The K Type Thermocouple

Overview of the K type thermocouples

Definition, applications, and general characteristics of the K Type thermocouple

What is a Type K Thermocouple?

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


K type thermocouple probe applications

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:

  • Market fields such as steel and iron to monitor temperature and chemistry throughout the steel production process.
  • To test temperatures associated with process plants, such as chemical production and oil refineries.
  • To test the safety of heating appliances.
  • Nuclear applications due to its relative radiation hardness.
  • Applications in the water, gas and food industry sectors.

Type K thermocouple accuracy

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.


Thermocouple type K colors codes

Type K wire is mainly available in two color codes:

  • yellow connectors and / or yellow and red conductors according to ANSI / ASTM E230;
  • green connector and / or green and white conductors according to IEC 60584.

Type k thermocouple temperature ranges

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:

  • If used for temperatures below 0 °C (32 °F) it is necessary to equip the thermocouple with a particular material to satisfy the accuracy of the data.
  • The maximum temperature limits suggested for Type K are based on the conductor size used.
  • Depending on the temperature range that the Type K thermocouple will run, it is necessary to use an adequate diameter of the conductor wires, as well as any extension cable.

K type insulation material

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:

  1. Simple isolation
  2. Mineral insulation

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.


K type thermocouple junctions

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.


Type K thermocouple sheath

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.


How to measure data from thermocouple K Type

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 in the K type thermocouple

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.


How to reduce the problems of the K type thermocouple

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.


Why choose a K type thermocouple

  • A major advantage of the K type thermocouple over other thermocouples is that it can operate in harsh environmental conditions and in various atmospheres.
  • It has a composition of Chromium and Aluminum wires with a temperature range from -200 °C to 1200 °C (-328 °F to 2192 °F) (with peaks up to 1300 °C / 2372 °F) and an output from -6.4 to 9 mV in the range of maximum temperature.
  • It can be used in a multitude of manufacturing processes due to its wide temperature range.
  • It can work in harsh environmental conditions and various atmospheres.
  • K-type has a longer life than J-type because J-type wire is made by iron (Fe) which oxidizes quickly, especially at higher temperatures.
  • They are economical and reliable.
  • They have a quick response.
  • They can be built in miniaturized diameters up to 0.25 mm.

Advantage of the K type thermocouple

  • Good linearity of the emf with respect to the measurement temperature
  • Provides good oxidation resistance below 1000 °C (1832 °F).
  • Highly stable output
  • Relatively cost-effective compared to other thermocouples.
  • Most stable among low-cost material thermocouples.

Disadvantage of the K type thermocouple

  • Not suitable for reducing atmospheres but resists metal vapors
  • Aging of the emf characteristic, compared to noble metal thermocouples (B, R and S).
  • Not suitable for vacuum applications due to chromium vaporization in the positive element.
  • The Green-Rotis phenomenon can occur due to the low oxygen level for thermocouples that are used between 815 °C and 1040 °C (1499 °F and 1904 °F).

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