Temperature Measurement Resistance Temperature Detector (RTD) Thermister Thermocouple

1. PREPARED BY : Kundan Parmar
Industrial Instrumentation
9-Oct-19

2.  Resistance Temperature Detector - RTD
 Thermister
 Thermocouple
Contents

3. Introduction
 RTD stands for Resistance Temperature Detector.
 RTDs are sometimes referred to generally as resistance thermometers.
 RTD is one of the most accurate temperature sensor.

4. Location of RTD

5. Principle of RTD
 To convert the Temperature variations into an equivalent change
in Resistance
 Also called as “ Resistance Thermometers”
 Resistance thermometer measure Resistance in terms of
Temperature
 Relation between resistance and temperature
Rt = R0 [ 1 + α1t + α2t2 ]

6.  Material used for sensing element
 Platinum : -260˚C to 110˚C
 Copper : 0˚C to 180˚C
 Nickel : -220˚C to 300˚C
 Tungsten : -200˚C to 1000˚C
Sensing Element

7. Construction & Working
 Sensing Element
 RTD

8. Bridge Circuit
R3 R2 = R1 (RA + RB + RS )
 2 Wire Bridge circuit
 Output

9. Types of RTD

10. Advantages of RTD
 Used for measurement of small as well as large temp. differences
 Measurement has a high accurate
 Measurement has high reproducibility
 RTD has good dynamic response
 Lots of flexibilities
 Temperature compensation not required
 Resistive element response time is of the order of 2 to 10 sec.
 Stability of the performance
 Limits of error are ±0.25 % of the scale
 Small size

11. Disadvantages of RTD
 Susceptibility to mechanical damage
 Need for lead wire resistance compensation
 Sometimes expensive
 Susceptibility to self-heating error
 Susceptibility to signal noise
 Unsuitability for bare use in electrically conducting substance
 Generally not repairable
 Need for power supply

12. Application of RTD
 In Automotive Industry – As audio amplifiers and engine oil
temperature sensors.
 In Communication and Instrumentation – As temperature sensors
and amplifiers.
 In Consumer Electronics – For small appliance controls and Fire
Detectors.
 In Industrial Electronics – For gas flow indicators and Plastic
laminating equipment.
 In Medical Electronics – For blood dialysis equipment and Infant
incubators

13. What is thermister ?
 A thermistor is a resistance thermometer, or a resistor
whose resistance is dependent on temperature.
 The term is a combination of “thermal”
and “resistor”.
 It is made of metallic oxides, pressed into a bead, disk,
or cylindrical shape and then encapsulated with an
impermeable material such as epoxy or glass.

14. Types of thermister
There are two types of thermisters:
 Negative Temperature Coefficient (NTC)
 Positive Temperature Coefficient (PTC).
Negative Temperature Coefficient (NTC) :
 With an NTC thermister, when the temperature increases,
resistance decreases. Conversely, when temperature decreases,
resistance increases. This type of thermister is used the most.

15. Positive Temperature Coefficient (PTC).
 A PTC thermistor works a little differently. When temperature increases, the
resistance increases, and when temperature decreases, resistance decreases.
This type of thermistor is generally used as a fuse.
Characteristic Curves of NTC & PTC :

16. Advantages Disadvantages
 Small size
 Very high sensitivity (Select range)
 Polarity insensitive
 No cold junction compensation
 Wide selection of sensors
 Inexpensive
 Fast response
 Not easily interchangeable
 High resistance, noise problems
 Non linear
 Unstable due to drift and decalibration
(especially at high temperatures)
 Narrow span
 Fragile

17.  Measurement of Temperature
 Control of Temperature
 Temperature Compensation
 It is used for the measurement of high-frequency power.
 The thermister measures the thermal conductivity.
 The thermister measures the pressure of the liquid.
Applications

19. Principle
As per the Peltier effect, when two dissimilar metals are joined
together to form two junctions, emf is generated within the circuit due to the
different temperatures of the two junctions of the circuit.

20. Working
 A thermocouple is comprised of at least two metals joined together to
form two junctions.
 One is connected to the body whose temperature is to be measured;
this is the hot or measuring junction.
 The other junction is connected to a body of known temperature; this
is the cold or reference junction

21. Types of Thermocouple
① TYPE E
② TYPE K
③ TYPE R
④ TYPE S
⑤ TYPE T

22. ADVANTAGE DISADVANTAGE
Wide temperature range
(-270℃ to 2700℃)
Slightly nonlinear
External DC source is not
required
Emf is small
Rugged construction Sometimes, the signal required
to be amplified
Less expensive Expansive accessories
They have good reproducibility Less accuracy
Advantages & Disadvantages

23. Applications
① Thermocouple have small heat capacity. Hence, they are suitable for
indication of rapidly changing temperature
② thermocouple are used in pyrometers
③ In measurement of surface temperature

24. Mobile Application
Symbol App Name About this App
Thermocouple
Reference,
Information and
Calculations