Thermocouples operate by generating a voltage when two dissimilar metals are joined and exposed to different temperatures. They are inexpensive, small, and accurate when used properly. Thermocouples work based on the Seebeck effect where a voltage is produced due to temperature differences between junctions. Tables are used to correlate the voltage measured to a specific temperature once the reference junction temperature is accounted for through laws such as the Law of Intermediate Temperatures. Common thermocouple types are T, J, and K, with the material chosen based on the required temperature range and accuracy.

1. Thermocouples
Most frequently used method to
measure temperatures with an
electrical output signal.

2. What are thermocouples?
• Thermocouples operate under the principle that a circuit
made by connecting two dissimilar metals produces a
measurable voltage (emf-electromotive force) when a
temperature gradient is imposed between one end and
the other.
• They are inexpensive, small, rugged and accurate when
used with an understanding of their peculiarities.

3. 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.

4. Let’s take a look at this circuit

5. How thermocouples work
• 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,
ε, the
thermoelectric
power, is a function
of temperature
only.
• Two wires begin
and end at the
same two
temperatures.
E = α(T − To) + β(T − To )
2
Generally, a second order Eqn. is used.

6. Material EMF versus Temperature
With reference to
the characteristics
of pure Platinum
emf
Temperature
Chromel
Iron
Copper
Platinum-Rhodium
Alumel
Constantan

7. Thermocouple Effect
• Any time a pair of dissimilar wires is joined
to make a circuit and a thermal gradient is
imposed, an emf voltage will be
generated.
– Twisted, soldered or welded junctions
are acceptable. Welding is most
common.
– Keep weld bead or solder bead
diameter within 10-15% of wire
diameter
– Welding is generally quicker than
soldering but both are equally
acceptable
– Voltage or EMF produced depends on:
• Types of materials used
• Temperature difference between
the measuring junction and the
reference junction

8. Thermocouple Tables
(EMF-Temperature)
• Thermocouple tables correlate temperature to
emf voltage.
– Need to keep in mind that the thermocouple
tables provide a voltage value with respect to
a reference temperature. Usually the
reference temperature is 0°C. If your
reference junction is not at 0°C, a correction
must be applied using the law of intermediate
temperatures.

9. Reference Temperature Systems and
Zone Boxes
• Ice Baths
– Accurate and
inexpensive
• Electronically Controlled
References
– Require periodic
calibration and are
generally not as stable
as ice baths, but are
more convenient.

10. Zone boxes
– A zone of uniform temperature
that insures all connections made
within the zone are at the same temperature.

11. What thermocouple materials
should be used?
• Depends on requirements:
– Temperature range?
– Required accuracy
– Chemical resistance issues
– Abrasion or vibration resistance
– Installation requirements (size of wire)
– Thermal conduction requirements

12. Thermocouple Material Vs EMF
Types T, J, and K are most commonly used thermocouples
(see Table 16.8 of the “Handbook”).

13. Simple TC Model “EMF-Temperature
Sketch”• Two materials
– Material A (+)
– Material B (-)
• Plus and minus refers
to how the emf changes
with temperature.
Number junctions around circuit and draw
emf
Temperature
3
1
2
3
1
2
T meter T junction
Measured
Emf
A
B

14. Law of Intermediate Metals
2) Insertion of an intermediate
metal into a thermocouple
circuit will not affect the emf
voltage output so long as the
two junctions are at the same
temperature and the material
is homogeneous.
– Permits soldered and welded
joints.

15. A Demonstration of the Law of
Intermediate Metals
emf
Temperature
3
1 2
T ref T 2 and 4
Measured
Emf
Fe
CFe (+)
C (-)
P (+)
54
6
Tcandle
1
2
T measured
3
5
4
6
Signs of the
materials used

16. Law of Intermediate
Temperatures
If a thermocouple circuit develops a net
emf1-2 for measuring junction
temperatures T1 and T2, and a net
emf2-3 for temperatures T2 and T3, then
it will develop a net voltage of emf1-3 =
emf1-2 + emf2-3 when the junctions are
at temperatures T1 and T3.
emf1-2+emf2-3= emf1-3
T2
T3 T1
T3 T2
T1

17. A Demonstration of the Law of
Intermediate Temperaturesemf
T 1 T 2
Fe
C
T 3
emf23
emf1-2+emf2-3= emf1-3
emf13
emf12

18. emf
T ref T hot
Measured
Emf
Fe
C
1
2
T measured
3
5
4
Hot Zone
1
2
3
4
A Demonstration of the Law of Intermediate
Temperatures

19. If a thermocouple circuit of materials A and C generates a net emfA-C
when exposed to temperatures T1 and T2, and a thermocouple of
materials C and B generates a net emfC-B for the same two
temperatures T1 and T2, then a thermocouple made from materials
A and B will develop a net voltage of
emfA-B = emfA-C + emfC-B
between temperatures T1 and T2.
• Sometimes useful in the calibration of different thermocouple
wires.

20. Single and multiplexing

21. Temperature Measurement
Errors
• Conduction
• Convection
• Radiation
• Response Time
• Noise
• Grounding issues and shorts, especially
on metal surfaces