Thermocouples are made when two conductors (wires) of different metals (alloys) are connected together to form a “junction.” This junction, or connection between the two conductors, is typically made by melting the two conductors together using a torch or a flash welding process. The size of the thermocouple is typically specified by the size of the two conductors, however, rather then the size of the junction formed where the conductors are melted together. The junction size is typically 2.5 time the wire diameter or less. Since the junction can vary somewhat, it is not the best way to specify the thermocouple size. So we us the wire size. Below are several of the most common ways to specify the size of a thermocouple:
Gauge (American Wire Gauge, or AWG)
Wire gauge is common in the US and has meaning in the electronic and electrical fields. It’s handy because it keeps you from having to say (or write) long decimal numbers like 0.005 inches in diameter when you can just say 36 gauge. However, it’s upside-down in that as gauge number goes up, wire diameter goes down. There is a ratio between the gauge size and the diameter in inches:
Wire Diameter (inches) = 0.005 * (92^((36-AWG)/39))
As messy as this is, we still use AWG to call out thermocouple wire size. Here is a table of some common wire gauge sizes and their diameters in inches:
We also size thermocouple wire by the diameter of the conductors. Each of the two conductors will be the same diameter, of course. See the above table for typical conductor conductor diameters use in the US.
Square Millimeters (mm²)
Most other countries in the world use what’s called cross sectional area to specify the wire size. This is nothing more then the area of the circle formed by the conductor if you were to look flat at the end of the conductor. You know the area of a circle is:
And since the rest of the world is metric, this area is in millimeters (mm²). Common wire sizes are in nice round mm² numbers which means common sizes do not match up well with the AWG sizes. The table below shows the mm² sizes for the AWG gauge sizes:
The most common thermocouple wire gauge sizes used for reflow or wave soldering in the US are: 30 and 36 AWG, and some 40 AWG
A common size in other countries is 0.03 mm², which as you can see from the table above is neither 30 nor 36, but real close to 32 AWG. The method used to specify a thermocouple size really depends on where (what country) you are buying it from. Although we can all convert, and most make equivalent sizes, what you will hear on the street will be AWG size in the US and area in millimeters most anywhere else in the world.