Transcript Temperature Fundamentals

RTD SENSOR
Fisher-Rosemount Korea
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Typical Assembly of Transmitter and Sensor
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2
Head
Headmount Transmitter
Extension
length
Cable Entry
Head connection
Instrument Connection
Thread
Thermowell
Immersion length
(U-length)
Stem
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Mounting arrangement
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RTD Sensors
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What is an RTD ?
–
–
–
Resistance Temperature Detector
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 know the
temperature!
Platinum
resistance changes
with temperature
Wire-wound sensing element
Rosemount’s
Series 78, 88
Fisher-Rosemount Korea
Thin-film sensing element
on ceramic substrate
Rosemount’s
Series 65 68, 58
Two common types of RTD elements:
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RTD Sensors
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What is a RTD Element ?
Class A RTD
Wire Wound
Class B RTD
Thin Film
M. I. Cable (Mineral
Insulated Cable), SST
or Inconel depending on
temperature
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Rosemount RTD Construction
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Sensor Differentiation
Explosion Proof Approvals
Calibration Services
RTD Technology:
Proprietary method of packing, O-ring
Molded rear
house assembly
Al2O3 packing for
protection against
vibration
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O-ring for protection
against moisture
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Rosemount RTD Differentiation
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Sensor Response Time Improved

Type of element
Platinum
Rosemount has externally
wound RTD for faster response time
– Thin-film has slightly faster response time than wirewound
–
–

Element packaging
Rosemount RTD’s are packed in aluminum oxide to provide
optimum thermal conduction within the sheath
– Grounded thermocouples are twice as fast as ungrounded
–

Sheath thickness and material
–
Rosemount uses 316SST and Inconel (for high temperatures)
for sheath; both are very good thermal conductors
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RTD Sensors Interchangeability
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Error in Degrees C.
SENSOR ACCURACY to DIN EN 60751:1995(DIN
Class B
IEC 751)
3
2.5
2
1.5
Class A
1
0.5
0
-200 -150 -100 -50
0
50
100 150 200 250 300 350 400 450 500
Temperature in Degrees C.
Fisher-Rosemount Korea
Class A = 0.15 + 0.002 |t|
Class B = 0.3 + 0.005 |t|
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RTD Sensor
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Why is Platinum used ?
It is the most stable & near linear resistance versus temperature
function when compared to other metals like thermistor,
Nickel & Balco
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Question
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What does it mean Pt100,  = 0.0385?
Pt = Platinum
 0.0385 = 0 deg. C the probe will read 100 ohms.
at 100 deg. C, it will read 138.5 ohms.
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RTD Sensors : Wiring
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4 - wire to 3 or 2 wire
For 3-wire systems use one white and two red leads.
Do not common White leads.
Insulate or terminate the unused wire in a manner
that avoids short circuiting to ground or earth
For 2-wire systems common both sets of leads
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Vol I - RTD Series
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Temperature vs  relationships and Tolerances confirm to IEC 751.
RED
RED
WHITE
WHITE
All Single Element Sensors
are supplied as 4 Wire
WHITE
WHITE
RED
GREEN
GREEN
BLACK
All Duplex Element Sensors
are supplied as 2 x 3 Wire

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0.25 inches sheath diameter
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Vol II - Series 65 RTD
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Temperature vs  relationships and Tolerances confirm to IEC 751.
RED
RED
WHITE
WHITE
Note:
Wire colors different
from Vol.1
RED
RED
All Single Element Sensors
are supplied as 4 Wire
BLACK
BLUE BLUE
GREEN
All Duplex Element Sensors
are supplied as 2 x 3 Wire




Range: -50 to 450°C
Standard 6mm sheath diameter
321 SST sheath material
Class B Tolerance
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RTD Sensors
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2, 3, & 4-Wire RTDs
Why use a 2-, 3-, or 4- wire RTD?
– 2-wire: Lowest cost -- rarely used due to high error from lead
wire resistance
– 3-wire: Good balance of cost and performance. Good lead
wire compensation.
– 4-wire: Theoretically the best lead wire compensation method
(fully compensates); the most accurate solution. Highest cost.
4-wire RTD
Red
Red
Sensing Element
(I.e. wire-wound, thin film)
White
White
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Black
Red
Red
Typically use copper wires
for extension from the
sensor
Green
Blue
Blue
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Temperature Sensor : Stability, Repeatability
& Linearity
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Resistance ()
How does these factors affect Your Measurement ?
 Inconsistent Quality
 Introduce downstream variation
 Waste raw material - rework or disposal !
 Lost Production
 High energy cost
 Lost profits
Temperature (oC)
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Temperature Sensor : Stability
TEMPERATURE

Stability is expressed as drift in the temperature reading per
unit time, as a drift in resistance per unit time expressed in
ohms or as a percentage of resistance.

Stability specifications are often define in terms of a
temperature exposure history.
IEC 751 defines Stability as the limit of drift after 250 hours of
exposure to full scale temperature.
< 0.005 Deg C/year for laboratory grade
<0.1 Deg C of FS/year for thin film Platinum RTD for normal
usage
15
<0.05 Deg C per 5-years for temperature ranges -40 to 125 Deg C
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Temperature Sensor : Repeatability
TEMPERATURE

Repeatability is the range of output that the PT RTD will give when
arriving at a target temperature in repetitive cycles from the same
direction.

Bi-directional repeatability is the output range when a target
temperature is approached from two directions.

Hysterisis is the difference in the mean values of the two
direction dependent output ranges when arriving at a target
temperature in repetitive cycles from both directions.
16
IEC 751 defines Repeatability to be the drift observed at 0 Deg C
after
a sensor experience 10 cycles over its full operating
temperature range.

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RTD Sensors
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Resistance changes are repeatable
How does a RTD works?
–
Resistance Temperature Detector
The resistance changes of the platinum wiring can be approximated by
an ideal curve -- the IEC 751
International Resistance
vs. Temperature Chart:
oC
0
10
20
30
Ohms
100.00
103.90
107.79
111.67
Resistance (Ohms)
–
350
300
250
150
100
50
0
-200
IEC 751
Page 3-2:
Sensors PDS (V.1)
Fisher-Rosemount Korea
IEC 751
200
0
200
400
600
800
Temperature (oC)
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Temperature Sensor : Linearity

18
Linearity is defined as the deviation of the output-versustemperature curve, from the best fit linear approximation of the
devices behaviour over the operating temperature range.
It is expressed as either a percentage of full scale output or as the
maximum deviation (either in Ohms or Deg C for an RTD)
Resistance ()

TEMPERATURE
Temperature (oC)
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RTD Sensor Joining Techniques
Joining Techniques Recommended Temperature
Range
Soldering
- 30 to 120 Deg C
Crimping
- 50 to 200 Deg C
Brazing
- 50 to 400 Deg C
Spot welding
Laser welding
- 50 to 600 Deg C
- 50 to 850 Deg C
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Comments
remove flux to prevent corrosion
more than 180 Deg C high melting
soldering material is used
max. allowable tensile strength of
the wires used are not exceeded
Do not ‘burn’ the RTD chip
during blazing process. Remove
flux to prevent corrosion.
Both spot & laser welding are
most secure & temperature
resistant. A high level of skill &
manufacturing know how with
strict adherence to manufacturing
parameters.
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Temperature : International Standards
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Standards
IEC 751
Comments
Defines Class A & B performance for 100 ohm
0.00385 alpha Pt RTDs.
DIN EN 60751 Matches IEC 751
BS-1904
Matches IEC 751
JIS C1604
Matches IEC 751 Adds 0.003916 alpha
ITS-90
Defines temeprature scales & transfer standards.
Parameter
Ro
Alpha
Range
Fisher-Rosemount Korea
IEC 751 Class A
100 ohm + - 0.06%
0.00385 + - 0.000063
minus 200 to 650 Deg C
IEC 751 Class B
100 ohm + - 0.12%
0.00385 + - 0.000063
minus 200 to 850 Deg C
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What other consideration?
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 The useful range of the thermocouple type.
 The sheath material maximum operating temperature.
 The de-rate upper temperature limit due to sensor diameter.
The same element with different sheath diameter will affect temperature limit as well.
Example : Thermocouple Type K
Sheath diameter : 5mm
Sheath material : Inconel
Measuring range : 0 to 1100deg. C
For this combination, the sensor will fail although maximum measuring range is
1372 deg. C.
 The temperature rating of other components such as
connectors, transition joints and wire.
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Sensor Lead-Wire Length : RTD



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Sensor Direct Wiring : Recommended Maximum 250 feet using 18 AWG
lead wire
For 3-wire RTD, maximum error 0.16 Deg F per 100 feet using 18
AWG lead wire.
For best RTD wiring practice to reduce error : use same specification &
run-length lead-wires
0
Average Wire Run Distance*
% of Total Points
Power
(200 Ft. Median; T/C)
644
244
Wire Direct
0
500
1000
1500
2000
2500
Wire Distance (feet)
* Break even distances for wire direct vs. 244E and 644
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Temperature Point Response Time
TEMPERATURE
Factors Affecting Temperature Point Response
Time
Thermowell

Sensor
 Thermowell
 Transmitter
 Process
75.4 °C
Sensor
Process
Transmitter
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Sensor Time Response
TEMPERATURE
Factors Affecting Sensor Response Time
OD
Type of element
–
Wirewound RTD
» externally or internally wound
– Thin-film RTD
– Thermocouple
Element packaging
–
Element coating, potting
–
Contact between element package & sheath
sheath
ceramic
bore
element
Al2O3
packing
Sheath thickness and material
Fisher-Rosemount Korea
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Sensor Time Response
TEMPERATURE
Industry
Average
Rosemount
(Series 78) wirewound RTD
6.0 - 7.0 s
4.7 s
(Series 65 & 68) thin-film RTD
5.0 - 5.5 s
3.38 s
Ungrounded thermocouples
<2s
<2s
Grounded thermocouples
<1s
<1s
* All results based on standard conditions: time required to reach 63.2%
sensor response for water flowing at 3 ft/sec. RTD response times shown
are the average + 6 sigma.
Fisher-Rosemount Korea
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Factors Affecting Response Time of
Sensors in Thermowells
TEMPERATURE
Thermowell design style
(thickness at tip)
–
Stepped is the fastest
Contact between sensor
sheath and thermowell
(x and y)
–
–
Spring loaded sensor ensures
contact at the tip (x=0)
Industry practice suggests using
thermally conductive fill can
significantly reduce time lag
x
y
Thermowell
Sensor
Assembly
Thermally
Conductive Fill
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Time Response for Sensors in
Thermowells
Tapered thermowell
26 seconds
Stepped thermowell
22 seconds
TEMPERATURE
Industry data shows stepped t-well with fill = 11 seconds
* Based on externally published data for time required to reach 63.2%
sensor response for water flowing at 3 ft/sec., with a sensor response
time of 5.5 seconds. Time response of assembled point is not additive.
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Factors Affecting Transmitter Response
Time
TEMPERATURE
Time response depends on element
(complexity of calculation)




2-wire RTD
440 3 & 4-wire RTD 520 Thermocouples 300 Above is good provided
than 2%
760 ms
920 ms
750 ms
the analog output changes less
Transmitter update time (output) every
500msec
Transmitter
75.4 °C
Process
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Process Factors in Temperature
Response Time
Velocity of the material
 Thermal conductivity of the material
 Density and viscosity of the material
Process time constants can be from
seconds to hours:
TEMPERATURE

75.4 °C
Water @ 3 fps t = 3.38 s
Air at 50 fps, 40-80oC = 38.0 s
Oil agitated in a bath: t = 43.0 s
Oil not agitated: t = >3 minutes
Process
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Response Time
TEMPERATURE
Sensor
< 1 to 4.7 sec
Sensor in Thermowell
22 to 26 sec
Transmitter
.5 to .9 sec
Process
Seconds to Hours
• Thermowells and process material/conditions have the greatest effect
on temperature point response time
• We are working on a method to predict time response of a
temperature point through modeling of the time response in two
known media
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Frequently Asked Questions
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Why should I buy an RTD when TC are more rugged & less expensive ?
RTDs are more accurate, drift less over time & resistance to noise.
Lower cost copper lead-wires compare the expensive TC-wires.
What is RTD & PRT ?
RTD is resistance temperature detector. It may use platinum, nickel or
copper for its element. A PRT is a platinum resistance thermometer, an
RTD that uses platinum for its element.
What is the maximum distance between a PRT & a recorder or
controller without using a transmitter ?
We recommend 250 feet using at least 18 AWG lead wire without a
transmitter. Further details can be check with recorder or controller
supplier.
Fisher-Rosemount Korea
FRKL/MAY 2000
Frequency Asked Questions
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32
How far can I run the signal with a temperature transmitter ?
The only limitation is the transmitter’s minimum voltage (12 VDC)
requirement at the terminals. A power supply must overcome the
lead wire resistance. A long lead wire can act as an atenna,
picking up stray electrical signal & causing RFI & EMI. Twisted
shielded wire should be used for long runs or if the wires run
next to other wires or electric motors.
What is the minimum immersion length for an RTD ?
Rule of thumb : At least 10-times greater than the diameter of
the sensor or thermowell plus the sensitive length of the PRT.
This is to minimise stem conduction errors caused by heat
conduction along the sheath or leadwires of a probe or along the
length of a thermowell. The heat conduction increases or
decreases the measured temperature depending on the
applications & environmental conditions.
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FRKL/MAY 2000
Frequently Asked Questions
TEMPERATURE
33
How is the calibration of a PRT confirmed ?
Simply measure the sensor’s ice point resistance (0 Deg C or 32
Deg F) and compare the measured value to its calibrated or
previous value. If the ice-point resistance increase, it is a sign
that the probe is being stressed (vibration or shocked) or that
the probe is used beyond its rated temperature. A decrease in
ice point resistance usually signals a problem with the moisture
seal on the element.
What is the difference between a .00385 & a .003902 probe ?
The only difference is the amount that the resistance changes
per degree of temperature. Both probes will read 100 ohm at
0 Deg C, but at 100 Deg C the .00385 probe will read
138.5 ohm and the .003902 probe will read 139.02 ohm.
The sensor must be properly matched to the easuring device to
obtain an accurate reading
Fisher-Rosemount Korea
FRKL/MAY 2000
Frequently Asked Questions
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34
How can I determine if I have a .00385 or .003902
temperature coefficient ?
The colour coding on the wires.
White, Red, Red - .00385
Red, White, White - .003902
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FRKL/MAY 2000