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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