This article outlines the definition, construction, working principle, material selection, advantage and disadvantage of a thermocouple.
Definition:
Thermocouple is a combination of two different metallic strips joined together in such a way to form a loop. When the two junctions are kept at different temperatures, there is an electric current in the loop and an emf is developed. The magnitude of emf developed depends on the metals and the temperature difference of hot and cold junction. Such combination of two metals is popularly known Thermocouple.
Thermocouple is used as a temperature sensing device in industries. A thermocupule sensor is modified version in which two different metals are joined at one end while the other end is connected to a read-out device or mili volt-meter. When the junction is heated or cooled, an emf is produced across the open end of metals which is very small in magnitude. This is the reason, a mili-voltmeter is connected to measure this generated emf. The measured emf is directly proportional to the temperature of the junction and hence the temperature can be calibrated with the emf measured. Thermocouple is an active transducer.
Construction:
A thermocouple basically comprises of wires (called thermoelement), Insulation, Sheath and means for external connection. One end of thermoelement made up of two different metals are welded together to form a junction. This junction is, in fact, the measuring point.
Figure below shows the construction of thermocouple.
The thermoelement is housed in a rigid metal sheath, generally made of Inconel. Measuring junction is formed at the bottom of the housing. Magnesium Oxide filling surrounds the thermoelement. This filling acts as a shock absorber and prevents the wire form vibrational damage. It also acts as a heat dissipating medium of hot junction.
The use of sheath slows the response appreciably as the increase the mass of the thermocouple. However, in applications where response time is the primary consideration, bare or thin sheathed thermocouples are used. The sensitivity of thermocouple can be increased by reducing the mass of the measuring junction. One method of reducing the mass is to butt weld the two thermocouple wires. In applications where the mechanical strength of butt weld is not sufficient, the two wires are twisted together and the ends are welded.
The measuring junction may either be connected to the sheath or not. On the basis of connection of junction point to the sheath, there are three different types of thermocouple construction: Ungrounded, Grounded and Exposed Thermocouple.
Grounded Thermocouple:
In grounded thermocouple, the junction is connected to the metallic sheath. This type of thermocouple is used for temperature measurement in corrosive environment. However, the measurement is free from noise.
Ungrounded Thermocouple:
In ungrounded thermocouple, the junction is not connected to the metallic sheath. It is widely used in high pressure application. The measurement is free from the effect of stray magnetic field in this type of sensor.
Exposed Thermocouple:
Exposed thermocouple has the fastest response and hence used for applications requiring fast response. It is used for temperature measurement of gas. But the major drawback is that thermoelement is very prone to corrosion as it is left exposed and hence not recommended.
Selection of Thermocouple Wire Material:
In industrial applications the choice of materials used to construct a thermocouple depends upon the temperature range to be measured, the kind of atmosphere to which the material will be exposed, the output emf and its stability, mechanical strength and the accuracy required for the measurement. Thermocouple materials are divided into two categories: Rare Metal Type using Platinum, Rhodium etc. and Base Metal Type.
Several combinations of dissimilar metals make good thermocouple for industrial use. These combinations apart from having linear response and high sensitivity, should be physically strong to withstand high temperature, rapid temperature changes, and the effect of corrosion and reducing atmosphere.
The material for common types of thermocouple are tabulated below.
|
Base Metal Type |
|||
| Positive Wire: | Copper | Iron | Chromel |
| Negative Wire: | Constantan | Constantan | Constantan |
| Temp. Range (°C): | -250 to 400 | -200 to 850 | -200 to 850 |
| Characteristics: | Resists oxidizing and Reducing atmosphere up to 350 °C.
Requires protection from acid fumes |
Low cost. Corrodes in presence of moisture, oxygen and Sulphur bearing gases. Suitable for reducing atmosphere. | Suitable for oxidizing but not for reducing atmosphere. |
|
Rare Metal Type |
|||
| Positive Wire: | Platinum 90% and Rhodium 10% | Tungsten 95% and Rhenium 5% | Phodium |
| Negative Wire: | Platinum | Tungsten 72% and Rhenium 26% | Iridium |
| Temp. Range (°C): | 0 to 1400 | 0 to 2600 | 0 to 2100 |
| Characteristics: | Low emf. Good for oxidizing atmosphere but poor for reducing atmosphere. | For use in non-oxidizing atmosphere only. | Low emf. Good for oxidizing atmosphere but poor for reducing atmosphere. |
Working Principle of Thermocouple:
A thermocouple works on See Beck Effect. See Beck effect tells that when two dissimilar metals are joined together to form a junction and if a temperature difference between the junction is maintained between the junctions, an emf is induced in it. This emf is called Thermo-electric emf. If the junction forms a closed loop then this emf will cause a current through the loop. This property is utilized to measure temperature of a system.
In Thermocouple sensor, one end of two dissimilar metals are joined together and the other end is connected to mili-voltmeter. This is shown in figure below.
Two different type of metal wires A & B are joined at one end to make a junction. At this junction, temperature is measured and called detecting junction. The other end is connected to mili-voltmeter to measure the emf E. The emf produced in a thermocouple is given by
E = a(ΔƟ) + b(ΔƟ)2
where (ΔƟ) = difference between the temperature of hot junction and the reference junction in °C and a & b are constants.
Since the thermoelectric emf depend upon the difference in temperature between the hot and the reference junction, the temperature of latter should remain absolutely constant in order that calibration between temperature and measured voltage holds good and there are no error on account of change in ambient temperature. The temperature of reference junction is controlled for this purpose. The reference junction is usually 0 °C by using ice bath. It should be noted at this pint that the combination of metal should be so chosen that a rise in temperature should always produce a linear rise in emf i.e. the value of “b” should be negligible.
Measurement of Thermocouple Output:
The output emf of a thermocouple as a result of temperature difference between the hot and cold junction can easily be measured using mili-voltmeter. The mili-voltmeter is connected across the cold leg. The deflection is proportional to the current flowing in the circuit. If the resistance of meter is Rm and that of external circuit is Re, the current in the circuit I = E/(Rm+Re).
In order to ensure sufficient current to deflect the movement, the resistance of meter should be small since the sensitivity of thermocouple is quite small and they produce an output voltage which is a few mV/100 °C.
Lead Compensation:
In many industrial applications, it is desirable to place the reference junction at a point far away from the detecting junction. The connecting wire from the thermocouple head to the meter are, therefore, very long and are usually not at the same temperature throughout their length. This causes error, which can be avoided by using connecting wires made of the same material as the thermocouple wires. The implementation of this arrangement may not be possible due to cost. Under this case, materials are chosen such that the relationship between emf and temperature is the same or almost same as that for thermocouple wires. These wires then called Compensating Leads.
Advantage:
Following are the advantages of thermocouple:
- Thermocouples are cheaper than the RTD.
- It follows the temperature changes with small time lag and as such are suitable for recording comparatively rapid changes in temperature.
- Thermocouples are very convenient for measuring the temperature at one particular point in a piece of apparatus.
Disadvantage:
- They have a lower accuracy and hence cannot be used for precision work.
- To ensure long life of thermocouple in their operating environment, they should be protected in an open or closed-end metal protecting tube or well. To prevent contamination of thermocouple, when precious metals like platinum or its alloy are being used, the protecting tube/well has to be made chemically inert and vacuum tight.
- The thermocouple is placed remote from measuring device. Connections are thus made by means of wire called extension wires. Maximum accuracy is assured when compensating wires are of the same material as that of thermocouple wires. The circuitry is, thus, very complex.