Temperature is the most important physical parameter for monitoring and controlling C & I in Industries. Accurate temperature measurement is vital in process industries such as Steel Industry, Cement Industry, Pharmaceutical Industry, Petrochemical Industry, Nuclear Industry, Power Industry, Laboratories and Furnace Industry.
The temperature sensor is a device that measures the temperature of air, water, steam, gases and converts an input signal into electrical or optical signals. These sensors are available in different forms, designs with various measuring technologies.
Common industrial temperature sensors are categorized into two types:-
1. Contact temperature sensor: Such a sensor requires direct physical contact with the detected object. It can use to determine solids, liquids, gases and vapour over a wide range of temperatures. It includes a thermocouple, RTD (resistance temperature detector), thermistors, etc.
2. Non-Contact Temperature Sensor: This type of sensor measures the temperature of the object surface without direct contact with the surface. It includes a pyrometer, thermal imagers and furnace monitoring system.
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Thermocouple | Definition, Working & Fact
- The thermocouple is a contact temperature sensing device.
- It operates under the principle that a circuit made by connecting two dissimilar metals produces a temperature-dependent voltage resulting from seeing back effect. This voltage can interpret in measuring temperature when a temperature gradient is imposed between both ends.
- The junction which kept into the process media to sense temperature is called the hot junction. The other junction, which is at the cold end of the thermocouple and almost always in some sort of measuring instrument, is called the cold junction.
- Thermocouples are available either as bare wire and MIMS (Mineral Insulated Metal Sheath), which offer low cost and response times is fast or built into probes. Customized Probes are available in a vast range and design, suitable for various measuring applications (industrial, scientific, food temperature, medical research etc.)
- A thermocouple works on the principle of seeing back effect or thermoelectric effect.
we observed that an electric current flows due to different no. Electron distributions in a closed circuit of two diverse metals if the two junctions are at distinct temperatures. The thermoelectric voltage generated depends on the metals used and on the temperature relationship between the junctions. Using the same metal type with temperature changes, the electron distribution due to heat will be the same temperature at the two junctions. The voltage produced at each junction cancels each other out, and no current flows in the circuit. When two dissimilar metals are used and two junctions are placed at a different temperature, different voltages are generated, and current flows in the circuit.
The measuring junction (often represent as the hot junction) is when two dissimilar metals of the thermocouple are welded together that which is exposed to be measured temperature. The reference junction, often known as the ‘cold’ junction, is the other connection that is kept at a constant temperature.
Thermocouple V/s RTD
- Operating principle
Thermocouples are pairs of dissimilar metal wires joining at one end, generating a net thermoelectric voltage in the middle of the open pair according to the temperature difference between the ends, whereas Resistance Temperature Detector uses a single metal. It is a temperature sensor that measures temperature. It operates on the measurement principle that a material’s electric resistance changes with temperature. Thermocouples are essential for measuring and maintaining a certain temperature in industrial processes that include producing any product.
- Temperature Range
The thermocouple is used to measure a wide range of temperature from -200°C to 2320°C whereas RTD generally operates in the range from -200°C to 850°C; therefore, temperature above 850, thermocouples are the only of choice contact temperature measurement devices.
- Response Time
To get a fast response time at step changes of temperature, a thermocouple is preferred as the best method.
- Power Supply
Compared to RTD, thermocouples are self-powered and require no external excitation because they produce a temperature-dependent voltage (EMF); thus, they are self-sufficient in their operation.
- Physical Size
Thermocouples are available in various sizes compared to other contact temperature sensors, starting from DIA 0.25mm to 12mm. A thermocouple can measure the temperature of a specific spot or small space..
- Cost
Thermocouples are less expensive and have better interchangeability in comparison with other temperature sensors.
- Resistance to vibration
Thermocouples resist shocks and vibration, whereas RTD is rather sensitive to vibration.
- Construction
Thermocouples are simple and robust and can withstand harsh environments compared to RTD. In thermocouple, no lead wire resistance problem occurs.
- Applications
Thermocouples are constructed with different metals like tungsten, rhenium, platinum, iron and rhodium. By using these high strength metals, thermocouples are used in a variety of demanding industrial applications such as Steel Industry, Cement Industry, Pharmaceutical Industry, Petrochemical Industry, Nuclear Industry, Power Industry, Laboratories and Furnace Industry etc. whereas for constructing RTDs, metals such as nickel, platinum and copper are used. RTDs are used to measure low temperature with high accuracy in applications such as Bearing, Turbine, Coal mill, Steam, Water, Air & Gases, Engine temperature and many other Industries. But to measure the temperature above 850°C in industrial, the thermocouple is used.
Comparison between thermocouple and RTD through table:-
| Basis for Comparison | Thermocouple | RTD |
| Temperature Range | -200°c to 2320°c | -200°c to 850°c |
| Response time | Medium to fast | Medium |
| Power Supply | Self Powered | Not Self Powered |
| Resistance to Vibration | Resist shocks and vibration | Sensitive to vibration |
| Construction | Simple and rugged | Have lead wire resistance problem |
| Physical Size | Small like 0.25mm | Large like 3.0mm |
| Self Heating | No self heating | self heating |
Conclusion
Thus, we conclude that the RTD and thermocouple have their applications in the respective fields according to the requirements of industrial temperature measurement. But, selecting the proper sensor type mainly depends on process criteria like size, accuracy, temperature range, self-healing, etc. Thermocouple has low cost, small size, durability, no abrasion and vibration resistance, higher operating temperature range, no self-heating, and quick response time, so thermocouples are more widely used in the industry to measure higher temperature.