Microgauge Thermocouple and its Assembly Technique

"Thermocouples are the most convenient way of extracting temperature from pretty much any environment. In the past, it wasn't really appreciated as a temperature measurement method due to its non-linear response to temperature variation. However, thanks to the advancement of microcontrollers with precision voltage transducers, this is no longer the case."

Thermocouples have several advantages over other temperature measurement techniques such as thermisters. Its primary advantage is the fact that it is extremely easy to make. Just tie up two different metal wires together, doesn't matter how you do it as long as two wires are maintaining contact. It also sticks out from other competitors due to its rugged performance. Its temperature range is not only wide enough to cover practically any engineering/scientific applications, but its performance is independent of the environment in which it is going to be installed. Whether it is vacuum, liquid submerged, or high pressure/temperature systems it will not let you down. The only downside is in its non-linearity and a requirement of cold-junction reference voltage, but, common, we live in the 21st century and it has been a long time since all those other peripheral devices became remarkably cheap and accessible. But one critical downside, this time rather unavoidable, if you say, is that unnecessarily thick thermocouple wires do make the reaction time of thermal measurement longer, degrading its overall performance in case faster data sampling is needed.

That's why there are micro-gauge thermocouples available online. They come in seriously sick diameters. The finest gauge I've seen out there has 0.001in. If you have no idea how thick that is, I have some pictures below which is rated at 0.002in. According to Omega, there reaction time over the range of 100 Celsius under still air is about 3 seconds! If the thermocouple is submerged into more highly conductive area, the response time will be easily under that limit. Under liquid water, its response should be almost instant.

This thermocouples, however, are very fragile and application of slightest tension will break the wires: a catastrophic failure for the purpose. I am going to talk about how I went around this problem by reinforcing its tensile strength by using Kapton tape. It is therefore completely vacuum compatible. Talk about the vacuum, it is truly the only measurement you can make in a cryogenic vacuum environment that is not limited by the domination of radiation heat transfer mode. 

First of all, wear your lab gloves. Let the sticky side of Kapton tape exposed and pointed upward. To do that I used a can duster as a support for one end as shown in the picture. 

Then stretch the tape to the length of micro-gauge thermocouple wires you have. Make sure that the sticky side is always pointing upward as you do. Place a support underneath so that it is not hanging. In the picture I used two can dusters to hold the tape and one box underneath to support it. The other end of tape is still attached to the tape roll. I didn't cut it.

For a quick comparison, here is a picture of 0.002 gauge thermocouple with a toothpick. It is a way finer than a human hair. 

Due to its almost non-existent tensile tolerance, the packaging sticker should be removed using a tweezer. A fine-tip tweezer works better. One thing I noticed is that when you peel it, you have a less chance of deforming the wires if you peel the sticker from the opposite side of the junction. I did it in backward in the picture so be careful.

Once the wires are detached, gently lay them out parallel onto the sticky side of tape. Make sure that the wires are as parallel as possible. This is important since you have less chance of trapping air later. When you do this, make sure that the thermal junction is not broken. It is also the only part of the wires that should not be attached to the tape. In the picture below, you can see wires already layed out parallel on the tape.

Now you have the entire thermocouple wires stuck on the tape in parallel (except the junction), now is the time to grab a regular gauge thermocouple wire of the same type to "reinforce" its end to "ease" the connectivity issue. Strip the wire and shape a hook at each end as shown in the picture below.

Now, connect the red wire to red wire and yellow to yellow (in case its is K-type). There is a trick I used to do this. The first connection is fairly easy. Make a loop out of a micro-gauge wire (but no need to tie it, just hold the end steady with tweezer after you make a loop) and use the hook from matching type wire with the looped wire to "hook" the loop. Gently turn the wire to wrap the micro-gauge wire around the hook. Once you make a decent amount of turns (~10), close the hook with tweezer so it won't escape. After this is done, it's time to stick the regular gauge wire to the tape and lift the other hook up orthogonal to the tape as shown in the picture below.

From now on, it is fairly easy. Grab the end of the rest fine gauge wire and gently wrap around the newly bent hook about ten times. Once this is done, collapse the hook so the wire cannot escape. Force the hook back to its original position so that it can stick to the tape again. 

Make sure that the wires are not touching each other other than at the thermal junction.

This is by far the most straight part. Cover the sticky end of tape with another layer of Kapton tape. Start from the opposite end (from thermal junction) as shown in the picture. When you are done covering all the tapes, you are done. But make sure that the thermal junction is not covered by the tape. The picture below shows a finished thermocouple system.

If you doubt that the wires will slip and break in the tape, you can fold the wire to prevent this.

Measure your thermocouple voltage to see if they are functioning properly. 

This picture shows just one example of how this can be used. I silver-pasted the thermal junction to a silicon chip so I can make a temperature measurement with minimum interference by thermal mass and radiation effect under cryogenic vacuum environment. 

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