1. RTD SENSOR
Temperature

2. Typical Assembly of Transmitter and Sensor
Head
Headmount Transmitter
Cable Entry
Extension
length
Head connection
Instrument Connection Thread
Thermowell
In many cases, particularly in Europe, the preferred method of assembly is to mount the transmitter directly to a sensor in a connection head. The head - mounted transmitter in this case is designed such that the head provides most of the protection from the environment.
RMD offers a range of head-mount transmitters from simple analogue fixed range (244R) through programmable analogue (244P, 244E) to HART (644).
Immersion length (U-length)
Stem
Mounting arrangement

3. RTD Sensors 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
Have one of the experienced sales people answer this.
Thin-film sensing element
on ceramic substrate
Wire-wound sensing element
Rosemount’s
Series 65 68, 58
Rosemount’s
Series 78, 88
Two common types of RTD elements:

4. RTD Sensors 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

5. Rosemount RTD Construction
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
O-ring for protection against moisture

6. Rosemount RTD Differentiation
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

7. RTD Sensors Interchangeability
Class B
Class A
Class A = |t|
Class B = |t|

8. RTD Sensor Why is Platinum used ?
It is the most stable & near linear resistance versus temperature
function when compared to other metals like thermistor,
Nickel & Balco

9. Question What does it mean Pt100,  = 0.0385? Pt = Platinum
 = 0 deg. C the probe will read 100 ohms.
at 100 deg. C, it will read ohms.

10. RTD Sensors : Wiring 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

11. Vol I - RTD Series
Temperature vs  relationships and Tolerances confirm to IEC 751.
RED
RED
WHITE
WHITE
All Single Element Sensors
are supplied as 4 Wire
WHITE
RED
GREEN
BLACK
All Duplex Element Sensors
are supplied as 2 x 3 Wire
Customers may remove excess leads as necessary to create 2 and 3-wire configurations as notified with shipping instructions. When converting to 3-wire configuration two leads should not be commoned or twisted together as this will cause a significant measurement error.
DUPLEX contains two platinum sensing resistors and are currently only available as 2 x 3-wire output leads. The duplex sensor type is often used where the customer requires redundancy allowing a higher reliability particularly where a second transmitter is also used. The 3244MV hot back-up feature can also be used with this sensor. In environments of high vibration belief in this approach to redundancy may be misplaced
0.25 inches sheath diameter

12. Vol II - Series 65 RTD
Temperature vs  relationships and Tolerances confirm to IEC 751.
RED
WHITE
Note:
Wire colors different from Vol.1
All Single Element Sensors
are supplied as 4 Wire
RED
BLACK
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
Customers may remove excess leads as necessary to create 2 and 3-wire configurations as notified with shipping instructions. When converting to 3-wire configuration two leads should not be commoned or twisted together as this will cause a significant measurement error.
DUPLEX contains two platinum sensing resistors and are currently only available as 2 x 3-wire output leads. The duplex sensor type is often used where the customer requires redundancy allowing a higher reliability particularly where a second transmitter is also used. The 3244MV hot back-up feature can also be used with this sensor. In environments of high vibration belief in this approach to redundancy may be misplaced

13. RTD Sensors 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
Typically use copper wires for extension from the sensor
Find the error in a 2-wire:
18 A.W.G. copper wire, Resistance = Ohms/ 1000 ft
-Assuming a length of 20 ft (6 meters), this adds 0.33oC error to the measurement. If the gage wire is larger (e.g. 22), the error shoots up to 0.84oC!
Red
White
Green
Blue
Black
Red
Sensing Element
(I.e. wire-wound, thin film)

14. Temperature Sensor : Stability, Repeatability & Linearity
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
IEC 751 Curve
Temperature (oC)
Resistance (W)

15. Temperature Sensor : Stability
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.
< Deg C/year for laboratory grade
<0.1 Deg C of FS/year for thin film Platinum RTD for normal usage
<0.05 Deg C per 5-years for temperature ranges -40 to 125 Deg C

16. Temperature Sensor : Repeatability
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.
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.

17. RTD Sensors 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 Ohms
IEC 751
Resistance (Ohms)
Have one of the experienced sales people answer this.
IEC 751
Temperature (oC)
Page 3-2:
Sensors PDS (V.1)

18. Temperature Sensor : Linearity
Linearity is defined as the deviation of the output-versus- temperature 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)
IEC 751 Curve
Temperature (oC)
Resistance (W)

19. RTD Sensor Joining Techniques
Soldering
Crimping
Brazing
Spot welding
Laser welding
Recommended Temperature
Range
- 30 to 120 Deg C
- 50 to 200 Deg C
- 50 to 400 Deg C
- 50 to 600 Deg C
- 50 to 850 Deg C
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.

20. Temperature : International Standards

21. What other consideration?
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.
Sensor elements for industrial applications must be packaged ruggedly. RMD standard sensors are all of mineral insulated cable design with 6 mm sheath diameter.
RTD ( Resistance Temperature Detector) sensors using platinum resistance elements provide the most stable and reliable measurement over the range to 500 C and should always be specified by choice. Sheath material is AISI 321 stainless steel. Model 65, using platinum film type element, has its’ range reduced to meet the requirements of the international specification IEC 751:1983 INDUSTRIAL PLATINUM RESISTANCE THERMOMETER SENSORS. ( including Amendments A1:1986 and A2:1995). Model 75, using a wire-wound element will meet IEC 751 over an extended temperature range. Grade B tolerance class is offered as standard with Grade A as an option.
Thermocouples must be offered for temperatures in excess of 500 C. Up to 1100 C type K should be specified. Sheath material must be suitable for these high temperatures and Inconel is available as standard. Thermocouples meet the requirements of IEC 584 :1995 THERMOCOUPLES, and are available to tolerance class 2 as standard. Other tolerance classes, requiring specially selected materials, are available as specials.
For temperatures above 1100 C thermocouples , such as types R or S, are available. These are always supplied as specials as the high temperature environment determines the many thermocouple and sheath material options that will be required.

22. Sensor Lead-Wire Length : RTD
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

23. Temperature Point Response Time
Factors Affecting Temperature Point Response Time
Thermowell
Sensor
Thermowell
Transmitter
Process
75.4 °C
Sensor
Process
Transmitter

24. Factors Affecting Sensor Response Time
Sensor Time Response
Factors Affecting Sensor Response Time
Type of element
Wirewound RTD
externally or internally wound
Thin-film RTD
Thermocouple
Element packaging
Element coating, potting
Contact between element package & sheath
Sheath thickness and material OD
ceramic
bore
element
sheath
Al2O3
packing

25. Sensor Time Response Industry Average Rosemount
(Series 78) wirewound RTD s 4.7 s
(Series 65 & 68) thin-film RTD s s
Ungrounded thermocouples < 2 s < 2 s
Grounded thermocouples < 1 s < 1 s
* 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.

26. Factors Affecting Response Time of Sensors in Thermowells
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

27. Time Response for Sensors in Thermowells
Tapered thermowell 26 seconds
Stepped thermowell 22 seconds
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.

28. Factors Affecting Transmitter Response Time
Time response depends on element (complexity of calculation)
2-wire RTD ms
3 & 4-wire RTD ms
Thermocouples ms
Above is good provided the analog output changes less than 2%
Transmitter update time (output) every 500msec
Process
Transmitter
75.4 °C

29. 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:
75.4 °C
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

30. Response Time 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

31. Frequently Asked Questions
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.

32. Frequency Asked Questions
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.

33. Frequently Asked Questions
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 & a 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 probe will read
138.5 ohm and the probe will read ohm.
The sensor must be properly matched to the easuring device to
obtain an accurate reading

34. Frequently Asked Questions
How can I determine if I have a or
temperature coefficient ?
The colour coding on the wires.
White, Red, Red
Red, White, White