What Is The Disadvantage Of Thermocouple Type K?
High Temperature Limitations
One of the main disadvantages of type K thermocouples is their relatively low upper temperature limit. The maximum continuous temperature limit for type K thermocouples is around 1,200°C (source). While suitable for many common industrial and scientific applications, this makes them unsuitable for extremely high temperature environments above 1,200°C.
For example, type K thermocouples would not work well for applications like gas turbines, rocket engines, or other systems that can reach over 1,200°C. The nickel-chromium wires used in the type K thermocouple would simply melt at temperatures exceeding their upper limit. For these ultra-high temperature applications, other thermocouple types like type R or type S are better suited to handle the extreme conditions without failing.
In summary, the relatively low maximum temperature limit of around 1,200°C is a key disadvantage of type K thermocouples compared to other thermocouple types. This renders them unsuitable for the most demanding high temperature applications that exceed their working range.
Lower Sensitivity
Type K thermocouples have a relatively low sensitivity of around 41 μV/°C compared to other common thermocouple types (source 1). This makes them less sensitive than types E, J, and T thermocouples, which have sensitivities between 50-70 μV/°C. The lower sensitivity limits the precision and accuracy possible with type K thermocouples, particularly when trying to measure small temperature changes.
According to one source, type T thermocouples have the highest accuracy of ±1°C or ±0.75%, followed by type E at ±1°C or ±1% (source 2). The lower sensitivity of type K, at 41 μV/°C, makes it more difficult to achieve this level of precision compared to other thermocouple types.
For applications requiring very precise temperature measurements, the lower sensitivity of type K thermocouples can be a disadvantage. Their reduced sensitivity caps their accuracy and ability to detect small temperature changes compared to alternatives.
Susceptibility to Oxidation
Type K thermocouples use nickel-chromium wires which are susceptible to oxidation at high temperatures. When exposed to oxidizing environments over 1000°C for extended periods, the chromium in the wire is selectively oxidized, forming an oxide layer on the surface of the wire [1]. This process, known as “green rot”, can affect the thermoelectric output and accuracy of the thermocouple.
The oxidation of the thermocouple wires causes a drift in EMF output over time as the chromel wire becomes depleted of chromium. Studies have shown that prolonged use above 1000°C in air can cause drifts of over 1°C in the temperature measurement [2]. This error continues to increase with higher temperatures and longer exposure times.
To reduce the effects of oxidation, type K thermocouples used for high temperature measurements may need to be replaced periodically. Using a protective tube or sheath around the wires can also help minimize exposure to oxygen.
[1] https://blog.wika.us/products/temperature-products/green-rot-affects-type-k-thermocouples/
[2] https://www.msm.cam.ac.uk/utc/thermocouple/pages/DriftInTypeKBareWiresThermocouples.html
Voltage Output
One of the main disadvantages of type K thermocouples is their relatively low voltage output compared to other common thermocouple types like types E, J, and T [1]. The voltage output of a thermocouple is dependent on the combination of metals used. Type K uses chromel (nickel-chromium alloy) and alumel (nickel-aluminum alloy), which produces lower thermoelectric voltages than other combinations.
For example, at 1000°C, a type K thermocouple will produce around 41 mV, while a type E can produce up to 76 mV. The lower voltage output of type K thermocouples makes them more susceptible to interference from electrical noise sources. This can decrease measurement accuracy compared to types with higher output voltages [2].
Cost
Type K thermocouples are less expensive than some other types like types B, R, and S. This is because type K uses relatively inexpensive nickel-chromium and nickel-aluminum alloys for its positive and negative legs. The materials for types B, R, and S can be more costly by comparison.
However, type K is more expensive than types J and T. This is because types J and T use even less expensive alloys like iron versus nickel and copper versus nickel-chromium. The iron and copper materials make types J and T cheaper to produce than type K.
Temperature Cycling Effects
Type K thermocouples are susceptible to changes in EMF output after being subjected to repeated temperature cycling. This phenomenon is caused by diffusion between the nickel and chromium conductors that make up the thermocouple (https://www.thermocouple-pt100atex.com/the-k-type-thermocouple/). At temperatures between 250°C and 600°C, but especially 300°C and 550°C, temperature cycling can result in hysteresis errors of several degrees Celsius (https://www.tc-inc.com/thermocouples/type-k-thermocouple.html). The EMF output may shift after temperature cycling due to the formation of metal carbides and secondary phases at the junction of the two conductors. These diffusion effects build up over repeated cycles and can lead to drift and inaccuracies in temperature measurement.
Radiation Effects
Type K thermocouples use nickel conductors that are sensitive to neutron irradiation, which can cause decalibration over time (ThermocoupleInfo). The nickel in the negative lead is particularly susceptible to radiation damage. As the thermocouple is exposed to neutron flux, the nickel undergoes transmutation into copper and other elements. This alters the thermoelectric properties and Seebeck coefficient of the conductors, leading to drift and inaccuracy in measurements (Falsetti). The degree of drift depends on the level of neutron fluence and can become significant in nuclear reactor monitoring applications.
To mitigate radiation effects, other thermocouple types such as N or R are often preferred for high radiation environments. Where type K must be used, the device may need frequent recalibration or replacement to maintain accuracy.
Melting Point of Ni-Cr
One key disadvantage of thermocouple type K is its relatively low melting point of around 1,350°C, which is the melting point of nickel-chromium (Ni-Cr) alloy (Wikipedia). This upper temperature limit arises because type K thermocouples use Ni-Cr as the negative leg. Although Ni-Cr alloys can withstand high temperatures, their melting point limits the maximum temperature that can be measured by type K to about 1,350°C (American Elements). Exceeding this temperature causes the thermocouple to fail.
Alternatives for High Temps
Type K thermocouples have an upper temperature limit of around 1,200°C. For applications above this temperature, other thermocouple types such as Types N, R, and S are better choices as they can measure temperatures up to 1,600°C or higher.
Type N uses a nickel-chromium alloy and nickel-silicon-magnesium alloy junction and can handle continuous use up to 1,300°C and brief spikes to 1,600°C. Type R uses platinum-rhodium alloys for both wires and is stable up to 1,600°C continuous use. Type S also utilizes platinum-rhodium and can reliably measure up to 1,600°C continuously with brief spikes over 1,700°C.
However, these alternatives are less common and more expensive than the widely used Type K. They require more exotic materials for the thermocouple wires and insulation. So while necessary for the highest temperature applications, Types N, R, and S may not make sense for uses below 1,200°C where Type K functions reliably.
Limited Uses in Nuclear
Type K thermocouples have limited uses in nuclear applications due to the radiation sensitivity of the nickel in the negative leg. The nickel can become damaged and decalibrated when exposed to neutron flux environments [1]. This limits the ability to use type K accurately for temperature measurement in nuclear reactors over an extended period of time. Other thermocouple types such as type N are more suitable for continuous use in nuclear applications.
In summary, while type K can be used for some nuclear applications in the short term, the radiation damage effects mean they have reduced accuracy and reliability compared to alternatives like type N for continuous long term nuclear temperature monitoring [2].
[1] https://www.tc-inc.com/thermocouples/type-k-thermocouple.html
[2] https://www.thermocoupleinfo.com/type-k-thermocouple.htm
