Accurate temperature control in industrial processes not only improves product quality but also ensures workplace safety and promotes increased production efficiency. To ensure accurate temperature control, the right thermocouple is essential. There are many types of thermocouples used in industrial heating processes. The two most common types are J-type and K-type. In general, J-type and K-type thermocouples are similar. However, K-type thermocouples have a wider temperature range and are more versatile. J-type thermocouples are also more stable.
This post will delve into the key differences between the two types in terms of measurement range, common application environments, varying accuracy, and different materials, among other factors. Let’s examine the differences between these two most commonly used thermocouple wires.
What is a K-Type Thermocouple?
The K-Type thermocouple is a base metal thermocouple with strong oxidation resistance. It is suitable for continuous use in oxidizing and inert gases. It has a short-term operating temperature of 1200°C and a long-term operating temperature of 1000°C. Its thermoelectric potential has a nearly linear relationship with temperature. K-Types thermocouple is the most widely used thermocouple.
However, it is not suitable for use with bare wire in vacuum, sulfur-containing, carbon-containing atmospheres, or atmospheres undergoing redox cycles. When the oxygen partial pressure is low, the chromium in the nickel-chromium electrode will preferentially oxidize, which causes a significant change in the thermoelectric potential, but the metal gas has little effect on this. So, it is often used with a metal protective tube.
Features
- Wide temperature range;
- High cost-effectiveness;
- Good linearity;
- Environmental adaptability.
What is a J-Type Thermocouple?
The iron-copper-nickel thermocouple (J-Type thermocouple), also known as the iron-constantan thermocouple. It is another inexpensive, low-cost metal thermocouple. The positive electrode (JP) of the J-Type thermocouple has a nominal chemical composition of pure iron. The negative electrode (JN) is a copper-nickel alloy. Its nominal chemical composition is 55% copper and 45% nickel, with small but significant amounts of manganese, cobalt, and iron. Although the J-Type thermocouple is called Constantan, it is different from the Constantan of nickel-chromium-Constantan and copper-Constantan. And it cannot be replaced with EN or TN.
Features:
- Good linearity;
- High thermoelectromotive force;
- High sensitivity;
- Good stability and uniformity;
- Affordable price.
Their Different Materials
The K-type thermocouple (nickel-chromium-nickel-aluminum/nickel-silicon) uses a positive electrode (KP) (chromel, containing 90% nickel and 10% chromium) and a negative electrode (KN) (alumel or nisil, containing 95% nickel and an aluminum/silicon/manganese alloy).
This material combination ensures linear output over the entire temperature range of -200°C to +1260°C. And a thermoelectromotive force of approximately 41 μV/°C, significantly higher than the J-type’s 51 μV/°C.
Notably, there are two variations of the KN electrode material: traditional alumel (containing aluminum and manganese) and improved nisil (containing silicon). The latter offers approximately 20% better high-temperature stability, but also comes at a 15% higher cost.
The J-type thermocouple (iron-constantan) consists of a positive electrode (JP) (pure iron) and a negative electrode (JN) (constantan, 55% copper and 45% nickel). The introduction of an iron electrode provides exceptional sensitivity in the 0-760°C range. However, iron accelerates oxidation above 538°C, directly limiting its high-temperature performance. Experimental data show that the thermoelectromotive force of the J-type thermocouple is 10.78 mV at 200°C, while that of the K-type is only 8.13 mV. This difference is even more pronounced at lower temperatures.
Their Different Applications
The J-type thermocouple is suitable for the following environments:
- Suitable for use in vacuum, oxidizing, reducing, and inert atmospheres;
- Excellent stability in hydrogen and carbon monoxide atmospheres;
- Not suitable for use in sulfur-containing environments or exposed to high-temperature oxidizing conditions.
The K-type thermocouple is suitable for the following environments:
- Furnace and heat treatment equipment temperature monitoring;
- Petrochemical industry pipeline temperature monitoring;
- Plastic extruder temperature control system monitoring.
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Different Measurement Range and Accuracy
The K-type thermocouple maintains an accuracy of ±2.2°C or ±0.75% (whichever is greater) over the standard range of -200°C to +1260°C, with a maximum short-term operating temperature of 1372°C.
The operating range of the J-type thermocouple is strictly limited to 0-760°C. Beyond this range, the error increases exponentially. A US NIST test report indicates that after 200 hours of continuous operation at 600°C. The drift of the J-type thermocouple is more than three times that of the K-type.
The K-type exhibits unique advantages in the cryogenic field. Even in a liquid nitrogen environment (-196°C), the K-type maintains an accuracy of ±5°C. However, the J-type exhibits nonlinear output below -100°C. The K-type is widely used in the aerospace industry to monitor cryogenic fuel storage precisely because of this stability.
Different Color
Thermocouples use the color of the insulation to determine the calibration of the thermocouple wire. However, this method only applies to thermocouple wire and does not apply to thermocouple extension wire.
- One red and one yellow wire is the K-type calibration.
- One red and one white wire is the J-type calibration.
How to Choose a Suitable Thermocouple?
Thermocouples are generally selected based on the operating temperature, with different calibrations selected. For temperatures between 1300°C and 1600°C, B-type thermocouples are generally used when high accuracy is required. For temperatures between 1100°C and 1300°C, S-type and R-type thermocouples are generally used. For temperatures between 400°C and 1100°C, K-type and N-type thermocouples are generally used. For temperatures within ±400°C, E-type and J-type thermocouples are generally used. For temperatures below 300°C and for measurements at negative temperatures, T-type thermocouples are generally used. At low temperatures, T-type thermocouples offer greater stability than other thermocouples. Sino-Inst has many years of experience in temperature measurement and has summarized the following aspects to help you choose the most suitable thermocouple.
When selecting a thermocouple, we should consider several other factors:
- Measured medium name, corrosiveness, temperature, pressure, etc.;
- Measurement element calibration: K-type, E-type, T-type, S-type, etc.;
- Quantity of measuring elements (single or double);
- Insertion depth for installation in pipes, tanks, or other containers;
- Sensor diameter;
- Connection method and dimensions to the pipe or container;
- Wiring method: junction box wiring or lead-out wiring;
- Output signal requirements: thermocouple mV or current.
What is the difference between K-type and E-type thermocouples?
K-type thermocouples are made of nickel-chromium alloy (NiCr) and nickel-aluminum alloy (NiAl). The positive electrode (measuring terminal) is a nickel-chromium alloy, and the negative electrode (reference terminal) is a nickel-aluminum alloy.
K-type thermocouples have a higher thermoelectric potential and a wider measurement range. It is suitable for temperatures from -200°C to 1260°C. Type E thermocouples are composed of nickel-chromium alloy (NiCr) and copper-nickel alloy (CuNi). The positive electrode (measuring end) is a nickel-chromium alloy, and the negative electrode (reference end) is a copper-nickel alloy.
Type E thermocouples have a lower thermoelectric potential and a narrower measurement range. It is suitable for temperatures between -200°C and 900°C.
Which type of thermocouple is most accurate?
Based on Sino-Inst's temperature measurement experience, the most accurate thermocouple is the type S (platinum-rhodium-10-platinum) thermocouple. The S-type thermocouple achieves the highest accuracy in normal, long-term operating temperatures of approximately 800°C to 1300°C. Temperature measurement accuracy is low within 800°C, and long-term use above 1300°C can easily damage the thermocouple.
Therefore, selecting the correct thermocouple calibration is crucial for ensuring accurate temperature measurements. Thermocouple accuracy varies under different temperature conditions. Different measurement temperatures result in different accuracy.
Why use a J-type thermocouple?
J-type thermocouples can be used in vacuum, oxidizing, reducing, and inert atmospheres. However, the positive iron oxidizes rapidly at high temperatures, limiting their operation temperature. They cannot be used directly in sulfiding atmospheres at high temperatures without protection.
J-type thermocouples are relatively inexpensive and exhibit good stability within these specific temperature ranges. They provide accurate temperature data. And they are simple and easy to use.
Which wire is positive on a J-type thermocouple?
The positive wire of a J-type thermocouple is iron (JP). The negative wire is copper-nickel alloy (JN). The positive and negative wires can be distinguished by color: the positive wire is typically green, and the negative wire is gray. Furthermore, the positive wire is attracted to magnets, while the negative wire is not.
How to Check a J-Type Thermocouple?
To determine the quality of a J-type thermocouple, you need to follow these steps:
1) Inspect the exterior for any abnormal changes or damage;
2) Perform an insulation test to ensure there are no shorts or breaks between the two metals;
3) Verify the thermocouple's sensitivity and tolerance using a standard temperature source;
4) Compare the deviation between the actual measured value and the theoretical value against the measurement range specified in the instruction manual to determine if the thermocouple is defective or needs adjustment.
In short, different types of thermocouples have different characteristics. I believe you have a general understanding of K-type and J-type thermocouples after reading this. If you have any further technical questions, please feel free to contact us.
Sino-Inst is a thermocouple supplier from China. If you need to purchase other temperature sensor products, please feel free to contact us. We are your best partner for temperature measurement.
