Thermocouples are one of the simplest sensors used in the industries for the purpose of temperature measurement. A thermocouple is typically formed by connecting two wires of diverse metals (or alloys) at one end. Between the two wires, a small output voltage is produced at a given temperature.
The working of a thermocouple is based on a Seebeck’s principle which was recognized by Thomas Johann Seebeck in the year 1821. The Seebeck’s principle says that whenever conductor material experiences a temperature difference i.e. temperature gradient, it produces voltage which can be measured by making use of another conductor. The second conductor material exposed to the same temperature difference would also produce a voltage which would be different from the first one. The difference between the two voltages is calculated and correlated to the corresponding changes in temperature i.e. temperature gradient. Hence, it is evident that a thermocouple is designed in such a way that it can only measure temperature differences and require a known reference temperature for accurate measurement.
A typical thermocouple circuit for temperature measurement is shown in the figure below:
It basically consists of two junctions: Measurement junction and Reference junction. A reference junction is formed at the point where the two wires are joined together and coupled to the measuring device. In actuality, this reference junction is basically made up of two junctions: one junction for each of the two wires. However, these two are regarded as one single thermal junction since they are supposed to be at the same temperature.
In general, three major phenomena are implicated in the functioning of a thermocouple circuit. They are:
- Seebeck effect: “The Seebeck effect describes the voltage or electromotive force (EMF) induced by the temperature difference (gradient) along the wire. The change in material EMF with respect to a change in temperature is called the Seebeck coefficient or thermoelectric sensitivity. This coefficient is usually a nonlinear function of temperature.”1 A small voltage is produced by this effect which tends to increase with the increase in the temperature gradient.
- Peltier effect: This effect is just opposite of Seebeck effect and hence illustrates the temperature gradient generated by the voltage.
- Thomson effect: This effect generally explains the relationship between the temperature gradient and EMF and vice versa within a homogeneous conductor.
- By means of techniques like soldering, brazing and welding, a third metal enters into a thermocouple circuit. However, it causes no adverse effect on thermocouple’s performance and stability since both the ends are maintained at equal temperatures.
- To maintain the calibration stability of a thermocouple above 1000°C temperatures, insulation and sheath materials for thermocouple need to be selected carefully. Besides this, the thickness of wire plays an important role in deciding the stability of a thermocouple at high temperatures.
- Earlier, an ice bath was used to maintain the reference junction temperature at 0°C. However, nowadays the ice bath has been replaced by a reference junction compensation arrangement in which reference junction temperature is measured with the help of an alternate temperature sensor i.e. an RTD or a thermistor.
- Since thermocouples generate low level output signals, their performance can be easily affected by electric or radio signal pickup from adjacent areas. Hence, extra care must be taken to prevent electrical interference from sources such as motors, power cable, transformers etc. One can reduce electric and magnetic interferences by means of a shielded cable or metal conduit and using a twisted pair of thermocouple wires.
- Some thermocouples are extremely sensitive to oxidation and external atmospheres. Hence, they must be operated in safe and secure operating environment so that their performance doesn’t get affected or degraded.
- Noble metal thermocouples consisting of platinum rhodium i.e. B, R and S types are inappropriate for use under neutron radiation.
- Thermocouples can work over an extensive range of temperature i.e. varying from approximately absolute zero to their melting point.
- Several grades of thermocouple wires are available in the market. Main grades among them are measurement grades and extension grades. Measurement grades are basically employed where temperature differences are considerable, since they offer highest purity whereas extension grades are used where the thermocouple needs to be connected to the measuring device.
Three types of thermocouple junctions are:
- Grounded Junction: It is also known as earthed junction. This type of junction is usually welded to the protective sheath which causes excellent transfer of heat from outside. A grounded junction is mainly used for applications where corrosive media and high pressures are involved. Grounded junction type is better than the other two types, since it provides faster response than the ungrounded junction type and extra protection than the exposed junction type.
- Ungrounded Junction: It is also known as insulated junction since it provides electrical isolation by detaching itself from the probe wall. This junction type is primarily used in corrosive environments. MgO powder is often used to physically insulate the thermocouple wire from its sheath. This insulation prevents the occurrence of spurious signals in temperature measuring circuits containing more than one thermocouple.
- Exposed: This type of thermocouple junction extends beyond the protective metal sheath and hence gets exposed to the adjacent surroundings. Although an exposed junction results in best response time, yet its use is prohibited in corrosive and high pressurized areas. It is mainly used for applications involving non-corrosive media where high sensitivity and fast response is needed.
Thermocouple Response Time
A thermocouple response time or time constant is generally defined as the time needed by the output voltage to reach 63% of maximum for the step change in temperature within a given set of conditions. The response time of a thermocouple depends upon following factors:
- Size and dimensions of the thermocouple
- Design and configuration of the sensor
- Properties of the medium in which the thermocouple is positioned
Exposed junction thermocouples give rapid response. Besides, smaller the sheath diameter of probe, quicker will be the response. However, the maximum temperature in that case will be reduced.
Selection of Thermocouple
Following points must be considered while selecting a thermocouple for a particular application in an industry:
- Range of temperature
- Physical properties i.e. Chemical resistance
- Abrasion resistance
- Shock and Vibration resistance
- Installation requirements i.e. Compatibility with the existing equipment
Thermocouples are preferred over other temperature measurement methods owing to their unique physical properties. Following are the major advantages offered by thermocouples:
- Extremely strong and robust
- Shock and vibration resistant
- Offers wide temperature range
- Easy to manufacture
- No power supply required for excitation
- No self heating involved
- Available in small sizes also
- High degree of versatility and flexibility
Major drawbacks associated with the use of thermocouples are mentioned below:
- Thermocouples generate a quite low level output signal. Besides, the resulting output tends to be non-linear due to which a sensitive and stable device is needed for temperature measurement. The measuring device should be capable enough to provide reference junction compensation and linearization as well.
- Installation of a thermocouple also necessitates tremendous care so that the possible noise sources could be diminished.
- Moreover, the hardware employed for measurement should also offer excellent noise rejection capability.
- In case of non-isolated systems, ground loops can give troubles. In order to eradicate this problem, the common mode range and rejection should be reasonably adequate.