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The evolution of technology mainly depends on the automation of tasks and reliability. One of the main components for automation is the sensor to collect information and take action accordingly. A sensor is a physical component that measures physical input from its surrounding environment and translates it into data that can be understood by either a machine or a human. Starting with a simple form of sensor like a glass thermometer, which shows visual data to end on electronic sensors that can convert data to electronic data, are the most common types of sensors.
The sensor as a device can convert stimuli such as heat, light, motion, and sound into electrical signals. These signals process through the electronic interface that converts them into a binary code and passes this on to a digital system to act.
Most industrial sensors organize into two groups: analog and digital. But there are a few types of sensors like temperature, Infra-Red, ultrasonics, pressure, proximity, and touch sensors that are frequently used in electronics applications. As per working classifications of the sensors, they divide into two groups: active and passive. Active sensors require an external excitation signal or a power signal. Passive sensors, on the other side, do not depend on any external power signal and directly produce output response.
The feeling of hot or cold is an important factor for human life. From ancient times, scientists work on different solutions to measure the temperature. Temperature is a measure in the form of energy in the material or body. The higher the energy, the hotter the surface is considered. The most basic way to measure the temperature is through the human body, we feel the ambient temperature, and base on that, we determine the weather is cold or hot.
In industry, the temperature is an essential factor to control the operation. Because of this importance, it requires measurement continuously, close monitoring, and control. Due to variation and disturbance in temperature, it may have a significant impact on product cost, end-product quality, equipment efficiency, and environmental safety.
The bimetallic temperature sensor works on the principle of the expansion. Two metal strips of different metal joined together to create a differential effect between them. Once the temperature changes its initiates the bending that can be used to measure the temperature in the form of a gauge or control the equipment in the form of thermostats. Both devices still operational and use in many applications.
A voltage is generated when there is a temperature difference between the two ends of a simple metal bar. More thermally charged electrons at the hot end begin to diffuse toward the cooler end. This redistribution of electrons creates a positive charge at the hotter end equal to a negative charge at the cooler end. This phenomenon is called the Seebeck effect, a type of thermoelectric effect which produces voltage in the open circuit.
A thermocouple works on the same principle. It consists of at least two metals joined together to form a junction. One metal is directly in contact with the surface whose temperature needs to be measured. This end is called the measuring junction. The other end of the junction is connected to the surface of the known temperature. This end is called a reference junction. At different temperatures, the thermoelectric current is initiated between them, which is directly proportional to the temperature difference.
Several factors must be considered when selecting the type of sensor to be used in a specific application:
Temperature range: Thermocouples have a wide range of temperature measurement starting from -200 to +2000 °C. Because of this, they are suitable for use in extreme temperature sensing applications.
Accuracy: For industrial applications accuracy is very important, so a thermocouple provides good accuracy as compared to thermistors. RTDs provide more accuracy than thermocouples but for a lower range of temperatures.
Response time: The response time for thermocouples is much higher than RTD due to the grounded effect. Also, thermocouples can be installed inside a smaller diameter sheath than RTDs. Due to these factors, the response time of a thermocouple is much higher.
Durability: thermocouples are very durable and often operate for long periods before they need to be serviced or replaced.
Power Consumption: RTD have higher power consumption then thermocouple. This feature extends its installation in industry.
Typical application in industry: RTD are commonly used with high precision and low temperature range application of gas and liquid flow. In contrast thermocouple are used in extreme conditions, such as those of a Cement kiln, Power Generator, industrial ovens, or high-capacity test equipment.
Classifications of Thermocouples
A thermocouple is a sensor that consists of two different types of metals, both of them joined together at one end. On heating or colling the junction, a voltage is created to correlate with the temperature. It’s manufactured, in different styles, such as thermocouple probes, probes with connectors, infrared thermocouple, bare wire thermocouple, or even just thermocouple wire.
For industrial use thermocouple, are available in different combination of metal or calibrations standard. Among them, the most common are base metal thermocouple know as type J, K, E, T, and N. For high-temperature calibration noble metal thermocouple type is used, and it includes type R, C, S, and GB. Each type has a different range of working conditions. In addition to the calibration standard, the maximum temperature varies with the diameter of the wire in use also. A thin wire will not support the full range of temperature range.
Depending on the required temperature range, vibration resistance, chemical resistance, response time, installation and equipment requirements, the end user can choose the appropriate thermocouple.
Type K Thermocouple: The most common type of thermocouple is K type it consists of Nickel-Chromium / Nickel-Alumel metal as a junction. The major benefits of K Type thermocouple are it’s inexpensive, accurate, reliable, and has a wide temperature range from -270 to 1260C. The extension wire is 0 to 200C. Standard Accuracy is +/- 2.2C or +/- .75%.
Type J Thermocouple: This type is also common. It consists of Iron and Constantan. Its temperature range is smaller from -210 to 760 C. The extension wire is 0 to 200 C. As compare to K type, its life span is smaller at higher temperatures. In terms of cost and reliability, it’s equivalent to K type. Standard accuracy is Standard: +/- 2.2C or +/- .75%.
Type T Thermocouple: This type is very stable. It consists of Copper and Constantan. Its temperature range is extremely low from -270 to 370 C. The extension wire is 0 to 200 C. Its use for application where low temperature is required such as ultra-low freezer and cryogenics. Standard accuracy is +/- 1.0C or +/- .75%.
Type E Thermocouple: This type has very strong signal strength and accuracy as compare to type J and K. It consists of Nickel and Constantan. Its temperature range is moderate from -270 to 870 C. The extension wire is 0 to 200 C. Standard accuracy is +/- 1.7C or +/- 0.5%.
N Thermocouple: This type has strong accuracy as K type. It
consists of Nicrosil and Nisil. The type N is slightly more expensive. Its temperature
range is moderate from -454 to 392 C. The extension wire is 0 to 200 C. Standard
accuracy is +/- 2.2C or +/- .75%.
Compare to the base metal, a noble metal thermocouple is more expensive. They can work at extremely high temperatures and with more accuracy.
Type S Thermocouple: This type installs in very high-temperature applications. It consists of Platinum Rhodium and Platinum. The type S use in the Biotech and Pharmaceutical industries. It’s another industrial application is a lower temperature with high accuracy and stability. Its temperature range is moderate from -50 to 1480 C. The extension wire is 0 to 200 C. Standard accuracy is +/- 1.5C or +/- .25%.
Type R Thermocouple: This type is used in very high-temperature applications and more expensive than S type due to a higher percentage of Rhodium. It consists of Platinum Rhodium and 13% Platinum. The Type R is similar to S in performance. It’s used in lower temperature with high accuracy and stability. Its temperature range is moderate from -50 to 1480 C. The extension wire is 0 to 200 C. Standard accuracy is +/- 1.5C or +/- .25%.
Type B Thermocouple: This type of sensor uses for extremely high-temperature applications. In a high-temperature environment, it will maintain accuracy and stability. It consists of Platinum Rhodium 30 and 6% Platinum Rhodium. Its temperature range is moderate from 0 to 1700 C. The extension wire is 0 to 100 C. Standard accuracy is +/- 0.5%.
Field Mounting of Thermocouple
Sheathed thermocouple contains an outer metallic sheath, which consists of the insulated leads embedded inside a high-density ceramic mixture. The Sheathed thermocouple probes are available with different junction types such as grounded, ungrounded, or exposed.
Ground Thermocouple: It’s the most common junction type. When both the sheath and wires of the thermocouple welded to form one junction, it becomes grounded. With this configuration, the response time is good due to direct contact with the sheath. It makes the transfer of heat effectively. The main drawback of this approach is the thermocouple more susceptible to the electrical signal influence. It happens because the sheath often comes into contact with the adjacent area, allowing interference.
Unground Thermocouple: When both the wires of the thermocouple are welded to form one junction but insulated from the sheath. The main benefit of this configuration is it reduces the risk of ground loops.
Exposed Thermocouple: When both the wires of the thermocouple weld to form one junction and directly inserted into the process. The main benefit of this configuration is response time is quick. The drawback of this configuration exposure to corrosion and degradation. This configuration is not recommended for normal operation.
Signal Termination with Control system
Thermocouple after field termination needs to connect with the control system for process monitoring and control. On the Control system side, for the termination of the sensor, two options available. One is to go directly with the Temperature input module. The other option is to use a temperature transmitter to convert the signal to 4 to 20 mA DC and terminate it on an analog module. Both options adopted in the industry based on the requirements like with direct connection, you do not need to maintain the intermediate device to convert the signal and provide flexibility even in case of any problem. In contrast, the thermocouple sensor output is low mV and, due to noise factors maybe get distorted. So, it’s better to convert the signal to 4-20 mA and use two wires for PLC analog input module termination. In an industrial environment, thermocouples are exposed to harsh environmental conditions. Therefore, often in industry 1 megaohm transistor is installed to detect the break or increase in the resistance of the wire.
Smart sensors are the future of industrial 4.0. In that regard, a long-range two-channel industrial IoT wireless thermocouple sensors have the option to measure the high and low temperature accordingly. These K types of wireless sensors are ideal for industrial boiler and food storage units. It comes with a long-life battery and, the data transmit up to two miles in line of sight.
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