Temperature measurement is a scientific process, and you may be surprised to learn that there are several different types of thermocouples in use today. The type of thermocouple we are talking about today—the K type thermocouple—is the most popular type of thermocouple because it is the most versatile.
According to Process Parameters, the K type thermocouple can be used for temperature measurement in harsh environments and has a long history of being used in both laboratory and industrial applications. Thermocouple type K can measure a wide range of temperatures making it suitable for industrial temperature measurement. It is also accurate and reliable.
How K Type Thermocouple Works
The working principle of type K thermocouple is similar to that of other thermocouple types. The type K thermocouple contains chromel and alumel conductors that make a junction at one end. It is this junction that is immersed or placed on an object to measure temperature.
A small DC voltage is created when the junction end senses a temperature change. The voltage produced varies linearly with the temperature.
Technical Information about Type K Thermocouples
Materials
The chromel conductor, the positive leg, consists of approximately 90% Nickel and 10% chromium. Alumel conductor, which is the negative leg, comprises roughly 95% nickel, 2% manganese, 2% aluminium and 1% silicon.
The positive leg is non-magnetic, while the negative leg is magnetic.
Temperature Range
Type K thermocouples can measure temperatures of between -200°C and 1260°C. However, above 800°C, drift and decalibration occur due to oxidation.
Accuracy
The standard accuracy of this type of thermocouple is +/- 2.2C or +/- 0.75%. The accuracy for special limits of error is +/- 1.1C or 0.4%.
Tolerance
Tolerance Class 1
± 1.5°C for temperatures between -40 and 375°C.
± 0.004×(t) °C or ± 0.4% (whichever is greater) for temperatures between 375 and 1000°C.
Tolerance Class 2
± 2.5°C for temperatures between -40 and 333°C
± 0.0075×(t) °C or ± 0.75% (whichever is greater) for temperatures between 333 and 1200°C.
Sensitivity
The K type thermocouple has a sensitivity of approximately 41µV/°C.
K Type Thermocouple Colour Codes
Green for the outer sheath, green for the positive conductor, and white for the negative conductor. This is the international IEC 60584-3 colour code standard for the K type thermocouple. UK’s colour code is red for the outer sheath, brown for the chromel conductor and blue for the alumel conductor.
K Type Thermocouple Sensor Styles
General Purpose Type K Thermocouples
These are the most diverse of the K type thermocouple sensor styles.
Mineral Insulated K Type Thermocouples
They are robust and flexible. The terminals can be made in various ways, such as heads, pot seals and cable connectors.
Heavy Duty K Type Thermocouples
They are specifically meant for heavy-duty applications such as furnaces, flues, Kilns, boilers and ovens. They have a fabricated protective sheath and can have a wide range of termination.
Bare Wire Type K Thermocouples
They have a simple construction and provide a fast response. They are suitable for OEM application, test and measurement and temperature mapping. Due to the exposure of the majority of the wire, they are not ideal for use in industrial applications.
Hand Held Type K Thermocouples
These are used in food industries to measure the temperatures of foods and beverages. They are also used in cold rooms. They are handheld and can measure temperatures by inserting or placing them on the surface of an object.
Pros and Cons of Type K Thermocouples
Pros
- Robust i.e. can measure a wide temperature range
- Oxidation resistance below 1000°C.
- Quick response
- Affordable
Cons
- Unsuitable for reducing air
- Susceptible to electromagnetic interference
Type K Thermocouple Junctions
Exposed Junction
During the welding together of the thermocouple wires, the junction is left exposed so that it can be inserted into the process. This type of junction offers a fast response. However, extreme conditions such as corrosion can degrade the junction. This junction style is not common and is not recommended.
Grounded Junction
In this junction style, the two conductors and the sheath are welded together. Grounded junctions have a fast response but are prone to electrical interference.
Ungrounded Junction
The positive and negative conductors are welded together. However, the sheath acts as insulation for the conductors.
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