When two metals having different work functions are placed together, a voltage is generated at the junction which is nearly proportional to the temperature. This junction is called a thermocouple.
Working Principle of Thermocouple
- Thomas Seebeck discovered in 1821 that when two dissimilar metals were in contact, a voltage was generated where the voltage was a function of temperature. The device, consisting of two dissimilar metals joined together, is called a thermocouple and the voltage is called the Seebeck voltage.
- If both ends of the two conductors are connected [Fig 1 (a)] and a temperature difference is maintained between the two junctions, a thermoelectric current will flow through the closed circuit (generation mode).
- If the circuit is opened [Fig (b)] an emf will appear across the open circuit (sensing mode). It is this emf that is measured in a thermocouple sensor.
For example, joining copper and constantan produces a voltage on the order of a few tens of millivolts with the positive potential at the copper side. An increase in temperature causes an increase in voltage.
Conductors a and b are homogeneous. Let junctions 1 and 2 are at temperatures T2 and T1.
Total emf is given by
- and are the absolute Seebeck coefficients given in mV/°C and are properties of the materials A, B.
- is the relative Seebeck coefficient of the material combination A and B, given in mV/°C
- The relative Seebeck coefficients are normally used.
Table 1: Absolute Seebeck coefficients for selected elements (Thermoelectric series)
Material | a [mV/°K] |
p-Silicon | 100 – 1000 |
Antimony (Sb) | 32 |
Iron (Fe) | 13.4 |
Gold (Au) | 0.1 |
Copper (Cu) | 0 |
Silver (Ag) | -0.2 |
Aluminum (Al) | -3.2 |
Platinum (Pt) | -5.9 |
Cobalt (Co) | -20.1 |
Nickel (Ni) | -20.4 |
Bismuth (Sb) | -72.8 |
n-Silicon | -100 to -1000 |
Joining Two Disimilar Metals
There are several methods of joining the two dissimilar metals.
- Weld the wires together
- This produces a brittle joint, and if not protected from stresses, this type of thermocouple can fracture and break apart.
- During the welding process, gases from the welding can diffuse into the metal and cause a change in the characteristic of the thermocouple.
- Solder the wires together
- This has the disadvantage of introducing a third dissimilar metal. Fortunately, if both sides of the thermocouple are at the same temperature, the Seebeck voltage due to thermocouple action between the two metals of the thermocouple and the solder will have equal and opposite voltages and the effect will cancel.
- Another disadvantage is that in many cases the temperatures to be measured are higher than the melting point of the solder and the thermocouple will come apart.
It would appear to be a simple matter to measure the Seebeck voltage and create an electronic thermometer. To do this, wires could be connected as shown in Figure to make the measurement.
Construction and Working
Assume that the meter uses copper wires as shown.
In this case, where the two copper wires come in contact there is no problem, but where the copper comes in contact with another metal, such as the constantan thermocouple wire, the two dissimilar metals create another thermocouple, which generates its own Seeback voltage.
For this example, copper interconnecting wires were used and the thermocouple was copper and constantan. The composition of the wires is immaterial, as any combination will produce these parasitic thermocouples with the problems of additional Seeback voltages.
It is an unavoidable fact that there will be at least two thermocouple junctions in the system. To contend with this, it is necessary that the temperature of one of the junctions be known and constant.
Therefore, there is a fixed offset voltage in the measuring system. In early times, it is mandatory to place this junction in a mixture of ice and water, thus stabilising the temperature to 0°C as shown in Figure 3.
In modern-age electronic reference junction is used and it is called the reference or cold junction because this junction was traditionally placed in the ice bath.
Nowadays, electronic devices are used to measure thermocouple voltages and to convert from the Seeback voltage to temperature, and to compensate for the reference junction.
Errors occurring During the Measurement using Thermocouple
Open Junction
- There are many sources of an open junction. Usually, the error introduced by an open junction is of such an extreme magnitude that an open junction is easily spotted.
- By simply measuring the resistance of the thermocouple, the open junction is easily identified.
Insulation Degradation
- The thermocouple is often used at very high temperatures. In some cases, the insulation can break down and causes a significant leakage resistance which will cause an error in the measurement of the Seeback voltage.
- In addition, chemicals in the insulation can defuse into the thermocouple wire and cause decalibration.
Thermal Conduction
- The thermocouple wire will shunt heat energy away from the source to be measured. For small temperature to be measured, small diameter thermocouple wire could be used.
- However, the small diameter wire is more susceptible to the effects. If a reasonable compromise between the degrading effects of small thermocouple wire and the loss of thermal energy and the resultant temperature error cannot be found, thermocouple extension wire can be used.
- This allows the thermocouple to be made of small diameter wire, while the extension wire covers majority of the connecting distance.
Galvanic Action
- Chemicals coming in contact with the thermocouple wire can cause a galvanic action. This resultant voltage can be as much as 100 times the Seebeck voltage, causing extreme errors.
Decalibration
- This error is a potentially serious fault, as it can cause slight error that may escape detection. Decalibration is due to altering the characteristics of the thermocouple wire, thus changing the Seeback voltage.
- This can be caused due to subjecting the wire to excessively high temperatures, diffusion of particles from the atmosphere into the wire, or by cold working the wire.
Types of Thermocouple
- The first material in each type (E, J, K, R, S and T) is positive, the second negative.
Table 2: Thermocouples (standard types and others) and some of their properties
Materials | Sensitivity [mV/°C] at 25°C. | Standard Type designation | Temperature range [°C] | Notes |
Copper/Constantan | 40.9 | T | -270 to 600 | Cu/60%Cu40%Ni |
Iron/Constantan | 51.7 | J | -270 to 1000 | Fe/60%Cu40%Ni |
40.6 | K | -270 to 1300 | 90%Ni10%Cr/55%Cu45%Ni | |
Chromel/Constantan | 60.9 | E | -200 to 1000 | 90%Ni10%Cr/60%Cu40%Ni |
Platinum(10%)/Rhodium-Platinum | 6.0 | S | 0 to 1450 | Pt/90%Pt10%Rh |
Platinum(13%)/Rhodium-Platinum | 6.0 | R | 0 to 1600 | Pt/87%Pt13%Rh |
Silver/Paladium | 10 | 200 to 600 | ||
Constantan/Tungsten | 42.1 | 0 to 800 | ||
Silicon/Aluminum | 446 | -40 to 150 | ||
Carbon/Silicon Carbide | 170 | 0 to 2000 |
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