Friday, January 17, 2014

Types of Thermocouple Wires

Thermocouples are metal junctions made of alloys with high heat and electrical conductivity, which determine temperatures by measuring heat induced electric voltage gradients. They find application wherever ambient temperature based automatic cutoffs and activations are required (such as in heaters or heat alarms). Thermocouples can be used to measure and control over a wide temperature range and as they are relatively cheap, they are popular in a variety of industrial and household applications but are generally used as thermopiles (multiple thermocouples connected in series or parallel). One drawback is that thermocouples are not very accurate for measuring differentials of less than 1K, although this can be partially compensated through instrumentation.

Types of Thermocouple Wires
Thermocouples vary by the metal alloy used in manufacturing, which determines their physical properties such as conductivity, magnetism and the temperature range that they can measure. They may also be classified based on the insulation type.

Standard Thermocouple Types
Type K is a general purpose thermocouple, and is made with chromel-alumel. It is inexpensive and has a wide temperature range from -200C to 1350C. It also has nickel, which makes it magnetic. Type E is a chromel-constantan non-magnetic thermocouple wire most suitable for cryogenic use. It can measure over a temperature range of -40C to 900C. Type J is made with iron-constantan. Although it has a relatively restricted temperature range (-40C to 750C) because of the presence of iron, it has a higher sensitivity. Type N thermocouple wire is a nicrosil-nisil alloy, and is most effective in measuring temperature ranges over 1200C. The composition also makes it less susceptible to high temperature oxidation. 

Low Sensitivity Thermocouple Wires
Low sensitivity thermocouple wires are made of platinum-rhodium alloy. Although having relatively low sensitivity, these are the most stable thermocouple materials available. Presence of platinum also makes them expensive and these are therefore only used in specialized situations such as measuring very high temperatures.

Type B is a platinum-rhodium thermocouple that can measure temperatures from 0C to 1800C. Type R and S are also made from platinum-rhodium (with varying percentage of rhodium), and measures temperatures up to 1600C.

Thermocouple wires of other alloys
Type T is a non-magnetic thermocouple made of copper-constantum and can measure temperature in the range of -200C to 350C. Type C thermocouple is made of tungsten and rhenium, with a very wide range of 0C to 2300C. Care should however be taken to not use it in the presence of oxygen and at temperatures above 260C, and are infact most suited for use in vacuum furnaces. Nickel is used in Type N thermocouples and can measure temperatures up to 1400C.

Thermocouple wires by insulation type
Metal sheathed thermocouple wires have magnesium oxide insulation and are covered with a metal sheath. Duplex insulated wires, which is the most common type, has a simple insulation that can vary by the application. Finally, a bare thermocouple wire however has no insulation.


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