The Construction of Thermocouples

A thermocouple is made of two wires formed from dissimilar metals, welded together at one end. The other ends of the wire are connected to an indicating instrument. Thermocouples are used as temperature sensors in many applications, such as heating and air conditioning systems, industrial ovens and furnaces, and medical equipment. The construction of a thermocouple varies depending upon its environmental and accuracy requirements.

Materials

There are two basic categories of thermocouples. Base metal thermocouples are made from inexpensive common metals like iron, copper, and nickel. The three most common types of base metal thermocouples are type J, K, and T. A type J thermocouple is made with a negative lead of constantan (a copper-nickel alloy) and a positive lead made of iron. A type K thermocouple has a negative lead made of alumel (a nickel-aluminum alloy) and a positive lead made of chromel (a nickel-chrome alloy). A type T thermocouple is made with a negative lead of constantan and a positive lead made from copper.

The other category of thermocouple is called rare metal, or noble metal thermocouple. The most common is a type S thermocouple. They are made with a negative lead of platinum and a positive lead made of a platinum-rhodium alloy. Due to its stability, type S is often used as a reference to calibrate other thermocouples with.

Junctions

The junction of a thermocouple refers to the point where the two dissimilar wires are connected to each other. Manufacturers differ in how the junctions are made. They can be butt or spot welded, crimped, or twisted. To improve physical strength of the junction, it can be twisted and welded. If the thermocouple is to be used at higher temperatures, crimping or twisting are generally not used.

Joins

In a perfect world, joins would not be used. Unfortunately, in the practical world, joins are a necessity. A join is a point where the wires from the thermocouple junction are connected with a plug to wires that are the same type (same alloys), but are not exactly the same. Joins are used because the output voltage from a thermocouple has to be sent to an instrument to be of any use. It is rarely possible to connect instrumentation directly to the junction wires.

Extension Leads

The wires used at joins to connect a thermocouple to an instrument are called extension leads. The extension leads must be of the same metal alloys as the thermocouple itself. For example, if the thermocouple is a type K, then the extension leads must also be type K. This method keeps the measurement error down to an acceptable level.

Sheaths

Thermocouples are generally used in environments that make them susceptible to physical damage. Sheaths are designed to protect them. The most common sheath is made of stainless steel. These sheaths protect thermocouples used to measure high temperatures up to about 1,000 degrees Celsius. The stainless steel sheaths are also used to protect against damage from caustic material, like acid. For thermocouples used at temperatures exceeding approximately 1,000 degrees Celsius, sheaths are made of ceramic material.

References

• www.omega: Thermocouples -- An Introduction
• "Traceable Temperatures, An Introduction to Temperature Measurement and Calibration", Second Edition; J.V. Nicholas and D.R. White; 2001