What Is The Difference Between Type E And Type K Thermocouple?

A thermocouple is a sensor consisting of two dissimilar electrical conductors joined together at one end. When the junctions at the end are heated or cooled, a voltage is generated that correlates to the temperature difference. This phenomenon is known as the thermoelectric effect or Seebeck effect, named after Thomas Johann Seebeck who accidentally discovered it in 1821 while experimenting with joined metals.[1]

Thermocouples are widely used as temperature sensors due to their simple construction, wide temperature range, and standardization. The two most common types of thermocouples are Type E and Type K.

Type E thermocouples use chromel (nickel-chromium alloy) and constantan (copper-nickel alloy) wires. They are inexpensive and standardized, with a range of -270°C to 1000°C. Type K thermocouples use chromel and alumel (nickel-aluminum alloy) wires. They are the most common general purpose thermocouple with a range of -270°C to 1372°C.

[1] OMEGA Engineering. “Introduction To Thermocouples.” https://www.jp.omega.com/techref/themointro.html

Type E Thermocouple

The Type E thermocouple is composed of chromel (nickel-chromium alloy) and constantan (copper-nickel alloy) [1]. It is suitable for use in oxidizing or inert atmospheres at high temperatures. Type E thermocouples have the following key characteristics:

• Temperature range: -454 to 1600°F (-270 to 870°C) for thermocouple grade wire; 32 to 392°F (0 to 200°C) for extension grade wire [2]
• Accuracy: ±1.7°F or ±0.9% (whichever is greater) [3]
• Common applications: food processing, petrochemical industries, ceramic kilns, glass furnaces

The high temperature capabilities and good oxidation resistance make Type E thermocouples well-suited for measurements in harsh, high heat environments. However, they have lower accuracy compared to other types of thermocouples.

Type K Thermocouple

The Type K thermocouple is composed of chromel and alumel conductors. Chromel is an alloy of nickel and chromium, while alumel is an alloy of nickel, manganese, aluminium and silicon [1]. This combination results in a thermocouple that is inexpensive and has a wide temperature range.

Type K thermocouples have a temperature range of -270°C to 1260°C (-454°F to 2300°F) [2]. They have an accuracy of +/- 2.2°C or +/-0.75% (whichever is greater) in the -40°C to 375°C (-40°F to 707°F) range. At higher temperatures, the accuracy degrades slightly to +/-2.2°C or +/- 2.0% [3].

Type K thermocouples are commonly used in high temperature industrial processes, as well as heating, ventilation, and air conditioning (HVAC) systems. They are also popular for monitoring exhaust gas temperatures in automobiles and combustion applications like furnaces and kilns.

Key Differences

There are a few key differences between Type E and Type K thermocouples:

Temperature range – Type E thermocouples have a recommended temperature range of -270 to 1000°C, while Type K thermocouples have a range of -270 to 1260°C¹. So Type K can withstand slightly higher temperatures.

Accuracy – Type E thermocouples are generally more accurate than Type K, with an accuracy of ±1.7°C or ±0.5% above 0°C, compared to ±2.2°C for Type K². The higher thermoelectric coefficient of Type E leads to a stronger signal.

Oxidation resistance – The positive leg of Type K (chromel) offers better oxidation resistance than Type E (chromel-constantan) at high temperatures. So Type K is more suitable for measurements in oxidizing atmospheres³.

Thermoelectric Effect

The thermoelectric effect is what enables thermocouples to work. It refers to phenomena where temperature differences create an electric potential or where applied electric potential creates a temperature difference.

The Seebeck effect is the phenomenon where electricity is generated between two dissimilar metals or semiconductors when their junctions are kept at different temperatures. It is named after Thomas Seebeck, who discovered it in 1821.

In a thermocouple, one junction is connected to the object being measured while the other is connected to a probe with a known reference temperature. The Seebeck effect causes a voltage to be generated proportional to the temperature difference. This voltage is measured and calibrated against known values to determine the target temperature.

The Seebeck effect works on the principle that heat flow through conductors causes charge carriers like electrons to move, generating a potential difference. The magnitude of the voltage depends on the materials used and the temperature difference. This predictable behavior is harnessed in thermocouples to accurately measure a wide range of temperatures.

Reference:https://www.youtube.com/watch?v=1r7a5L_qtoQ

Calibration

Proper calibration is critical for achieving accurate temperature measurements with thermocouples. Thermocouples produce a small electrical signal that must be converted into a temperature reading. This conversion relies on reference tables and compensation for errors like the cold junction effect. Calibration helps account for these factors and any deviations in the thermocouple itself.

Cold junction compensation is an important part of thermocouple calibration. This compensates for temperature differences between the measuring junction (probe tip) and the reference junction (terminal block). A cold junction sensor measures the reference temperature, allowing compensation of this error in the calibration process. Proper cold junction compensation is vital for precise temperature control and measurement.(1)

Regular recalibration per the manufacturer’s recommendations ensures the thermocouple system remains within specified accuracy limits. Proper calibration techniques, traceable to national standards, are essential for reliable temperature monitoring across many industrial applications.

Advantages of Type E

Type E thermocouples offer several advantages that make them preferable for certain applications:

Higher accuracy – Type E thermocouples provide higher accuracy than other common types like Type K and Type J. They have a stronger thermoelectric output signal over their temperature range, allowing more precise measurements [1]. Their calibration also tends to be more stable.

Wider temperature range – The recommended temperature range for Type E is -270 to 1000°C, which is wider than Type K (-270 to 1200°C) and Type J (-210 to 1200°C) [2]. This makes Type E suitable for very low cryogenic measurements.

Better oxidation resistance – The positive chromel leg in Type E has a higher chromium content compared to Type K. This gives it better resistance to oxidation, allowing it to operate longer in oxidizing atmospheres at high temperatures [3].

Advantages of Type K

Type K thermocouples have several key advantages that make them popular:

Lower cost – Type K thermocouples are generally less expensive than other types like Type E. The nickel-chromium and nickel-aluminum wires used in Type K are cheaper than the nickel-chromium and copper-nickel wires in Type E (RTDcn.com). This makes Type K the most affordable option.

More common – Due to the lower cost, Type K thermocouples are the most widely used general purpose thermocouple. Their ubiquity makes replacements and complementary equipment easier to find (RTDcn.com).

Easier to find replacements – The popularity of Type K thermocouples also means it’s easier to find replacements when an old one fails. Their wide availability and lower cost makes keeping spares handy more feasible.

Common Applications

Type E and Type K thermocouples are used in a variety of industrial applications. Here is an overview of the typical uses for each type:

Type E thermocouples are commonly used for:

• Cryogenic applications like liquid nitrogen or helium cooling due to Type E’s high accuracy at extremely low temperatures [1]
• Measuring temperature in reducing or vacuum environments since Type E is less prone to oxidation errors [2]
• Aerospace applications because Type E has a fast response time [2]

Type K thermocouples are commonly used for:

• General purpose temperature measurement from -200 to +1200°C [3]
• High temperature furnaces, boilers, and ovens due to Type K’s wide temperature range [3]
• Most common in heating, ventilation, and air conditioning (HVAC) systems because Type K is inexpensive [3]

Conclusion

In summary, the key differences between Type E and Type K thermocouples are:

• Type E has higher accuracy and stability at lower temperatures up to 1000°F, whereas Type K is more stable at very high temperatures above 1000°F
• Type E uses Chromel and Constantan wires, while Type K uses Chromel and Alumel
• Type E generates higher output voltage than Type K
• Type K can withstand higher temperatures up to 2300°F compared to Type E’s 1600°F limit
• Type K is less expensive than Type E

Based on these differences, here are some recommendations for usage:

• Use Type E for precise temperature measurement and control under 1000°F
• Choose Type K for very high temperature applications above 1000°F where accuracy is less critical
• Select Type E when accuracy and stability are priorities, and cost is less of a concern
• Type K is a good general purpose thermocouple given its cost and temperature range

In summary, consider accuracy needs, temperature range, and budget when selecting between these two common thermocouple types for an application.