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This week's Fire Safety Technical Note briefly introduces an essential piece of fire science equipment.

A thermocouple is a temperature sensor that is used extensively throughout fire testing and fire science research. The thermocouple provides temperature monitoring throughout a test period and outputs a result of temperature vs time. 

This temperature vs time output is a useful measure as we can begin to understand the conditions that occupants and/or elements of the building structure may be exposed to during a fire event.

Thermocouples are made up of two wires that are joined together to form a junction. The two wires are dissimilar – i.e. made of different metals with different thermal properties. 

By heating the junction a voltage is measured that corresponds to the differential between the heated junction (the ‘hot’ junction) and a base junction that isn’t heated (the ‘cold’ junction). This voltage is then converted to a temperature reading.

There are three key types of thermocouple: beaded, probe and surface.

• A beaded thermocouple consists of two wires joined together by a welded bead. Beaded thermocouples are commonly used for measurements of gas temperatures; however, due to the exposed sensor elements, can be susceptible to damage from corrosive liquid samples or from solids that may hold an electrical charge.
• Thermocouple probes encapsulate the wires into a metal tube or sheath. Thermocouple sheaths are generally stainless steel with the tip being either grounded, ungrounded or exposed.
• A surface thermocouple measures temperature over a flat surface, providing an accurate measurement over a solid material. Surface thermocouples can also be mounted on a mechanism that rotates, allowing for measurement of a moving surface.

Thermocouple types are rated for different temperature ranges, for example a Type-B thermocouple has a relatively small range of application for high temperatures whereas a Type-T thermocouple has a wider range but is only suitable for low temperatures up to around 350 degC.

A Type-K thermocouple has a very wide range of application and provides coverage from a relatively low temperature up to more than 1200degC. It is due to this range of application that Type-K thermocouples are commonly used in fire testing and research. 

Approximate temperature ranges are given below (though it is noted that all devices have a tolerance and may vary depending on manufacturer):

• Type-B thermocouple suitable for 1350 degC to 1700 degC
• Type-C thermocouple suitable for 1650 degC to 2300 degC
• Type-E thermocouple suitable for 95 degC to 900 degC
• Type-J thermocouple suitable for 95 degC to 750 degC
• Type-K thermocouple suitable for 95 degC to 1250 degC
• Type-N thermocouple suitable for 650 degC to 1250 degC
• Type-R thermocouple suitable for 850 degC to 1450 degC
• Type-S thermocouple suitable for 980 degC to 1450 degC
• Type-T thermocouple suitable for -200 degC to 350 degC

Thermocouples are used extensively in fire testing and in fire research. Many items of test apparatus incorporate thermocouples at strategic places, for example the BS 8414 test rig which requires Type-K thermocouples that are mineral insulated with a nominal diameter of 1.5mm to measure temperature increase on a building facade.

Thermocouples can also be used for monitoring within fire safety systems, for example integrated into a smoke/heat detector to give the emergency services a real-time record of fire conditions.

Multiple thermocouples are often used in a single fire test to measure the conditions at various locations and heights throughout an enclosure.