ThermocouplesMost 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.

Let’s take a look at this circuit

Material EMF versus Temperature

With reference tothe characteristicsof 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 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

Thermocouples - standard types

Table 3.4. Thermocouples (standard types and others) and some of their propertiesMaterials Sensitivity

[V/C]at 25C.

StandardTypedesignation

Temperaturerange [C]

Notes

Copper/Constantan 40.9 T 270 to 600 Cu/60%Cu40%NiIron/Constantan 51.7 J 270 to

1000Fe/60%Cu40%Ni

Chromel/Alumel 40.6 K 270 to1300

90%Ni10%Cr/55%Cu45%Ni

Chromel/Constantan 60.9 E 200 to1000

90%Ni10%Cr/60%Cu40%Ni

Platinum(10%)/Rhodium-Platinum 6.0 S to 1450 Pt/90%Pt10%RhPlatinum(13%)/Rhodium-Platinum 6.0 R to 1600 Pt/87%Pt13%RhSilver/Paladium 10 200 to 600Constantan/Tungsten 42.1 0 to 800Silicon/Aluminum 446 40 to 150Carbon/Silicon Carbide 170 0 to 2000Note: sensitivity is the relative Seebeck coefficient.

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