Thermocouple(TC) - All you need to know

What is a Thermocouple?

The American Society for Testing and Materials (ASTM) has defined the term thermocouple as follows:

Thermocouple, n. - in thermometry, the sensor of a thermoelectric thermometer, consisting of electrically conducting circuit elements of two different thermoelectric characteristics joined at a junction. [Vol. 14.03, E 344 - 02 § 3.1 (2007).]

Put differently, a thermocouple occurs when any two different kinds of metals joined at a junction are exposed to a temperature gradient. When the two different metals are exposed to a temperature gradient they generate a very small electrical charge, commonly measured in millivolts, that correlates to the temperature to which the elements are exposed. This phenomenon is sometimes referred to as the Seebeck effect.

The junction that is put into the process in which temperature is being measured is called the HOT JUNCTION. The other junction which is at the last point of thermocouple material and which is almost always at some kind of measuring instrument, is called the COLD JUNCTION.

Thermocouples can be made of very common materials such as iron and nickel. Thermocouples can also be made of rare and expensive materials such as platinum and rhodium.

Cold Junction Compensation 

In the above example, one end of the thermocouple is @ 1000° and the other end is @ 100° so the difference is 900°. If we wanted to measure the temperature in a furnace, we could use a thermocouple to do so. If the above example were used, the temperature inside the furnace is 1000° and the temperature outside is 100°, the thermocouple would indicate a difference in temperature between the inside and outside of 900°. The only problem with the example above is that we want to know the temperature inside the furnace, not the difference between the outside and the inside. To do this with a thermocouple, we need to apply “Cold Junction Compensation”. To apply this cold junction compensation, all we need to know is the temperature of the cold junction.

The measuring instrument normally does this cold junction compensation. The instrument measures the temperature at the point where the thermocouple attaches and adds that temperature back in to the equation as per the above example. The instrument then displays the result of this equation. It is important to maintain thermocouple material throughout the circuit as in the case of a sensor that is located some distance from the measuring instrument. Specially coded extension wire is normally used.

In the above example, thermocouple extension wire was not used in the circuit and so an error has occurred due to incorrect cold junction compensation.

What types of thermocouples have been recognized as reliable?

Although any two different metals can be joined to form a thermocouple, scientists and temperature professionals have recognized that it is preferable to use certain combinations of metals in order to reliably measure temperature. These reliable combinations of metals are called thermocouple types (they are also informally referred to from time to time as thermocouple calibrations).

The ASTM has defined the term thermocouple type as follows:

Thermocouple type, n. - a nominal thermoelectric class of thermoelement materials that, used as a pair, have a standardized relationship and tolerance between relative Seebeck EMF and temperature, physical characteristics and an assigned type letter designator and color code. [Vol. 14.03, E 344 - 02 § 3.2 (2007).]

European standards are set by the IEC which uses different color code designation for thermocouples but largely sticks with the same letter designations. In the United States, different letter and color code designations are defined for each thermocouple type by the ANSI/ASTM E 230 standard.

So how do I know what types of thermocouples are used for what?

The ASTM and IEC have recognized the following types of thermocouples. Typical uses for these thermocouple types are set out below.

1. Type J Thermocouple (Most Common): This thermocouple consists of an Iron and a Constantan leg and is perhaps the most common thermocouple in use in the United States. The bare Type J thermocouple may be used in vacuum, reducing, oxidizing and inert atmospheres. Heavier gauge is wire recommended for use above 1000 deg. F since the iron leg of this thermocouple oxidizes rapidly at high temperatures.

2. Type K Thermocouple (Most Common Real Hot): This thermocouple consists of a Chromel and an Alumel leg. This thermocouple is recommended for oxidizing or inert atmospheres up to 2300 deg. F. Cycling above and below 1800 deg. F is not recommended due to EMF alteration from hysteresis. This thermocouple is fairly accurate and stable at high temperatures.

3. Type N Thermocouple (A Newer, Better Type K): This thermocouple consists of a Nicrosil and a Nisil leg. This thermocouple is recommended for the same range as a Type K. It has better resistance to degradation due to temperature cycling, green rot and hysteresis than the Type K and is typically very cost competitive with the Type K.

4. Type T Thermocouple (Most Common Real Cold): This thermocouple consists of a Copper and a Constantan leg. It may be used in vacuum, oxidizing, reducing and inert atmospheres. It maintains good resistance to corrosion in most atmospheres and high stability at sub-zero temperatures.

5. Type E Thermocouple (Most Common Power Application): This thermocouple consists of one Chromel leg and one Constantan leg. This thermocouple is not subject to corrosion in most atmospheres. The Type E also has the highest EMF per degree of any standard thermocouple type. However, this thermocouple must be protected from sulfurous atmospheres.

6. Type B, R & S Thermocouples (Most Common Real, Real Hot): Platinum & Rhodium Thermocouples. Recommended for use in oxidizing or inert atmospheres. Reducing atmospheres may cause excessive grain growth and drift in calibration of these thermocouples. Types R & S may be used up to 1480 C. Type B may be used up to 1700 C.

7. Type C Thermocouple (For the Hottest of Environments): Tungsten and Rhenium thermocouple. Recommended for use in vacuum, high purity hydrogen or pure inert atmospheres. May be used at extremely high temperatures (2316 C). This thermocouple, however, is inherently brittle. 

More Comprehensive Information on Other Types of Thermocouples are available below

Thermocouple Reference Tables 

Tables have been established worldwide that show temperature vs. millivolt output figures for the various accepted thermocouple combinations or “types”. These reference tables are all based on a reference or cold junction temperature of 32°F (0°C), which is the freezing point of pure water. All manufacturers follow these reference tables, which are published in ASTM document E-230.

Working Principle

The working principle of thermocouple is based on three effects, discovered by Seebeck, Peltier and Thomson. They are as follows:

1) Seebeck effect: The Seebeck effect states that when two different or unlike metals are joined together at two junctions, an electromotive force (emf) is generated at the two junctions. The amount of emf generated is different for different combinations of the metals.

2) Peltier effect: As per the Peltier effect, when two dissimilar metals are joined together to form two junctions, emf is generated within the circuit due to the different temperatures of the two junctions of the circuit.

3) Thomson effect: As per the Thomson effect, when two unlike metals are joined together forming two junctions, the potential exists within the circuit due to temperature gradient along the entire length of the conductors within the circuit.

In most of the cases the emf suggested by the Thomson effect is very small and it can be neglected by making proper selection of the metals. The Peltier effect plays a prominent role in the working principle of the thermocouple.

Thermocouple Temperature Range

Thermocouple Color Code

Thermocouple Characteristics & Composition

Limits of Error

Accuracy of temperature sensors is referred to as limits of error and apply only to brand new, un-used temperature sensors. Once a sensor is exposed to elevated temperatures, there is no guaranteed accuracy. All manufacturers adhere to these limits, which are establish by ASTM and are covered under their publication ASTM E –230. The Limits of Error tables appear in the SensorTec catalog as well as many competing manufactures catalogs.

Thermocouple Types (In More Detail)

There are several different recognized thermocouple types available. Each type has different useful temperature ranges as well as different recommended applications. ASTM, which is recognized in the United States as the authority for temperature measurement, has established guidelines for the different thermocouple types. These guidelines cover composition, color codes, and manufacturing specifications. 

Base Metal Thermocouples

Base metal thermocouple types are composed of common, inexpensive metals such as nickel, iron and copper. The thermocouple types E, J, K, N and T are among this group and are the most commonly used type of thermocouple. Each leg of these different thermocouples is composed of a special alloy, which is usually referred to by their common names.

Type E

The type E thermocouple is composed of a positive leg of chromel (nickel/10% chromium) and a negative leg of constantan (nickel/45% copper). The temperature range for this thermocouple is –330 to 1600°F (-200 to 900°C). The type E thermocouple has the highest millivolt (EMF) output of all established thermocouple types. Type E sensors can be used in sub-zero, oxidizing or inert applications but should not be used in sulfurous, vacuum or low oxygen atmospheres. The color code for type E is purple for positive and red for negative.

Type J

Type J thermocouples have an iron positive leg and a constantan negative leg. Type J thermocouples have a useful temperature range of 32 to 1400°F (0 to 750°C) and can be used in vacuum, oxidizing, reducing and inert atmospheres. Due to the oxidation (rusting) problems associated with the iron leg, care must be used when choosing this type for use in oxidizing environments above 1000°F. The color code for type J is white for positive and red for negative.

Type K

The type K thermocouple has a Chromel positive leg and an Alumel (nickel/ 5% aluminum and silicon) negative leg. The temperature range for type K alloys is –328 to 2282°F (-200 to 1250°C). Type K sensors are recommended for use in oxidizing or completely inert environments. Type K and type E should not be used in sulfurous environments. Because type K has better oxidation resistance then types E, J and T, its main area of usage is at temperatures above 1000°F but vacuum and low oxygen conditions should be avoided.

Type N

Type N thermocouples are made with a Nicrosil (nickel – 14% chromium – 1.5 % silicon) positive leg and a Nisil (nickel – 4.5% silicon - .1% magnesium) negative leg. The temperature range for Type N is –450 to 2372°F (-270 to 1300°C) and the color code is orange for positive and red for negative. Type N is very similar to Type K except that it is less susceptible to selective oxidation. Type N should not be used in vacuum and or reducing environments in an unsheathed design.

Type T

Type T thermocouples are made with a copper positive leg and a constantan negative leg. The temperature range for type T is –328 – 662°F (-200 to 350°C) and the color code is blue for positive and red for negative. Type T sensors can be used in oxidizing (below 700°F), reducing or inert applications.

Noble Metal Thermocouples

Noble metal thermocouples are manufactured with wire that is made with precious or “noble” metals like Platinum and Rhodium. Noble metal thermocouples are for use in oxidizing or inert applications and must be used with a ceramic protection tube surrounding the thermocouple element. These sensors are usually fragile and must not be used in applications that are reducing or in applications that contain metallic vapors.

Type R

Type R thermocouples are made with a platinum/13% rhodium positive leg and a pure platinum negative leg. The temperature range for type R is 32 – 2642°F (0 to 1450°C) and the color code is black for positive and red for negative.

Type S

Type S thermocouples are made with a platinum/10% rhodium positive leg and a pure platinum negative leg. The temperature range for type S is 32 – 2642°F (0 to 1450°C) and the color code is black for positive and red for negative.

Type B

Type B thermocouples are made with a platinum/30% rhodium positive leg and a platinum/6% Rhodium negative leg. The temperature range for type r is 32 – 3092°F (0 to 1700°C) and the color code is gray for positive and red for negative

Refractory Metal Thermocouples

Refractory metal thermocouples are manufactured with wire that is made from the exotic metals tungsten and Rhenium. These metals are expensive, difficult to manufacture and wire made with these metals are very brittle. These thermocouples are intended to be used in vacuum furnaces at extremely high temperatures and must never be used in the presence of oxygen at temperatures above 500°F. There are several different combinations of alloys that have been used in the past but only one generally used at this time.

Type C

The type C thermocouple is made with a tungsten/5% rhenium positive leg and tungsten 26% rhenium negative leg and has a temperature range of 32 – 4208°F (0 – 2320°C). The color code for this type is white with red tracer for positive leg and red for the negative leg.