The type K thermocouple is part of the family of base metal thermocouples composed of metal wires in Nickel-Chromium (Ni-Cr) (+) and Nickel-Aluminum (Ni-Al) (-).
The type K thermocouples can be used in many applications, especially at industrial market, as they are nickel based and have a good corrosion resistance.
It is the most widely used type of thermocouple; its low cost and a wide temperature range make it a versatile temperature sensor.
They are suitable for continuous exposure at temperatures around 1100 ° C (2012 °F), with peaks of 1250 ° C / 1300 ° C (2192 °F/ 2372 °F) for short exposures.
The element has a sensitivity of 41 µV / ° C
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They are mainly used for applications at temperatures above 550 °C (1022 °F) up to the maximum operating temperature of the thermocouple K.
As mentioned, this type of thermocouple is suitable for various kind of applications, especially at industrial field.
Some examples of use:
Despite their widely used, Type K thermocouples are not as stable as other common metal sensors. They have a tendency to drift in an amount proportional to the temperature to which they are exposed; the higher the temperature, the greater the drift.
At temperatures between 250 °C (482 °F) and 600 °C (1112 °F), in particular at 300 °C (572 °F) and 550 °C (1022 °F), the thermal hysteresis can cause errors of various degrees.
At high temperatures they can lead to important variations in the output signal due to the reduction of chromium present in the positive conductor (oxidation).
The accuracy of a type K thermocouple is generally a maximum of +/- 2.5 °C (36,5 °F) or +/- 0.75%, whichever is greater (EN 60584-1). However, there can be differences between different thermocouples, even from the same production batch due to deviations in the alloys.
If exposed in oxidizing atmospheres in the temperature range between 815 °C (1499 °F) and 1040 °C (1904 °F), oxidation can be the cause of a thermal drift of its calibration.
Prolonged exposure above 427 °C (800,6 °F) accelerates the aging of the thermocouple.
Type K wire is mainly available in two color codes:
The temperature range of a type K thermocouple varies between -200 °C to 1200 °C (-328 °F to 2192 °F), up to peaks of 1250 °C / 1300 °C (2282 °F / 2372 °F) for short exposures.
However, there are some caveats:
In the K type thermocouple basically two types of insulation are used that differentiate the thermocouple, above all in terms of response speed and robustness:
Simple insulation: ceramic insulators are used, a very light and extremely effective insulation product as it can withstand high temperatures of 1260 °C (2300 °F).
Mineral insulation: compressed magnesium oxide (MgO) is used when high rigidity and excellent electrical insulation are required. Combined with a correct metal sheath, this type of insulation responds quickly to changes in temperature and is very durable to process stress.
Mineral oxide insulation is recommended when the thermocouples must be immersed in liquids, in the presence of high humidity, corrosive gases, high pressures or vibrations. In addition, the MgO insulation provides the thermocouple with a greater response speed.
The connection of the two conductors of different metals is called “junction”. The junction that is exposed to the temperature to be measured is called the “hot junction” or “measuring junction”.
Three types of hot junction:
Exposed hot Junction: The thermocouple wires are welded together and exposed directly to the process. Response time is very fast but exposed wires are more prone to corrosion and degradation. Unless the application requires exposed junction, this type of processing is not recommended in corrosive environments or extreme conditions.
Grounded hot Junction: In this case both the thermocouple wires and the sheath are all welded together to form a single junction at the tip of the probe. The response time is fast enough because the thermocouple makes direct contact with the sheath, allowing the heat to transfer easily. A disadvantage is that the thermocouple is more susceptible to electrical interference as the sheath often comes into contact with the surrounding air providing interference. The grounded hot junction thermocouple is used in corrosive environments or under high pressure conditions.
Insulated hot joint: The joint is completely isolated from the protective sheath with response times that are slightly longer than the other two types of joints. The thermocouple with the insulated hot junction is ideal for processes with the presence of parasitic electromotive forces that cause potential distortions of the measurements.
316 Stainless Steel: Maximum temperature: 900 °C (1652 °F). Better corrosion resistance than austenitic stainless steel. Widely used in the food and chemical industry. It is subject to harmful precipitation of carbide between 480 °C and 870 °C (896 °F and 1598 °F).
316L Stainless Steel: Maximum temperature: 900 °C (1652 °F). Features like 316 except the low carbon version allows for better welding and fabrication.
304 Stainless Steel: Maximum temperature: 900 °C (1652 °F). Widely used in the food, beverage, chemical and other industries where corrosion resistance is required. It is subject to harmful precipitation of carbide in the range from 480 °C and 870 °C (896 °F and 1598 °F).
310 Stainless Steel: Maximum temperature: 1150 °C (2102 °F). Mechanical and corrosion resistance similar but better than 304 SS. Excellent heat resistance. This alloy contains 25% chromium, 20% nickel.
Inconel 600: Maximum temperature: 1175 °C (2147 °F). Good high temperature resistance, corrosion resistance, chloride ion stress corrosion resistance and oxidation resistance at high temperatures. Not recommended in environments containing sulfur.
Nicrobell: Maximum temperature: 1300 °C (2372 °F). Excellent resistance to high temperatures, excellent resistance to oxidation, generally superior to stainless steels. Suitable for use in reducing, oxidizing and vacuum atmospheres. It can be used in sulphureous atmospheres at reduced temperatures.
The thermocouple K Type has a millivolt output signal; therefore, to carry out the measurement it can be connected to a voltmeter, or to a data-logger or to a data acquisition card, capable in turn of converting the voltage values measured into a temperature through a voltage-temperature conversion circuit which uses the formula, and the coefficients present in the reference standard.
All types of instruments capable of measuring a thermocouple must consider the measurement of the cold or reference junction, whether it is made through the ice bath or electronically simulated, to obtain the correct temperature reading and not affected by the splice / link error.
The chromium element is subject to what is known as “green rot“. When this happens, the chromium oxidizes and turns green and corroded.
This occurs in environments with reduced oxygen in temperatures between 815 °C and 1040 °C (1499 °F and 1904 °F).
Such oxygen-depleted environments are called reducing and type K thermocouples should never be used in reducing or cyclically oxidizing atmospheres.
Also, they should not be used in sulfuric environments because they become brittle and will break quickly.
The presence of chromium makes them unsuitable for vacuum, except for short periods of time as vaporization could occur.
Problems can be minimized by using Type K thermocouples within the recommended temperatures and environments.
Careful calibration, installation with the appropriate connectors and cables, and the use of compensation circuits also serve as useful aids.
The K thermocouples made according to special process requirements can be built with particular types of insulation and equipped with annealing cycles above their operating temperatures.
Another shortness is to replace frequently the K thermocouple or, in cases where the problem became a constant, replace them with Type N Thermocouples which solve many problems related to K type.
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