Temperature is essential in any process, so it's crucial to accurately measure it. There are various types of sensors for temperature measurement, with the thermocouple being one of them.
Thermocouple definition(Thermocouple sensor)
The thermocouple is a sensor that is used to measure the temperature of any system. Two wire legs made of various metals make up a thermocouple. The wire's legs are joined by welding at one end, forming a junction. The temperature is gauged at this intersection. A voltage is produced when the junction's temperature changes. The temperature can then be calculated from the voltage.
The thermocouple extension wire must comply to IEC-60584 or ISA/ANSI MC96.1 specifications. In order to avoid noise interference, thermocouple signals, which are low-level signals, should be appropriately shielded and grounded. Insulated hot junctions must have a minimum insulation resistance of 50 megohms at room temperature and a minimum resistance of 2 megohms at 4508C (8508F). The terminal wire must have the necessary markings, such as "+," "-," and "ground." The several circuits in the terminal heads must be easily recognized. The technician must verify that the lead wires' polarity and continuity are the same throughout all of the junction boxes from the thermocouple head to the control center,
Measurement Principle of thermocouple
The circuit shown in the figure is the basis of thermocouple theory. If two dissimilar metals, are linked at both ends(A & B) and have two distinct temperatures T1 and T2, a current will flow through this close loop. When in use, connection B would go to a voltage-measuring device, and connection A would be the measuring junction. The sensor output is an electromotive force, which is a very tiny voltage.
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| Thermocouple basic principle |
Types of thermocouples.
Thermocouples come in a variety of forms, each with special qualities that make it ideal for a particular application in terms of temperature range, durability, vibration resistance, chemical resistance, and application compatibility.The most popular types of thermocouples are "Base Metal" thermocouples, which are Type J, K, T, and E.
Noble metal thermocouples of types R, S, and B are employed in high-temperature applications.
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| Reference- okazaki |
K-type thermocouple
Type K thermocouples are the most commonly used temperature sensors today mainly because it is reliable, inexpensive, and have the broadest operating temperature range. It is made up of two legs: a magnetic negative leg and a nonmagnetic positive leg. K-type thermocouples use typical base metals, allowing them to perform at high temperatures and provide the largest operating temperature range.
One of the key advantages of the K type thermocouple over other thermocouples is that it can perform in harsh climatic conditions and diverse atmospheres.
It has an integrated composition of Chromel and Alumel wires, a temperature range of -270 °C to 1260°C, and an output of -6.4 to 9 mV over the maximum temperature range.
Alumel
Alumel is an alloy made up of around 95% nickel, 2% aluminum, 2% manganese, and 1% silicon. It is a magnetic alloy used in the negative conductors of ANSI Type K thermocouples and thermocouple extension wires.
Chromel
Chromel is an alloy composed of roughly 90% nickel and 10% chromium by weight that is used to create the positive conductors of ANSI Type E (chromel-constantan) and K (chromel-alumel) thermocouples.
How to identify thermocouple type?
Thermocouples come in a variety of forms, each having special qualities of its own. The colors of the wire insulation of a thermocouple's wire can be used to quickly identify its type.
- In ANSI color coding, the red wire of a thermocouple is negative and the positive wire is color coded to the type of thermocouple.
- A type K thermocouple has one yellow color for the positive wire.
- Type J thermocouple has one red colored wire and one white wire.
- A type T thermocouple has one red wire and one blue wire. Type T thermocouples have a useful temperature range of negative 200 to 350 degrees Celsius.
- A type N thermocouple has one red wire and one orange wire.
- Finally, a type E thermocouple has one red wire and one purple wire.
How to test thermocouple?
We can use a digital multimeter for testing and checking a thermocouple. Since it is a voltage generator, by checking the voltage across the thermocouple's two legs we can test this temperature-measuring device.
How to calibrate a thermocouple?
Actually, there is no thermocouple calibration, but the thermocouple transducer calibration must be done.
What is a thermocouple used for?
It is a temperature-measuring device and the most applicable device as a temperature sensor in industrial as well as residential areas.
