Thermocouples have insulation over the conductors and an outer jacket

All thermocouples are made up of two conductors, each of a different metal alloy. These two conductors must remain electrically separate from each other until they reach the ” hot junction” where the two conductors connect together. This junction is where temperature is measured and the voltage (typically less the 50 mV) that junction produces is a function (a nasty mathematical function) of the temperature. It is critically important that the two conductors remain separate from each other back to the measuring instrument, just like any electrical circuit.

This is where the insulation comes into play, and if this were a pair of wires used in a device that only reaches normal room temperatures, most any standard insulation used on wire would be fine. However, these two conductors are typically exposed to temperatures near 300 ºC (572ºF) in many soldering processes, so a much higher temperature insulation must be used.

There are two places the insulation is used: 1) over each of the two conductors, and then 2) a jacket to wrap the two insulated conductors together. The two insulations are often called out as one material “over” another. So if the conductors are insulated by Teflon and the jacket is also Teflon, then the thermocouple wire insulation is called Teflon (jacket) over Teflon. Thermocouple wire can have a different jacket over the conductors, but the two conductors are almost always insulated by the same material, even if not the same colors.

The colors of the insulation have meaning, and of course there are standards for the colors that vary from one country to the next. That will be the subject of another blog. For now, since most solder process profiling equipment uses type “K” thermocouples, the colors according to ANSI Standard ISA-MC96.1-1982, here in the US, are yellow for the positive conductor and red for the negative. The jacket color has meaning as well, where the jacket is made from a material that can be colored. Brown typically refers to a “thermocouple grade” of alloy conductor material, meaning you can use it to make a thermocouple at any point along its length simply by cutting it and welding the two conductors together. If the jacket is yellow (for type “K”), the wire is of an “extension grade” which means it’s only good at room temp for extending the good thermocouple grade wire over long lengths.

Here are some typical insulation types used in most soldering processes:

Natural Teflon shows through the color of the wire insulation inside

Natural Teflon® (AKA: PFA, TFE, PTFE, T) – All forms of Teflon are about the same. They were produced because of their ability to extrude, form or take a color. Most all have the same basic
characteristics. The natural Teflon jacket is clear and hard to see, but it’s there holding the two conductors together. If this were a larger size wire, the jacket would be brown, to indicate it is real thermocouple grade wire.

Max Temp: 260ºC (500ºF)

Advantages:

 Smooth clean and neat

• Can be colored for easy ID
• Easy to strip insulation
• Can be applied to most any size wire
• Low cost

 Disadvantages:

• Some solder process can reach the temperature limit
• Burned Teflon is said to be bad for you
• Very low negative (-) on the triboelectric series

Kapton insulation over thermocouple wire

Kapton® (AKA: Polyimide, K) – Natural Kapton is brown or amber colored and is ofter spiral wrapped and fused together as a jack around the two conductors.

Max Temp: 316ºC (600ºF)

Advantages:

• Smooth clean and neat
• Can take most an solder process temperature

Disadvantages:

• Very stiff and likes to tangle
• Cannot be extruded onto wire smaller then 30 AWG
• Very difficult to color for identification
• Costs more then Teflon

Fiberglass insulation over thermocouple wire

Fiber Glass (AKA: Glass, or Glass braid, G) – Fiber glass is braided onto the conductors and over all as a jacket. The glass braid is sometimes saturated with a material that helps prevent the braid from fraying. Max Temp: 482ºC (900ºF)

Advantages:

• No problem taking solder temperatures
• Very flexible
• Can be colored for identification
• Low cost

Disadvantages:

• Can fray and look bad quickly
• Glass fibers can break and get into things
• Harder to strip the insulation
• Color fades to brown with heating
• Glass can be damaged from wear or over bending

Stainless steel braid over glass insulated thermocouple wire

• Can be added to most any insulation type
• Can be used to bundle several thermocouple pairs
• Adds strength and durability
• Creates electrical shield when properly grounded

Disadvantages:

• Makes the thermocouple much stiffer
• Adds to the cost
• Does not increase temperature limit
• Can contact the thermocouple if not dressed at the ends properly
• Sharp barbs at the ends will find their way into you fingers

Conclusion and Recommendation

For most soldering process I use the fiber glass insulation. If you take care not to kink or tight bend them, they will last a good long time. It will take the temperature, even temperatures required for lead-free soldering, no problem.

Teflon may do fine as well, but you run the risk of the insulation getting soft at high temperatures. This is fine as long as you don’t bundle them too tightly. Under pressure, the wires will press through the Teflon insulation and begin to short from one channel to another. Kept apart and treated well they will work fine.

Kapton will last a long time in most solder applications, but I can’t stand they way it holds a shape and wants to coil back up, making it easy to kink. They are limited to no less then 0.010″ diameter (30 AWG ) so they will not work where smaller sizes are needed. They work do well in the baking/food industry.

The stainless over braid makes for a very durable thermocouple. However, it ends up too big for soldering process. These tend to get used mostly in the paint and powder coating industries, where they get rough treatment and a lot of paint powder or spray. You can get thermocouples here from ECD.[poll id="2"]