It shows working of RTD and Thermocouple and some characteristics along with merits and demerits

1. TRANSDUCER

2. What is TRANSDUCER ?
“A device which convert the one form of energy
into another form of the energy”
OR
“A device which converts a non-electrical quantity
into an electrical quantity”
Functions:
1. Detects or senses the presence , magnitude and
changes in physical quantity being measured.
2.Provides a proportional electrical signal

3. CLASSIFICATION OF TRANSDUCERS
A] Broadly classified in TWO groups;
1] Active Transducer or Self generating type
transducer :- develop their own voltage or current
(eg. Thermocouple and thrmopile , piezoelectric
pick up , photovoltaic cell etc.)
2] Passive Transducers or Externally powered
transducers :- derive power from external source
(e.g RTD , Thermistor, potentiometric devices etc)

4. B] Based on type of output
1] Analog transducer:- convert i/p physical quantity
into an analogous o/p which is contineous function of
time. (e.g Strain guage , Thermocouple , Thermistor,
LVDT etc)
2] Digital transducer :- convert i/p physical quantity
into an electrical o/p which may be in pulse form

5. C] Based on Electrical principle involved
1.Variable-resistance type: a. Strain and pressure
gauges b. Thermistors, resistance thermometers c.
Photoconductive cell
2.Variable –inductance type- a. LVDT b. Reluctance
pick up c. Eddy current gauge
3.Variable-capacitance type- a. capacitor microphone
b. Pressure gauge c. Dielectric gauge
4.Voltage generating type- a. Thermocouple b.
Photovoltaic cell c. Rotational motion tachometer d.
Piezzoelectric pick up
5. Voltage divider type- a. Potentiometer position
sensor b. Pressure actuated voltage divider

6. Selection of Transducer:
1. Ranges available 7. Maximum depth
2.Squaring system 8. Linearity and hysteresis
3. Sensitivity 9. Output for zero input
4. Maximum working temp. 10. Temp. coe. Of zero drift
5. Method of cooling 11. Natural frequency
6. Mounting details

7. Temperature Measurement using RTD
(Resistance Temperature Detector)

8. The Birth of RTD
• The classical resistance temperature detector (RTD)
construction using platinum was proposed by C.H. Meyers in
1932.
• He wound a helical coil of platinum on a crossed mica web and
mounted the assembly inside a glass tube.
• This construction minimized strain on the wire while
maximizing resistance.

9. Platinum Sensing Resistors
• Platinum, with its wide temperature range and stability, has
become the preferred element material for resistance
thermometers.
• Platinum sensing resistors are available with alternative Ro
values, for example 10, 25 and 100 Ohms.
• A working form of resistance thermometer sensor is defined in
IEC and DIN specifications.
• This forms the basis of most industrial and laboratory
electrical thermometers.
• The platinum sensing resistor, Pt100 to IEC 751 is dominant in
many parts of the world.
• Its advantages include chemical stability, relative ease of
manufacture, the availability of wire in a highly pure form and
excellent reproducibility of its electrical characteristic.
• The result is a truly interchangeable sensing resistor which is
widely commercially available at a reasonable cost.

10. RTD

11. Resistance Measurement
Several different bridge circuits are used to
determine the resistance. Bridge circuits help
improve the accuracy of the measurements
significantly. Bridge output voltage is a function of
the RTD resistance.

12. FROM THE EQUATION OF RESISTANCE,
• R = resistance
• ρ = specific resistance of the
conductor material
• L = the length of the
conductor in meters
• A = the area of the conductor
in square meters
• When a strain produced by a
force is applied on the wires,
L increase and A decrease.

13. 0
50
100
150
200
250
300
350
400
-300 -100 100 300 500 700 900
t, o
C
R,Ohm
Platinum RTD
Copper RTD

15. COMPARISON OF ELEMENT TYPES

16. Calendar—Van Dusen equation
      3
0 01.0101.001.0101.01 TTTTTRR  
• Where, a,b and dare calibration constants, dependent
on the purity of the platinum which is country
dependent.
•The dominant constant is a which has a value of either
0.003921/°C for the so-called U.S. calibration curve, or
• 0.003851/°C for the "European" calibration curve.
•RTD sensors corresponding to either curve are available.
•For the U.S. calibration curve, d = 1.49, b = 0 for T < 0
and b = 0.11 for T>0.
 432
0 1001 TCTCTBTARR 

17. Properties of RTD
1.Linearity of resistance
2. Resist to corrosion and oxidation under the temp. range
3. To provide reproducible and consistent results
4. Good sensitivity
5.High sensitivity i.e unit can be fabricated in compact and
convenient size.
6.No change of phase or state within a reasonable temperature
range
Note:- Industrial thermometers are usually made as Platinum ,
Nickel , Copper and basically for precise measurement Platinum
is preferred because physically stable , high electrical resistance
characteristics .
*Platinum resistance thermometer to its accuracy, stability,
sensitivity temp range from b.p of Oxygen -182.9 deg. C to
freezing point of antimony 630.5 deg. C

18. Advantages of RTD
1.Resist to corrosion and oxidation
2. Physically stable
3. Measurement is very accurate
4. It has a lot flexibility
5. It does not require a reference junction
6. Temp. sensitive resistance element can be replaced and
installed easily
7. High working signal level
8. The limits of error of resistance element are claimed to be +/-
0.25 % of the scale range
9.The accuracy of measuring circuit can be checked easily by
substituting a std resistor.
10. Self heating effect can be reduced to negligible with proper
design
11.The resistance element response time is order of 2 to 10
seconds

19. Dis-advantages of RTD
1. It is more costly
2. They suffer from time lag
3. Leakage current is possible between resistance element to
ground
4. Resistance change due to temperature changes of measuring
resistors

20. Temperature Relationships
(°F) = 9/5*(°C) +32
(°C) = 5/9*[(°F) –32]
(°F) = (°R) – 459.67
(°C) = (K) – 273.15

21. THERMOCOUPLE

22. A thermocouple is in simplest form, consist of two
dissimilar metals or alloys which develop emf when
the reference and measuring junction are at different
temperatures.
The reference or cold junction is maintained at
constant temp. such as 0 deg cen.
Fig shows a simple circuit of thermocouple and temp
measuring device.
In industrial installations , equipment's are equipped
with automatic compensating devices for temp.
changes of the reference junction , thus eliminating the
necessity of maintaining this junction at constant
temp.

23. • Seebeck effect
1. If two wires of dissimilar metals are joined at both
ends and one end is heated, current will flow.
2. If the circuit is broken, there will be an open circuit
voltage across the wires.
3. Voltage is a function of temperature and metal types.
4. For small DT’s, the relationship with temperature is
linear
5. For larger DT’s, non-linearities may occur.
V T  

24. Properties of Thermocouple:
1. The temp and e.m.f relation should be linear and
reproducible
2. Adequate thermo e.m.f per degree of temp. change to
facilitate detection and measurement
3. It should be strong for withstand high temp.
4. It should maintain its calibration without drift for long
period of time.
5. Cost should be reasonable
6. It should have long life

25. TYPES OF THERMOCOUPLE

26. Thermocouple Types
• Type B – very poor below 50ºC; reference junction temperature not
important since voltage output is about the same from 0 to 42 ºC
• Type E – good for low temperatures since dV/dT is high for low
temperatures
• Type J – cheap because one wire is iron; high sensitivity but also high
uncertainty (iron impurities cause inaccuracy)
• Type T – good accuracy but low max temperature (400 ºC); one lead is
copper, making connections easier; watch for heat being conducted
along the copper wire, changing your surface temp
• Type K – popular type since it has decent accuracy and a wide
temperature range; some instability (drift) over time
• Type N – most stable over time when exposed to elevated
temperatures for long periods

27. Advantages of Thermocouple:
1. Better response
2.Higher range of temp. measurements
3. Sensing element can be easily installed
4. Cheap
5. Very convenient for measuring the temp. at
one particular point in a piece of appratus.

28. Disadvantages of Thermocouple:
1. Low accuracy
2. Circuit is very complex
3. For long life they need to be amply
protected.

29. Choice Between RTDs, Thermocouples, Thermisters
• Cost – thermocouples are cheapest by far, followed by RTDs
• Accuracy – RTDs or thermistors
• Sensitivity – thermistors
• Speed – thermistors
• Stability at high temperatures – not thermistors
• Size – thermocouples and thermistors can be made quite small
• Temperature range – thermocouples have the highest range, followed by RTDs
• Ruggedness – thermocouples are best if your system will be taking a lot of
abuse

30. Thank You