There are many different kinds of thermocouple wire insulations. Is there an ESD concern using these?

To understand the ESD threat, one must first understand that all insulators can be an ESD threat, yet electronic circuits cannot function without insulators. Electrons are what “charge” a material and conductors allow the charges to be carried away removing the potential difference, NOT the electrons, but the difference in the number of electrons between the conductors. In other words, conductors let electrons even out so current (electrons) has no reason to flow and risk damage to sensitive parts. Insulators can build up electrons and because they are insulators, they stay there and huge potential differences can build. One source of this build up is caused by mechanical moving of different materials against each other, called “triboelectric” charging.

Triboelectric charging takes place when two materials come together, or rubbed together and then are separated. “Tribo” means to rub. This process of charging is caused when one material loses electrons, thus making it more positive, and the other gains electrons, thus making it more negative.

The triboelectric series is a list that ranks materials according to their propensity to gain or lose electrons. Steel is near the middle of the list and these materials do not show strong tendency to behave either way. Note that the propensity of a material to become more positive or more negative after charging has nothing to do with the level of conductivity of the material.

These tests are not exact science and are not easy to do, so different tests sometimes yield different results in determining the placement of a material on the triboelectric series. The triboelectric series shown in the table is a product of a merging of several triboelectric series I found on the web.

Conclusion and Recommendation

The charging that can occur with these materials only happens when they are different and somewhat distant from each other in the series. So materials near the middle of the series will help generate a charge on those either positive or negative, while somewhat uncharged themselves. Also remember, the charging does not happen unless there is some mechanical action, like rubbing, between the two. That is why the attraction between a balloon and your hair gets stronger with rubbing.

These materials by themselves are threat unless they are in the presence of others at opposite ends of the series and there is mechanical action between them. So don’t run out and get rid of all the Teflon or Kapton tape. Just know that good ESD practices should be employed when working with these materials. And keep this at opposite ends of the series away from each other. 

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"]

The main factors affecting thermocouple response time are thermocouple bead size and the conducting medium including attachment method.

Thermocouples response time is measured as a “time constant.” The time constant is defined as the time required for a thermocouple’s voltage to reach 63.2% of its final value in response to a sudden change in temperature. It takes five time constants for the voltage to approach 100% of the new temperature value.

Thermocouples attached to a heavy mass will respond much slower than one that is left free standing because its value is governed by the temperature of the large mass.  A free standing (exposed or bare wire) thermocouple’s response time is a function of the wire size (or mass of the thermocouple bead) and the conducting medium.  A thermocouple of a given size will react much faster if the conducting medium is water compared to still air.

Here are some typical time constants of various free standing thermocouple bead sizes (bead size is typically 2 times the diameter of the wire) in these conducting mediums:

    Wire (AWG)    Bead Size (inches)      Still Air (sec)        Water (sec)
           42                    0.003                              0.07                      0.003
           40                    0.005                              0.25                       0.02
           36                    0.010                                 1                            0.05
           30                    0.020                                4                            0.17

NOTE: Remember it takes five time constants for a thermocouple to reach 100% of the final temperature value so the above time constants must be multiplied by 5 to get the total time.

So the most common sizes (30 AWG or smaller) of thermocouples used to attach to surfaces or components will have fast enough response time to accuracelly measure the temperatures of reflow solder process which tend to change no faster then 5 degrees/second. If one wishes to measure the air temperature, 36 AWG is common since the air is always moving, and the chart reflects “still air” response times.

The main reason for selecting thermocouples of a specific size is to match the size of the surface or point where the thermocouple is to sense temperature. 36 AWG is a good compromise between cost, size, and strength. Much smaller and it is too easy to break. Much bigger and it may be bigger then the component or attachment point.

One other factor in selecting thermocouples is the heat source. In wave soldering, the heat is typically from the bottom of the assembly and the thermocouples are attached on top. Here the thermocouples will be cooler then bottom of the assembly causing them the sink the heat as it flows from the bottom to the top. Smaller thremocouples will reduce  the heat sinking effect.

In reflow soldering, both the top and the bottom of the assembly are heated at about the same rate causing the thermocouple wire and its bead to heat as fast as the assembly.  Some times the thermocouple can heat faster then the assembly because it is closer to the heat source and can act as a heat source to the component. This is often true where there is exposed thermocouple wire where the insulation has pulled back from the bead more then 0.5 inches. Keeping the insulation closer to the bead prevents this in most cases.

There is more than one way to specify thermocouple size.

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:

AWG            Diameter (inches)
22                   0.0253
24                   0.0201
26                   0.0159
28                   0.0126
30                   0.0100
32                   0.008
34                   0.0063
36                   0.005
38                   0.004
40                   0.0031

• Wire diameter
  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:

Area =∏*radius²

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:
AWG                mm²
22                   0.326
24                   0.205
26                   0.129
28                   0.081
30                   0.051
32                   0.032
34                   0.020
36                   0.013
38                   0.008
40                   0.005

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.

Box-whisker plots are a great way to show how stable the different thermocouple attachment methods are relative to each other.

The top dot is the Maximum value and the bottom dot is the Minimum value from the data set. The top of the box is the 75th Percentile (AKA: 3rd Quartile) and the bottom of the box is the 25th Percentile (AKA: 1st Quartile). This makes the Median, the red dot, the 50th Percentile (AKA: 2nd Quartile). Percentile is a number describing the data set such that the K-th Percentile is a number such that K % of all data values are less and (100 – K) % are larger than it, or to be more precise, at least K% of the sorted values are less than or equal to it and at least (100 – K) % of the values are greater than or equal to it.