Thermocouples

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

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.

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.

Lets take a look at this circuit

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.

Generally, a second order Eqn. is used.

Material EMF versus Temperature

With reference to

the characteristics

of pure Platinum

emf

Temperature

Chromel

Iron

Copper

Platinum-Rhodium

Alumel

Constantan

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 diameterWelding is generally quicker than soldering but both are equally acceptableVoltage or EMF produced depends on:Types of materials usedTemperature difference between the measuring junction and the reference junction

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 0C. If your reference junction is not at 0C, a correction must be applied using the law of intermediate temperatures.

Reference Temperature Systems and Zone Boxes

Ice BathsAccurate and inexpensiveElectronically Controlled ReferencesRequire periodic calibration and are generally not as stable as ice baths, but are more convenient.

Zone boxes

A zone of uniform temperature

that insures all connections made

within the zone are at the same temperature.

What thermocouple materials should be used?

Depends on requirements:Temperature range?Required accuracyChemical resistance issuesAbrasion or vibration resistanceInstallation requirements (size of wire)Thermal conduction requirements

Thermocouple Material Vs EMF

Types T, J, and K are most commonly used thermocouples (see Table 16.8 of the Handbook).

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

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.

A Demonstration of the Law of Intermediate Metals

emf

Temperature

3

1

2

T ref

T 2 and 4

Measured Emf

Fe

C

Fe (+)

C (-)

P (+)

5

4

6

Tcandle

1

2

T measured

3

5

4

6

Signs of the

materials used

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

A Demonstration of the Law of Intermediate Temperatures

emf

T 1

T 2

Fe

C

T 3

emf23

emf1-2+ emf2-3= emf1-3

emf13

emf12

A Demonstration of the Law of Intermediate Temperatures

emf

T ref

T hot

Measured Emf

Fe

C

1

2

T measured

3

5

4

Hot Zone

1

2

3

4

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.

Single and multiplexing

Temperature Measurement Errors

ConductionConvectionRadiationResponse TimeNoiseGrounding issues and shorts, especially on metal surfaces

E

=

a

(

T

-

T

o

)

+

b

(

T

-

T

o

)

2