K type thermocouple is one of the most widely used primary temperature-measuring elements. K-type thermocouple’s wide temperature range and cost-effectiveness make it one of the most widely used thermocouples.
This article describes K type thermocouple construction, temperature range, pros, cons, and applications.
Table of contents
Construction of a K type thermocouple
K type thermocouple is made from two alloys, which are chromel and alumel. The composition of the material is as follows: chrome has 90% nickel and 10% chromium. In contrast, alumel has 95% nickel, 2% manganese, 2% aluminum, and 1% silicon. Chromel is used in the positive leg, and alumel is used in the negative leg of the thermocouple.
Also, one important point is that the chromel is non-magnetic, and alumel is magnetic. Hence, many of you might come across addressing K type thermocouple’s legs in terms of their magnetic properties, i.e., magnetic leg and non-magnetic leg.
One of the properties of the chrome material is that it develops green rot because of the presence of the chromium metal. The green rot occurs when the thermocouple is exposed to the temperature range of 800° C to 1260° C with a low concentration of oxygen gas. Because of this green rot, the measurement can become inaccurate, and even drifts can be observed during the measurement.
Hence, a K type thermocouple should be used in oxygen-rich atmospheres. Also, sulfurous environments can cause damage to the thermocouple, which ultimately makes the thermocouple material brittle. Hence, this type of thermocouple should not be used in sulfurous environments. The chromium material present in the alloy used for making these thermocouples vaporizes in vacuums.
Hence, K type thermocouples should not be used for continuous use in a vacuum environment.
Temperature Range of K type thermocouple
K type thermocouple has the widest temperature range compared to all other thermocouples. This type of thermocouple can measure temperatures from -200 °C to 1350°C or, in other words, the temperature range of the thermocouple is 1550°C. This wide temperature range of this thermocouple is because of the use of base alloys in its construction.
Nickel-chromium / Nickel-Alumel thermocouples are generally used for temperature measurements above 540°C. To get the most accurate output, it should be used in the temperature range of -200°C to 1260°C.
One important thing to note is that this type of thermocouple has a magnetic leg. Hence, at a certain temperature, it will face the issue of the Curie point (Curie point is the point at which any magnetic material used in the thermocouple will produce a change in output at a certain temperature).
The Curie point for a type K thermocouple is around 354°C. So, when the temperature reaches nearly 354°C, a change in the output will be observed in the K type thermocouple. The sensitivity of the thermocouple is 41 µV/°C.
The general temperature range of type K thermocouple is -200 to 1260°C (-326 to 2300°F). However, there are some specific conditions to this:
- If a type thermocouple is used below 0°C, it can not give the specified accuracy. Specific materials are required to construct it to meet the accuracy.
- The upper limit of temperature depends on the size of the conductors used for the thermocouple.
The following table shows the relationship between the conductor size and the K-type thermocouple temperature range.
| Wire Gauge | Diameter | Maximum Suggested Temperature |
| #8 AWG | 0.128” | 1280°C/2300°F |
| #14 AWG | 0.064” | 1090°C/2000°F |
| #20 AWG | 0.032” | 980°C/1800°F |
| #24 AWG | 0.020” | 870°C/1600°F |
| #28 AWG | 0.013” | 870°C/1600°F |
| #30 AWG | 0.010” | 760°C/1400°F |
K type Thermocouple Insulation Material
It has two types of insulation. One is ceramic bead insulation, and the other is mineral insulation with an outer metal sheath.
Ceramic Bead Insulation
The ceramic bead insulation is used because of its lightweight. Also, other properties like low or very little heat retaining ability, low thermal conductivity, and effective insulations make this insulation more suitable for k type thermocouple. It can sustain the temperature up to 1260 °C.
Mineral Insulation with an Outer Metal Sheath
This type of insulation has a mineral insulation with an outer metal sheath in which Magnesium Oxide (MgO) is present. MgO responds very quickly to temperature changes, and the durability of MgO is very high. MgO insulation is highly recommended to be used with a K type thermocouple when it is immersed in liquids with high moisture or corrosive gas or a high-pressure environment is present.
Color codes and specifications of K Type Thermocouple
Color Codes
Specifications
| Properties | Specifications |
| Code | ANSI: K IEC: K |
| Alloy Combination | + Lead:NICKELCHROMIUM Ni-Cr – Lead: NICKEL-ALUMINUM Ni-AI (magnetic) |
| Color Coding(Thermocouple Grade) |  |
| Color Coding(Extension Grade) |  |
| Maximum Useful Temperature Range ++ | K type Thermocouple Grade: –328 to 2282°F –200 to 1250°C Extension Grade: 32 to 392°F 0 to 200°C |
| Maximum Thermocouple Grade Temperature Range | –454 to 2501°F –270 to 1372°C |
| EMF (mV) Over Max. Temperature Range | –6.458 to 54.886 |
| Standard Limits of Error (above O°C) | greater of 2.2°C or 0.75% |
| Special Limits of Error (above O°C | greater of 1.1°C or 0.4 |
| International IEC 584-3 Color code |  |
Voltage of Type K Thermocouples
| Temperature | Output |
| 0°C | 0.000 mV |
| 100°C | 4.096 mV |
| 200°C | 8.138 mV |
| 300°C | 12.209 mV |
| 400°C | 16.397 mV |
| 500°C | 20.644 mV |
| 600°C | 24.905 mV |
| 700°C | 29.129 mV |
| 800°C | 33.275 mV |
| 900°C | 37.326 mV |
| 1000°C | 41.276 mV |
| 1100°C | 45.119 mV |
| 1200°C | 48.838 mV |
Advantages
The advantages are listed below.
- It is one of the most inexpensive temperature measurement elements.
- It has the widest temperature measurement range.
- It has a good linearity of millivolts generated vs. temperature to be measured.
- It is rugged. Hence, it can be used in various atmospheric conditions and harsh environments.
- K-type thermocouples can be used almost everywhere because of their wide temperature measurement range.
- The life span of this thermocouple is longer when compared to other thermocouples, like E-type thermocouples with iron in their composition, making them oxidize rapidly.
- It has stable output, accurate measurement, and fast response. It is smaller in size.
- The linearity provided by this thermocouple is also good.
Disadvantages
The disadvantages are listed below.
- K-type thermocouple is not suitable for atmospheres where reduction reactions can occur.
- While using this thermocouple, aging can cause a little problem. This aging causes a change in the EMF generated.
- The presence of chromium makes this thermocouple unfavorable for use in vacuum conditions because chromium vaporizes in a vacuum.
- When this thermocouple is exposed to a low oxygen concentration, green rot can occur, which can damage the thermocouple element.
- It cannot be used in sulfur-rich environments either.
Applications of K type thermocouple
K-type thermocouple is suitable for high-temperature measurement and widely used for temperature measurement in the following sectors.
- Steel plants
- Refineries
- Petrochemical complex
- Nuclear applications