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