Thermocouples are used for industrial temperature measurement in many industries. They come in many different types. This article focuses on K-type thermocouples. We hope it will be helpful.
What is a K-type Thermocouple?
A K-type thermocouple is a commonly used temperature sensor. It is typically used with display instruments, recording instruments, and electronic controllers. A K-type thermocouple typically consists of a temperature-sensing element, a mounting fixture, and a junction box.
K-type thermocouples can directly measure the surface temperature of liquids, vapors, and gases, as well as solids, in various production processes. Its measurement ranges from -400°C to 1300°C. The two primary metals used in K-type thermocouples are nickel-chromium alloy (NiCr) and nickel-aluminum alloy (NiAl).
K-type thermocouples are currently the most widely used, low-cost metal thermocouples. Their usage is comparable to all other thermocouple types combined. The nominal chemical composition of the positive electrode (KP) is Ni: Cr = 90:10. And the nominal chemical composition of the negative electrode (KN) is Ni: Si = 97:3.
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Advantages:
- High accuracy
- Good linearity
- Large thermoelectric potential
- High sensitivity
- Wide measuring range
- Strong oxidation resistance
- Affordable price
- Can be used in oxidizing inert atmospheres
Disadvantages:
K-type thermocouples cannot be used directly at high temperatures in sulfur, reducing, or alternating reducing and oxidizing atmospheres, or in a vacuum environment. They are also not recommended for use in weakly oxidizing atmospheres.
K-Type Thermocouple Working Principle:
Two conductors of different compositions form a closed circuit. When a temperature gradient exists between the two ends, current flows through the circuit. This generates an electromotive force (electromotive force) between the two ends, known as the Seebeck effect. The two homogeneous conductors of different compositions are called thermocouples. The end with the higher temperature is the working end. The end with the lower temperature is the free end. The free end is usually kept at a constant temperature.
Based on the functional relationship between thermoelectromotive force and temperature, a Thermocouple indexing table is constructed. This scale is obtained when the free end temperature is 0°C, and different thermocouples have different scales. When a third metal is connected to the thermocouple circuit, as long as the temperatures of both junctions of that material are the same, the thermoelectromotive force generated by the thermocouple remains unchanged, unaffected by the presence of the third metal.
Therefore, when measuring temperature with a thermocouple, a measuring instrument can be connected to obtain the temperature of the measured medium by measuring the thermoelectromotive force.
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K-Type Thermocouple Industrial Applications
Based on our years of experience, we have summarized the application areas of thermocouples.
K-type thermocouples are used in petrochemicals, steelmaking, and ceramics production. They are often used to measure key parameters. such as furnace and pipeline temperatures. These industries require high accuracy and stability in temperature measurement. And K-type thermocouples precisely meet these requirements. In the steelmaking industry, K-type thermocouples are used to measure the temperature of equipment. such as blast furnaces and converters, providing important data for production process control.
K-type thermocouples are also used in laboratories. They are commonly used to measure the temperature of equipment. such as high-temperature furnaces and electric heaters. These devices are often used in experimental research in fields. such as materials science and chemical engineering. K-type thermocouples enable researchers to obtain accurate temperature data. They provide strong support for the accuracy and reliability of experimental results.
K-type thermocouples are also used in industries. such as food processing, plastics processing, and aerospace. In the aerospace industry, K-type thermocouples can be used to measure the temperature of high-temperature components. such as engines and combustion chambers. They provide a critical assurance for aircraft safety.
In the food processing industry, K-type thermocouples can be used to monitor the temperature of baking, cooling, and heating processes.
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K-Type Thermocouple Compensating Conductor Usage Characteristics
Thermocouple compensation wire is a type of wire used to connect thermocouples to measuring instruments. By extending the cold junction of the thermocouple, its thermoelectromotive force (EMF) within a certain temperature range (typically 0-200°C) is equal to the nominal value of the thermocouple. Thereby, it compensates for measurement errors caused by temperature variations at the junction.
Compensation wire is based on the thermocouple intermediate temperature law. And it uses a compensating alloy wire and insulation material (such as fluoroplastic or polyvinyl chloride) that matches the thermocouple. Compensating conductors can reduce temperature and potential interference and improve signal transmission stability.
1. Maximum Conductor Operating Temperature:
Heat-resistant Grade: Polyethylene propylene (F46) 200°C. Soluble polytetrafluoroethylene 260°C. Silicone rubber 180°C
Ordinary Grade: Polyvinyl chloride 70°C and 105°C. Cross-linked polyethylene 90°C. Low-smoke halogen-free flame-retardant polyolefin 70°C. Cross-linked at 90°C and 125°C.
2. Low Ambient Temperature:
PVC Sheath: -40°C for fixed installation, -15°C for non-fixed installation.
Plastic Insulation and Sheath: -60°C for fixed installation. -20°C for non-fixed installation.
3. Allowable Cable Bend Radius:
Minimum 6 times the cable’s outer diameter for unarmored cables. Minimum 12 times the cable’s outer diameter for copper-tape shielded or steel-tape armored cables. Minimum 10 times the cable’s outer diameter for plastic insulation and sheath.
K-type thermocouple extension wire is the most commonly used thermocouple. Copper-copper-nickel 40 extension wire is almost always used to connect it to regulating and display instruments. This type of extension wire is inexpensive.
K-Type Thermocouple Wiring:
Correct wiring of K-type thermocouples is crucial. Maintaining proper polarity is crucial. The terminal block will have “+” and “-“ markings. But it is not all manufacturers do, so technicians must rely on experience to determine the proper polarity. Correct wiring directly affects the accuracy of subsequent temperature data. K-type thermocouples can be distinguished by magnets.
Universal terminal blocks cannot be used for thermocouple wiring. This is because the current guide bars within the terminal blocks are typically made of a specialized copper material. such as electrolytic copper, which is different from the copper used in the thermocouple. Unlike conventional combination terminal blocks, which are supplied individually, thermocouple terminal blocks are supplied in pairs. The current guide bars in each pair of thermocouple terminals are made of different metals.
How to Choose a K-Type Thermocouple?
Selecting the right K-type thermocouple requires careful consideration of several factors. We typically consider the following:
- Temperature range
- Ambient conditions
- Protection tube material
- Probe diameter and length
- Mounting method
- Connection method
- Accuracy requirements
- Budget and manufacturer
Careful evaluation of these factors ensures that the thermocouple you choose meets your specific application needs. And a right thermocouple provides accurate temperature measurements. If you have questions about selecting a K-type thermocouple, you can contact Sino-Inst’s technical staff for a free custom measurement solution.
What is the difference between a K-type and a J-type thermocouple?
K-type thermocouples are made of nickel-chromium (NiCr) and nickel-aluminum (NiAl) alloys. J-type thermocouples, on the other hand, are made of iron (Fe) and copper-nickel (CuNi).
K-type thermocouples are typically used for temperature measurements between -200°C and 1372°C.
J-type thermocouples have a temperature range of -40°C to 760°C.
Generally speaking, K-type thermocouples offer higher accuracy than J-type thermocouples.
What is the temperature limit for a Type K thermocouple?
The maximum temperature measurement range for a Type K thermocouple is approximately 1300°C. If a higher temperature range is required, other types of thermocouples can be used.
How to test a Type K thermocouple with a multimeter?
1. Before testing with a multimeter, visually inspect the protective tube for corrosion and leaks. Then, use the multimeter to measure continuity. The resistance of an assembled thermocouple is generally no greater than 2 ohms. The resistance of a wired thermocouple is generally no greater than 50 ohms. A value greater than 1K indicates a fault.
2. Use a multimeter to measure the resistance. Any resistance greater than 100K indicates a fault.
3. Use the multimeter's ohm measurement method to test the thermocouple. Adjust the resistance, connect the two terminals, and lightly heat them with a lighter. If the multimeter needle noticeably increases or decreases, it indicates a good condition. If the needle does not move, it indicates a fault. You can also use the multimeter's millivolt range to measure the voltage across the terminals. If there is no voltage, it indicates a fault.
All in all, the K-type thermocouple is a commonly used temperature sensor in industry. This post has provided some information about K-type thermocouples. And I believe you’ve gained a general understanding of them.
If you have any other technical questions, please feel free to contact us. Our technical engineers are available 24/7 to provide technical support.
