Standards For Thermocouples
A thermocouple is a device that is formed by the junction between dissimilar metals. It develops a small voltage that varies with its temperature, and the output is monitored to provide an accurate indication of the surrounding temperature. Several different metals and alloys are used for the junction, and each combination results in a unique thermocouple type with its own temperature range and sensitivity.
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Thermocouple Materials
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Common metal elements that are used for thermocouple junctions are copper, platinum and iron. A number of metal alloys are also used. Nickel is a component of constantan (with copper), chromel (chromium) and alumel (aluminum). Nickel is also used in Nicrosil (Chromium-Nickel-Silicon-Magnesium) and Nisil (Nickel-Magnesium). There are also Platinum-Rhodium and Tungsten-Rhenium junctions.
Selection
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Selecting the proper thermocouple type depends on a number of factors including the temperature range, physical and chemical environment, required accuracy and cost. In order to work properly, the wires that connect the thermocouple to the measuring instrument must be made of the same alloys as the junction. This can complicate the design of the monitoring system and greatly add to the cost, as all wiring including connector pins must adhere to this requirement. The cost for long runs of platinum wire can add up quickly.
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Standard Types
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Type B thermocouples use two different mixtures of platinum and rhodium, while type C uses similar mixes of tungsten and rhenium. These types are useful for very high temperatures from about 2000 to 4000 degrees F.
Type E thermocouples uses chromel and constantan, type J has iron and constantan and type K uses chromel and alumel. These types have an operating range of about 200 to 2000 degrees F.
Type N devices have nicrosil and nisil and types R and S use different mixes of platinum and rhodium for one side of the junction and platinum for the other. These sensors are used for about 1200 to 2500 degrees F.
Type T thermocouples are made of copper and constantan, which is useful over the lowest temperature range of -300 to 600 degrees F.
Color Coding
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There are several standards for the color coding of wire insulation, and they differ from country to country. In the U.S., J type thermocouples have a black outer jacket with white and red wire insulation, type K uses a yellow jacket with yellow and red wires, type T has a blue jacket with blue and red wires, type B is gray with gray and red insulation and types R and S are green on the outside jacket with black and red wire insulation.
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