Download - Therm Sensor
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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
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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.
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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.
Temperature MeasurementTemperature MeasurementTechnologyTechnology
Expansion & Contraction of FILLED THERMAL FLUIDS
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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.
Temperature MeasurementTemperature MeasurementTechnologyTechnology
Change in RESONANT FREQUENCY of crystal in response to change in TEMPERATURE
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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.
Temperature MeasurementTemperature MeasurementTechnologyTechnology
Collection of THERMAL RADIATION from an object subjected to HEAT
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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 -
Temperature MeasurementTemperature MeasurementTechnologyTechnology
Change in RESISTANCE with response to change in TEMPERATURE
Moderate
Poor (Logarithmic)Negative
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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!
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Temperature SensorsTemperature SensorsRTDsRTDs
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
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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
Temperature SensorsTemperature SensorsRTDsRTDs
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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
Temperature SensorsTemperature SensorsRTDsRTDs
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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
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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
Temperature SensorsTemperature SensorsRTDsRTDs
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Temperature SensorsTemperature SensorsRTDsRTDs
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Temperature SensorsTemperature SensorsRTDsRTDs
T t ST t S
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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!
Temperature SensorsTemperature SensorsRTDsRTDs
T t ST t S
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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
Temperature SensorsTemperature SensorsRTDsRTDs
EN 60751 Tolerances
Pt 100, = 0.00385
ST t S
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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
Temperature SensorsTemperature SensorsRTDsRTDs
T t ST t S
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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
Temperature SensorsTemperature SensorsRTDsRTDs
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
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!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 ??
Temperature SensorsTemperature SensorsRTDsRTDs
T t ST t S
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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
Temperature SensorsTemperature SensorsRTDsRTDs
Sensor Matching - eliminates sensor interchangeability error
T t STemperature Sensors
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Temperature SensorsTemperature SensorsRTDsRTDs
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
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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
Temperature SensorsTemperature SensorsRTDsRTDs
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
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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.
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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
Temperature SensorsTemperature SensorsThermocouplesThermocouples
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Temperature SensorsTemperature SensorsThermocouplesThermocouples
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Temperature SensorsTemperature SensorsThermocouplesThermocouples
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Temperature SensorsTemperature SensorsThermocouplesThermocouples
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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
Temperature SensorsTemperature SensorsThermocouplesThermocouples
Hot-Junction Configurations
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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
Temperature SensorsTemperature SensorsThermocouplesThermocouples
Hot-Junction Configurations
Dual
Ungrounded,Un-isolated
Dual
Ungrounded,
Isolated
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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
Temperature SensorsTemperature SensorsThermocouplesThermocouples
Temperature SensorsTemperature Sensors
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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
Temperature SensorsTemperature SensorsThermocouplesThermocouples
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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
+ -
+ -
+ -
Temperature SensorsTemperature SensorsThermocouplesThermocouples
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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!
Temperature SensorsTemperature SensorsThermocouplesThermocouples
Other Types
Temperature SensorsTemperature Sensors
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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
Temperature SensorsTemperature SensorsThermocouplesThermocouples
T t ST t S
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Temperature SensorsTemperature SensorsThermocouplesThermocouples
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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!
Temperature SensorsTemperature SensorsThermocouplesThermocouples
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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
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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
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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
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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
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42SEMICONDUCTOR TEMPERATURE SENSORSEMICONDUCTOR TEMPERATURE SENSOR
LM35
Precision Centigrade Temperature Sensors
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43SEMICONDUCTOR TEMPERATURE SENSORSEMICONDUCTOR TEMPERATURE SENSOR
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
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44cold junction compensatorcold junction compensator
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45SEMICONDUCTOR TEMPERATURE SENSORSEMICONDUCTOR TEMPERATURE SENSOR
Temperature transmitterTemperature transmitter
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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
Temperature transmitterTemperature transmitter
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Sensor
Thermowell
Transmitter Process
Process
Transmitter
Thermowell
Sensor75.4 C
ppFactors Affecting Response TimeFactors Affecting Response Time
Temperature transmitterTemperature transmitter
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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
ppFactors Affecting Response TimeFactors Affecting Response Time
Temperature transmitterTemperature transmitter
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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
ppFactors Affecting Response TimeFactors Affecting Response Time
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
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Thermowells and process material/conditions havethe greatest effect on temperature point responsetime
ppFactors Affecting Response TimeFactors Affecting Response Time
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
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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
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52Distributed Temperature SensingDistributed Temperature Sensing
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
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53Distributed Temperature SensingDistributed Temperature Sensing
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.