypical installation, where the fibre is looped around a building or in aprocess area, distances of several kilometres are more common.

Di t ib t d T t S iDi t ib t d T t S i


52/53


MEASUREMENT VARIABLES The measurements themselves depend onfour variables, or parameters. These include:

Distance, or range: the distance over which the measurements will bemade

Speed: the time required for each measurement

Temperature resolution: the size of temperature changes that will be

detected

Spatial resolution: the smallest distance over which a change intemperature can be detected.

WHAT ARE THE ADVANTAGES? The flexibility and speed ofmeasurements offered by distributed sensing systems offer great potential

in a wide range of applications. A fibre laid around every room on everyfloor can provide a complete picture of temperature throughout a building,making it possible to more precisely control heating and air conditioningsystems. The same cable can also serve as a very effective fire detectionsystem capable of detecting the location of a fire very precisely.

Di t ib t d T t S iDi t ib t d T t S i
http://wlow.net/wlink.php?qq=range&index=3http://wlow.net/wlink.php?qq=range&index=3


53/53


Measuring principle Raman effect

Physical measurement dimensions, such as temperature or pressure and tensileforces, can affect glass fibres and locally change the characteristics of lighttransmission in the fibre. As a result of the damping of the light in the quartz glassfibres through scattering, the location of an external physical effect can bedetermined so that the optical fibre can be employed as a linear sensor. Lightscattering, also known as Raman scattering, occurs in the optical fibre. Unlike

incident light, this scattered light undergoes a spectral shift by an amount equivalentto the resonance frequency of the lattice oscillation. The light scattered back fromthe fibre optic therefore contains three different spectral shares:

the Rayleigh scattering with the wavelength of the laser source used,

the Stokes line components with the higher wavelength in which photons aregenerated, and

the anti-Stokes line components with a lower wavelength than the Rayleighscattering, in which photons are destroyed.

Therm Sensor

Documents