Hello, readers welcome to the new post. Here we will learn about the Introduction to Thermocouples. The thermocouple is an electrical instrument that has two different conductors to make an electric junction. Its operation is based on the seeback effect and make voltage used to find the temperature value. It works as a temperature sensor. Thermocouples are less costly and used for measuring the different values of temperatures.
Its best feature is that then other temperature measuring devices it not need any power source to get start operation. But has some disadvantages that can not measure the value of temperature for some certain centigrade. In this post, we will discuss all the details parameters of this temperature sensor. So let’s get started.
Introduction to Thermocouples
- The thermocouple is sensor used for measuring the temperature. It has two different metallic wires to make a junction-like configuration.
- If these two metallic wires get heated and cooled voltage is formed and this voltage give a value of temperatures changed.
- It is a very simple device and less expensive and mostly used for temperature measuring.
- Different types of structures it has such as thermocouple probes, probes with connectors infrared thermocouples.
- There are different types of thermocouples on basis of construction thermocouples created with copper called T thermocouples.
- A thermocouple made with Iron is called J thermocouple and another type that is made with alumel and chrome is called K thermocouple that is commonly used.
- If more then one thermocouples are created with use of different materials called a thermopile.
Thermocouple Working Principle
- The working principle of thermocouples is based on the seedback effect. According to seeback effect when 2 different metallic wires join to make a junction voltage is induced in the close circuit that voltage is directly proportional to temperature.
- In the below figure, you can see a circuit of thermocouples that measure temperature. Two different metallic wires connect at 2 junctions denoted as X and Y.
- X is the measuring junction and Y is the reference point. In this circuit there is PMMC module is connect
- If these two junctions have different values of temperature cold junction has zero centigrade and value of measuring junction is not known that has to measure.
- Volts or EMF will be generated due to the different temperature values
- EMF generated has values in millivolts. For measuring the EMF PMMC instrument connected in the circuit is used
- If both junctions have the same value of temperature EMF produced will be the same. So zero current will flow and no movement in the meter
- If the temperature at junctions is not same current will pass in the meter and deflection can seen in meter.
- EMF generated is in direct relation to the temperature difference and current is also in direct relation to temperature difference. Hence meter can measure temperature directly
Advantages of Thermocouple
- It is used for temperature measuring applications since it carries the variation in temperature through a small time lag.
- It is the very finest option for measuring the temperature at a single point.
Disadvantages of Thermocouple
- Its accuracy is very low and not used for high-value measurements.
- It must avoid contamination to work for a larger time interval.
- They have to be placed at a larger distance from the measuring point that decreases the faults in errors.
Thermocouple Construction
- Thermocouples are made with the use of two different metallic materials with different thermoelectric features. They are connected at one endpoint.
- The other two ends of the metals are open. The connected points are called hot junctions, and separated ends or open ends are cold or reference junctions.
- Metals are connected through welding, twisting with each other.
- The diameter of the metal is used to define the temperature range and response time of instruments. For high temperatures, larger dia metals are used, and it helps to minimize the response time. Before designing, these factors are considered.
Thermocouple Types
Type K
- A Type K thermocouple is a low-cost thermocouple that comes with different temperature measuring values. It has high accuracy and low cost. Its other name is the chrome-aluminum thermocouple.
- Its positive lead is made with the use of chrome, which is a mixture of 90% nickel and 10% chromium.
- The negative lead is created with aluminum, which is a mixture of 95% nickel and 2% magnesium. 2% aluminum, and 1% silicon.
- Positive and negative leads are of the different thermocouples.
- Type K thermocouple temperature measuring range is -270 °C to 1260 °C and provides an accuracy of ±2.2 °C.
Type T
- Type T thermocouple used for measuring low-temperature values. It is best for use in oxidizing conditions.
- Its opposite pin is made with copper, and the negative is made with Constantan, which is a combination of 55% copper and 45% nickel.
- Its temperature measuring range is -270 °C to 370 °C, and accuracy is ±1.0 °C.
Type J
- Type J thermocouple is also a common type used and best for use in oxidizing conditions with high sensitivity. Its positive lead is made with iron, and its negative lead is made with Constantan.
- Its temperature measuring range is -210 °C to 760 °C and provides an accuracy of ±2.2 °C with sensitivity of 50 μV/˚C.
Type S
- This type of thermocouple is used for high-temperature applications. It is also used for low-temperature conditions since it has high accuracy. Its structure is made with rare metals that make it high cost.
- Its positive pin is made with 90% platinum and 10% rhodium. The negative pin is made with the use of pure platinum. Its temperature measuring range is 50 °C to 1480 °C with an accuracy of ±1.5 °C.
Type E
- This type of thermocouple is sensitive and provides high accuracy and good signal strength. It is not commonly used like other types and has non-magnetic features.
- Its positive lead is made with chrome, which is a combination of 90% nickel and 10% chromium. The negative lead is made of Constantan. Its temperature measuring range is -270 °C to 870 °C.
Type | Positive Lead Material | Negative Lead Material | Temperature Range | Accuracy | Positive Lead Color |
Negative Lead Color
|
Type K | Chrome | Alumel | -270°C to 1260°C | ±2.2 °C | Yellow | Red |
Type T | Copper | Constantan | -270 °C to 370 °C | ±1.0 °C | Blue | Red |
Type J | Iron | Constantan | -210°C to 760°C | ±2.2 °C | White | Red |
Type S | Platinum 90%, Rhodium 10% | Platinum | -50 °C to 1480 °C | ±1.5 °C | Black | Red |
Type E | Chromel | Constantan | -270 °C to 870 °C | ±1.7 °C | Purple | Red |
Type N | Nicrosil | Nisil | -270°C to 1260°C | ±2.2 °C | Orange | Red |
Type R | Platinum 87%, Rhodium 13% | Platinum | -50 °C to 1480 °C | ±1.5 °C | Black | Red |
Type B | Platinum 70%, Rhodium 30% | Platinum 94%, Rhodium 6% | 0°C to 1700°C | ±0.5 % | — | — |
Thermocouples vs. RTDs
Temperature Range
- Thermocouples are best to work at high temperatures, and some new RTDs have about 90 percent of RTDs. Thermocouples are used for measuring about 25000 oC.
Cost
- Thermocouples lost cost more than RTD, and RTD prices are 2 to 3 times more than thermocouples having the same temperature range.
Sensitivity
- Thermocouples are high-speed working, and the grounded, fast-working thermocouple responded about 3 times Pr100 RTD.
Accuracy
- RTD provides more accurate results than thermocouples, and RTD accuracy is 0.1 C, compared to 1 C for most thermocouples. Some thermocouples have about RTD accuracy. Factors that affect sensor accuracy are linearity, repeatability, or stability.
Linearity
- The relation of temperature resistance for the RTD sensor is linear for sensor ranges, and the thermocouple comes with an ‘S’-type plot.
Stability
- RTD readings are stable for a long time, and thermocouple readings drift since of chemical changes of sensors.
FAQS
What is the difference between Type N and K thermocouples?
- Thermocouple N is a new type that was created with the use of Type K. It comes with high stability with time and high cost as compared to K for the same temperature ranges. It provides a sensitive value of 39 µV/°C, less compared to type K.
Which is better, RTD or Type K thermocouple?
- Thermocouples come with a larger temperature range than RTD sensors. If there is a need for high-temperature conditions, the thermocouple is best to use. RTD is used for moderate temperature conditions and has reliable results.
What are the physical properties of a thermocouple?
- The thermocouple is best to use for measuring high temperatures in remote locations. Its physical features are temperature differences of metals and potential differences that are generated.
What are the properties of a thermocouple sensor?
- It is best to use for cryogenic conditions and sensitivity and temperature range based on the iron quantity used. The sensitivity is about 15 μV/K for low temperatures, and the low temperature values are 1.2 and 4.2 K.
That is all about the Introduction to Thermocouples all details have been explained. If you have any query ask in the comments. Thanks for reading have a good day see you in next post.