What is a Thermocouple? How does a Thermocouple work?

A thermocouple is a sensor used to measure temperature in a number of processes. Thermocouples consist of two wire legs made from dissimilar metals which are fixed together at one end, creating a junction. When this junction experiences a change in temperature a voltage is created, this voltage can then be measured and referenced back to the temperature.

Thermocouples are available in different combinations of metals, each with its own unique characteristics in terms of temperature range, durability, vibration resistance, chemical resistance, and application compatibility.

• Base metal thermocouples or type K, J, T, & E are relatively low cost and therefore the most popular thermocouples, they are commonly used in a broad range of low to medium temperature applications.

• Noble metal thermocouples or type R, S, and B have greater resistance than base metal thermocouples, however they have platinum conductors, making them far more expensive. They are commonly used in high temperature applications.

The graph below shows the thermocouple characteristics and relationship of temperature vs. mV output for all main base metal and nobel thermocouple types. This shows that Thermocouple Sensors have a relatively linear output. 

A thermocouple construction consists of two conductors welded together at the measuring point and insulated from each other along the length, which will usually have an outer protection sheath.

Materials such as PVC, silicon rubber, FEP or PTFE insulators are most commonly used in applications up to 250°C, glass fibre or ceramic is used for temperatures above 250°C. For medium range temperatures (400 to 1200°C) the thermocouple protection sheath is often a metal such as stainless steel or alloy 600 and can be a machined pocket, designed to fit into a particular process. At high temperatures normally above 1100°C ceramic sheaths are used.

Often more consideration goes into selecting and designing protection sheaths than sensors as they are more expensive and must provide protection from chemical or physical attack. Sheaths are often made to order as they are designed specifically to fit into a particular process or application.

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