thermocouples - mullanathermocouples principle of operation • in, 1821 t. j. seebeck observed the...
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Thermocouples
Most frequently used method to
measure temperatures with an
electrical output signal.
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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.
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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.
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Let’s take a look at this circuit
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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, 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.
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Material EMF versus Temperature
With reference to
the characteristics
of pure Platinum
emf
Temperature
Chromel
Iron
Copper
Platinum-Rhodium
Alumel
Constantan
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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
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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.
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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.
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Zone boxes
– A zone of uniform temperature
that insures all connections made
within the zone are at the same temperature.
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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
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Thermocouple Material Vs EMFTypes T, J, and K are most commonly used thermocouples (see
Table 16.8 of the “Handbook”).
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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
EmfA
B
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Law of Intermediate Metals2) 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.
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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
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Law of Intermediate
TemperaturesIf 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
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A Demonstration of the Law of
Intermediate Temperaturesem
f
T 1 T 2
Fe
C
T 3
emf23
emf1-2+ emf2-3= emf1-3
emf13
emf12
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emf
T ref T hot
Measured
Emf
Fe
C
1
2
T measured
3
5
4
Hot Zone
12
3
4
A Demonstration of the Law of Intermediate
Temperatures
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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.
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Single and multiplexing
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Temperature Measurement
Errors• Conduction
• Convection
• Radiation
• Response Time
• Noise
• Grounding issues and shorts, especially
on metal surfaces