BSM LAB

1. Instrumentation Lab.
 RTD
 Thermistors
 Thermocouple PRESENTED BY:
Gaurav

2. Topics -

3.  Overview
 Sensitivity
 Response Time
 Construction
 Signal Conditioning
 Dissipation Constant
 Range
Resistance Temperature Devices

4. Overview
A resistance-temperature detector (RTD) is a
temperature sensor that is based on the principle
of metal resistance increasing with temperature.
Metals used in these devices
1. platinum, which is very repeatable, quite
sensitive, and very expensive,
2. nickel, which is not quite as repeatable, more
sensitive, and less expensive.

5. Sensitivity
 RTD sensitivity can be noted from typical
values of , the linear fractional change in
resistance with temperature
 For platinum, this number is typically on the
order of 0.004/°C, and
 for nickel a typical value is 0.005/°C.
Thus, with platinum, for example, a change
of only 0.4 W would be expected for a 100-W
RTD if the temperature is changed by 1°C.

6. Response Time
 In general, RTD has a response time of 0.5 to 5
seconds or more.
 The slowness of response is due principally to the
slowness of thermal conductivity in bringing the
device into thermal equilibrium with its environment.
 Generally, time constants are specified either for a
"free air" condition (or its equivalent) or an "oil bath"
condition (or its equivalent).

7. Construction
An RTD, is simply a length of wire whose resistance
is to be monitored as a function of temperature.
The construction is typically such that the wire is
wound on a form (in a coil) to achieve small size and
improve thermal conductivity to decrease response
time.
 In many cases, the coil is protected from the
environment by a sheath or protective tube that
inevitably increases response time but may be
necessary in hostile environments.

8. Resistance temperature devices (RTD) are
either a metal film deposited on a former or are
wire-wound resistors.
The devices are then sealed in a glassceramic
composite material.
The electrical resistance of pure metals is
positive, increasing linearly with temperature
These devices are accurate and can be used to
measure temperatures from - 300 to 1400°F (-
170 to 780°C).
Construction

9. Figure :- compensation lines in this typical RTD
signal-conditioning circuit

10. Figure :- Block diagram for RTD Circuit

11. Dissipation Constant
where
DT = temperature rise because of self-heating in °C
P = power dissipated in the RTD from the circuit in W
PD = dissipation constant of the RTD in W/°C

12. Range
The effective range of RTDs depends
principally on the type of wire used as the
active element.
Thus, a typical platinum RTD may have a
range of -100°C to 650°C,
RTD constructed from nickel might
typically have a specified range of -180°C to
300°C.

13. RTD Applications
Air conditioning and
refrigeration servicing
Furnace servicing
Foodservice processing
Medical research
Textile production

14. RTDs
• Most stable over time
• Most accurate
• Most repeatable
temperature measurement
• Very resistant to
contamination/
• corrosion of the RTD
element
• High cost
• Slowest response time
• Low sensitivity to small
temperature changes
• Sensitive to vibration
(strains the platinum
element wire)
• Decalibration if used
beyond sensor’s
temperature ratings
• Somewhat fragile
Advantages Disadvantages

15. Thermocuple
Introduction
 Thermocuple Principal of Operation
 Working of Thermocuple
 Applications of thermocuple
 Advantages and Disadvantages of thermocuple

16. Thermocouples
Two wires of different
metal alloys.
Converts thermal energy
into electrical energy.
Requires a temperature
difference between
measuring junction and
reference junction.
Easy to use and obtain.

17. Thermocouple measures temperature difference
(T1 – T2) between two junctions
Copper
Copper
Constantan Voltage output
T1
T2
- Easy to construct. Just twist together Copper and
Constantan wires, and solder.
- Beautifully suited to measuring temperature
differences directly.
- Requires knowledge of temperature at T2 (“reference”
temp) to get actual temperature at T1.

18. Thermocouples Principle of Operation
• In, 1821 T. J. Seebeck observed the existence of an
electromotive force (EMF) at the junction formed between
two dissimilar metals (Seebeck effect).
– Seebeck effect is actually the combined result of two
other phenomena, Thomson and Peltier effects.
• Thomson observed the existence of an EMF due
to the contact of two dissimilar metals at the
junction temperature.
• Peltier discovered that temperature gradients
along conductors in a circuit generate an EMF.
• The Thomson effect is normally much smaller
than the Peltier effect.

19. • It is generally reasonable to assume that the emf is
generated in the wires, not in the junction. The
signal is generated when dT/dx is not zero.
• When the materials are homogeneous, e, the
thermoelectric power, is a function of temperature
only.
• Two wires begin and end at the same two
temperatures.
E  (T  To)  (T  To )2
Generally, a second order Eqn. is used.
Working of Thermocuple

20. Thermocouple Applications
 Plastic injection molding
machinery
 Food processing equipment
 Deicing
 Semiconductor processing
 Heat treating
 Medical equipment
 Industrial heat treating
 Packaging equipment

21. Thermocouples
 Simple, Rugged
 High temperature
operation
 Low cost
 No resistance lead wire
problems
 Point temperature sensing
 Fastest response to
temperature changes
 Least stable, least
repeatable
 Low sensitivity to small
temperature changes
 Extension wire must be of
the same thermocouple
type
 Wire may pick up radiated
electrical noise if not
shielded
 Lowest accuracy
Advantages Disadvantages

22. Thermistors
• A thermistor is a type of resistor used to measure
temperature changes, relying on the change in its
resistance with changing temperature.
• Thermistor is a combination of the words thermal and
resistor.
• The Thermistor was invented by Samuel Ruben in
1930.

23. Thermistors
• A semiconductor used as a temperature sensor.
• Mixture of metal oxides pressed into a bead, wafer or
other shape.
• Beads can be very small, less than 1 mm in some cases.
• The resistance decreases as temperature increases,
negative temperature coefficient (NTC) thermistor.
Figure :- circuit diagram for thermistors

24. Thermistors
Thermistor materials have a temperature
coefficient of resistance (α) given by
α = ∆R / Rs (1/ ∆T)
where
∆R is the change in resistance due to a
temperature change ∆T
RS is the material resistance at the
reference temperature

25. Thermistors
 Thermistors have high sensitivity which can be up to 10
percent change per degree Celsius, making them the most
sensitive temperature elements available, but with very
nonlinear characteristics.
 The typical response times is 0.5 to 5 s with an operating
range from - 50 to typically 300°C. Devices are available with
the temperature range extended to 500°C.
 Thermistors are low cost and manufactured in a wide range
of shapes, sizes, and values.
 When in use care has to be taken to minimize the effects of
internal heating.

26. A non linear decrease in resistance as
temperature increase

27. Thermistors
• High sensitivity to small
temperature changes
• Temperature
measurements become
more stable with use
• Copper or nickel
extension wires can be
used
• Limited temperature
range
• Fragile
• Some initial accuracy
“drift”
• Decalibration if used
beyond the sensor’s
temperature ratings
• Lack of standards for
replacement
Advantages Disadvantages

28. Applications:
1. Temperature measurement.
2. Time delay (self heating from large current ‘opens’
the thermistor so it can be used as a slow switch).
Heating = i2 R where R is the resistance and i is the
current.
3. Surge suppression when a circuit is first energized.
Current needs to flow through the thermistor for
awhile to heat it so that it ‘opens’, and acts again as
a switch.

29. Resistance v/s Temperature

30. References
 Fundamental of Instrumentation Control &
Process by William C. Dunn
 PatArnott, ATMS 360 Atmospheric
Instrumentation
 Temperature Sensors By Grant Stucker
 http://google.com/images/thermocuple
 Wikipedia/