High Temperature Thermocouple Wire - Page 1
Forum section: electronics / projects

author: bostonman
author_rank: Super Contributor
timestamp: March 14, 2022, 04:29:47 am
content: I have some k-type thermocouples and recently I wanted to measure around 325 degrees C. Not thinking, and because my temperature meter is rated for somewhere around 1372 degrees C, I thought the thermocouple was rated for that too (or something a bit less).Needless to say, the insulation melted.Now I'm thinking of buying a role of high temperature k-type, but have two questions.The existing thermocouple wire has a bead of solder on the end. If I attempt to use solder and touch it to high temp, it's going to melt. Do I need to add solder or can I just twist the wires at the end (or what is the common practice for measuring high temperatures)?Does anyone have any recommendations on a quality high temperature wire? I saw various stuff on Amazon for relatively cheap, but wanted to check if anyone had input.On another note, I'd also like a way to mount the thermocouple to things like a hot plate or a soldering iron so I can measure temperature. Does high temp tape exist that may work? I've tried using Kapton, but it doesn't stick after about 200 degrees C.

author: magic
author_rank: Super Contributor
timestamp: March 14, 2022, 05:22:47 am
content: I have modded one cheap-ass thermocouple: removed the original insulation, applied glass fiber tubes except for the final 1.5cm near the tip. This thermocouple was used to roughly monitor temperature of small objects suspended on the tip and held in fire. It still appears functional in boiling water test (I think it read at least 98°C last time I tried).

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 14, 2022, 05:51:09 am
content: Should be a weld bead, not solder??You can twist, but it's preferable to weld. If you can arrange a modest to high current, this might be doable without much work.Example: kinda sorta spot weld it, between the faces of two graphite rods. Can be pencil leads (soft). Should take somewhere north of 10A and not many volts. Worst you can do is heat the pencil leads so hard, they glow red hot (you may want to remove them from the pencil body first). Use PPE. Heavy gloves and a heat resistant work surface will suffice.Arc welding is also feasible. Pencil lead once again, only need one this time, and higher applied voltage. Somewhere above say 5A and 30V should be enough (depends on available voltage; lower current is possible at higher voltages). Use a series ballast resistor (or inductance for AC). Use the thermocouple leads themselves as one terminal, and graphite for the other. Touch to the thermocouple tip, then drag away, quickly at first, slower until it's arced just long enough to do the job. Avert your eyes from the arc flash, it's too bright to see anything anyway. (This should at least be less exposure than needed to sunburn your eyes, but, why risk it, eh?) Try it a few times until you see a glowing metal bead cooling off on the end. Can always trim back the wire and try again. Or melt the bead off entirely, hah... Again, do this over a heat resistant surface. A clean steel table will do (but be careful where you're arcing), or unglazed tile, say.This can even be done with mains AC (carefully!), or a high voltage supply if you're into that sort of thing (something capable of fat enough arcs to melt wires, obviously).Or spot welding with a capacitor discharge, yet another option. Probably need on the order of 25V and some 10mF+. Not too hard to find, a small "computer grade" cap will fit that. Electrodes don't need to be graphite, can be whatever. Basically dump enough current that the wire end exceeds its fusing current, and it'll melt and ball up into a bead.None of these methods are fantastic, because it's done in air and the weld bead will be full of oxides (and maybe carbides too). It won't be very strong. Fortunately, it's not exactly a load-bearing part! Clean it up shiny, keep strain relief around the tip, and it'll last a long time.Polyimide works fine up there, but you need silicone adhesive. Guessing you had rubber or acrylic which decomposed. Or use epoxy, the high temp stuff, not that 5 minute junk. (JB Weld is probably the hottest commonly available formulation.) Obviously, epoxy is not such a temporary solution. For higher still, dunno, fiberglass wrappings and alumina or other refractory cements probably?If you want quality, Amazon is the last place you should be looking?I'm not sure offhand what suppliers to go to for high temperature stuff (furnace and industrial sorts of things say), but McMaster Carr has no shortage of just about anything remotely mechanical. e.g.https://www.mcmaster.com/9251T93/Have also seen ceramic fiber insulated thermocouple wire for industrial furnaces; must not be cheap. Had scrounged a small spool of the stuff years ago but lost it somewhere along the way... shame.Guessing you're on a rather tight budget, hence the Amazon drop? Offhand I see brand name (Omega is probably the biggest) with ceramic covering and short lengths (<1m) in the $30 range.https://www.farnell.com/datasheets/2339597.pdfNeedless to say, anything McMaster will be out of your league too then...Hmm..."Nextel covering", the hell is that. Going to guess they aren't wrapping them with phones. Ah, a 3M product, alumina (or with silica and boria) fiber. Have seen (and used!) ceramic wool for many years but somehow hadn't seen the straight thread kind before. Jesus, up to 406ksi tensile, too. Use it straight-up, use it in plastic composites (higher strength, lower dielectric loss than glass fiber!), use it in ceramic composites (basically clay molded around fiber, or other more technical-ceramic ), yeah, that'll do it.https://multimedia.3m.com/mws/media/1440890O/3m-nextel-ceramic-textiles-fibers-and-composites.pdfTim

author: Berni
author_rank: Super Contributor
timestamp: March 14, 2022, 06:32:08 am
content: There are thermocuples with special high temperature fiberglass insulation. Some of them might get discolored at high temperature but should still work fine. Main thing is the insulation doesn't melt.As others have said thermociple wires are welded together, so you need to get it >1000 °C before that will let go. If you try soldering these wires with regular 60/40 solder you will find that it doesn't stick properly. This is why terminating thermocouples is better done with crimping.No idea what the best way to weld these together is. Capacitor discharge is certainly one way, tho this proabobly won't give you a nice round ball on the end. One thing to try in my opinion is a high voltage supply to melt them. Twist the wires together, connect both wires to ground, then bring a high voltage wire close to draw an arc to the end of the twisted pair of wires. The arc should melt the end into a nice round ball, at what point you can pull away the high voltage wire and let it cool. You might be able to get a cleaner result under inert gas (In case you have a bottle of nitrogen or argon handy)

author: magic
author_rank: Super Contributor
timestamp: March 14, 2022, 06:43:40 am
content: Quote from: Berni on March 14, 2022, 06:32:08 amUnless you use dilute phosphoric acid flux.But it's still crap because it melts easily.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 14, 2022, 09:25:50 am
content: Twisting the wires (mainly for mechanical stability) and than a small spot weld from a capacitor discharge (mainly for reliable electrical contact) is possibly.The DIY spot weld tends to be not that mechanically stable.Another option is to spotweld both wires to a washer (or between 2). This can help to attach the TC to the point to measure.

author: richard.cs
author_rank: Super Contributor
timestamp: March 14, 2022, 09:26:08 am
content: Not mentioned so far I don't think, but you can also use ceramic or glass beads as insulation for the hot end. You can buy special ones for the purpose, but cheap jewellery beads seem to work pretty well.

author: jonpaul
author_rank: Super Contributor
timestamp: March 14, 2022, 09:42:11 am
content: Bonjour been using K TC for decades no issues to get ready-made TC from Fluke, Omega,Fischer etc in guage #32...36 with Kapton or Téflon insulation or fiberglass wovenThe insulated TC wire is in précision TC grade and extension grade, lower quality and calibrationConnectors can be specific modifed bannana plugs, normal or miniature yellow Type K blade connectors that match most TC meters, eg Fluke.Suggest that you Avoid Chinese vendors.Formfabricating your own TCs, such as to twist or solder the wire, they will be unreliable and uncalibratied, and the wires will become oxidized with heat and loose the TC connection.You cannot effectively solder Chromel....Alumel, the alloy cant take solder and the calibration is wrong.The only accepted TC fabrication method is welding, eg using a small capacitive discharge or tack welder.In 1980s we made 100s from a spool of Oméga TC wire.For just a few TC, easier to just buy them ready made,$5...25 ea depending on insulation legnth and connectorsBon courageJonKon

author: jpanhalt
author_rank: Super Contributor
timestamp: March 14, 2022, 10:55:39 am
content: Just adding to the conversation on building a TC... I have a decent battery tabe CD welder. It only sort of worked, and the weld was not reliable. Maybe my wire was too thick. It was the standard 24 awg wire, Teflon and Kynar(?) insulated, that came with a Fluke meter.* In the chemistry lab, we had hydrogen torches. They worked fine, but I no longer have access to them.I ended up using my TiG welder. My solution for holding the twisted leads and ground was a copper plate with a roughly cut slot (attachment 1). Ground was to the copper block on which it sat. The arc was started on the copper and moved across the end of the TC leads. Used relative low current and HF for starting. The twist doesn't need to be much. Its purpose is just to hold the wire closely together. Hold the wires very close to the end. Most of the exposed wire ends up in the bead. Examples from my practice welds are shown (attachment 2).*I got my wire from a science surplus dealer in Idaho. It is identified as Omega TFE-K-24-SLE thermocouple wire. Cost: $24 for 25 feet.

author: bostonman
author_rank: Super Contributor
timestamp: March 14, 2022, 01:33:19 pm
content: I'll go through these responses more thorough tonight.The only thermocouples I've dealt with personally are the ones that came with this meter (in the past I've normally used thermistors at previous jobs and maybe some thermocouples).Recently I found one thermocouple was failing and seemed like a bad connection. After cracking open the yellow plastic plug, I realized I must have turned the plug over time and caused the wires to twist so they shorted at the plug end (a silly design if you ask me). I have a bag of yellow plugs, just need the thermocouple wire, so thought buying a roll would be practical.A roll would allow me to cut to length and probably save on cost since I'm not buying made up thermocouples.Once I realized my temp measurements could exceed solder melting point (I assumed that was a bead of solder and not weld), I began questioning how I'll make these. Again, I'll read through these responses deeper later, but seems buying pre-made or devising some quick spot weld setup are the two solutions.

author: Kleinstein
author_rank: Super Contributor
timestamp: March 14, 2022, 04:19:01 pm
content: Thermocouple wire can be surprisingly expensive - it can be worth it if you really need a lot or rather long wires. One would usually need 2 spools as one needs 2 alloys. The only exception may be the few types that use copper for one side (e.g. type T). For the copper constantan pair, the constantan for a thermocouple has different requitements to the constantan sold for resistors. So the easier to get resistor wire would cause added uncertainty for a thermocouple.Ready made TCs may still be a cheaper alternative, as one can get them in smaller quanteties and the more frequent offers also help to find a good price.With bare wire one can be lucky and find a spool / pair, but less choices there.Edit: For some uses there are also ready made TC probes with a stainless steel (or Hasteloy) outer shield. These are a bit more expensive (e.g. $20-$40), but can be very convenient as the TC is protected and isolated and can still be quite thin (down to 1 mm).

author: jonpaul
author_rank: Super Contributor
timestamp: March 14, 2022, 07:17:13 pm
content: Klinestein, Omega, etc all supply ready-made TC wire and extension wire, in all typesThe paired insulated TC wire is colored code and available in various gages and insulation.One adds the required connectors and tack welds TC junction.Fluke, Oméga, ThermoFischer offer the wire (100' spools) as well as finished TC with connectors.I have never seen unpaired TC wireJon

author: jpanhalt
author_rank: Super Contributor
timestamp: March 14, 2022, 08:22:43 pm
content: Out of curiosity and tired after doing my taxes, I looked up some flame temperatures:Oxy-acetylene 6300°FOxy-MAPP 5300°FOxy-hydrogen 5100°FThus, oxy-MAPP should work fine and is available at home improvement stores. Since MAPP (and acetylene) can be smokey, I would keep the flame lean. It will not be as quick as an electric arc, but a little heat sink as I used with the slit copper should work to protect the insulation.Also Googled TC wire in bulk. About $0.50/ft in 100 ft increments with teflon and $2.00 per foot with very high temp ceramic insulation. Not too bad for a DIY.Omega connectors are expensive, but also use chromel and alumel for the contacts. Much cheaper ones just use plated copper. It probably doesn't matter that much, but the Omega female connectors are very nice compared to what one can DIY. That is, a couple of years ago, I bought a bunch of different connectors for flat pins, and nothing compared to the Omega ones. If interested, I can share those results.

author: bostonman
author_rank: Super Contributor
timestamp: March 15, 2022, 12:58:10 am
content: Hmm now I'm uncertain which direction to go. I thought buying just the theromcouple wire would be better because I can make new ones should they break.Does an easy way exist to spot weld?Otherwise, maybe I'll take the advice and buy some directly from Omega or another reputable company. I don't need anything over about 450 degrees C, but, because I had considered a spool (or some length), figured I'd splurge and buy higher temp.

author: jpanhalt
author_rank: Super Contributor
timestamp: March 15, 2022, 06:21:38 am
content: I suspect you can get an inexpensive oxy-MAPP torch (e.g., Bernzomatic) cheaper and quicker than you can make or buy a sufficiently powerful CD welder. Moreover, that torch has other uses.Spot welding is not hard to do, but the equipment is a bit more expensive. An inverter-type HF spot welder might work. But is the experimentation time and risk of failure worth it?Here's an Amazon oxy kit for $30. It's a knockoff of the branded kit I have for miniature welding/brazing.https://www.amazon.com/Jewelry-Welding-Soldering-Acetylene-Portable/dp/B07PP7GTRB/ref=asc_df_B07PP7GTRB/?tag=hyprod-20&linkCode=df0&hvadid=532326880957&hvpos=&hvnetw=g&hvrand=17146790166188653518&hvpone=&hvptwo=&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=9015391&hvtargid=pla-1436688001838&psc=1

author: Berni
author_rank: Super Contributor
timestamp: March 15, 2022, 06:30:07 am
content: By the way you can make a high voltage supply capable of melting some wire out of a TV flyback transformer coupled with a simple two transistor ZVS oscillator to push some 100W into it.You can also draw an arc by interrupting any high DC current. Found that by accident once when testing a electronic load by feeding it with a big lab supply. I think it was feeding it something long the lines of 40V at 20A. I noticed one of the big crocodile clips was getting pretty warm, so i tried to wiggle it a bit, so that it might find a better bite with lower resistance. What happened instead is that the clip slipped off and lost connection, in the process drawing a pretty big and blinding arc accompanied by the characteristic DC arc hissing sound. It scared me enough to just drop everything and jump away. Only damage was a slightly melted corner on the clip. I know it sounds like a stupid thing to do, i was just expecting it to make a slight crackle when loosing connection (that you tend to see at 12V), never expected this thing to turn into a full on arc welder just from a clip slipping off. So if you actually wanted to make a welder out of this you can probably do it with a large enugh stack of batteries and a big enough resistive/inductive load.

author: BradC
author_rank: Super Contributor
timestamp: March 15, 2022, 06:31:23 am
content: Quote from: jpanhalt on March 14, 2022, 08:22:43 pmProper MAPP has not been available for years except in the odd rare old stock goldmine. The stuff they sell now is predominantly propylene. Hotter than propane, but not within cooee of proper MAPP.

author: jpanhalt
author_rank: Super Contributor
timestamp: March 15, 2022, 07:18:25 am
content: Quote from: BradC on March 15, 2022, 06:31:23 amInteresting. Early Fall, I redid a bunch of 1-1/4" copper plumbing using my Bernzomatic and MAPP cylinders I had bought years ago (I had three and two were unused). As I got near the end of the job, I bought a replacement spare. Same yellow color, and I didn't read the label carefully. I mistakenly thought the "Pro" in MAP Pro was just sales puffery (i.e., "professional"). It's not. Oh well, M cylinders of acetylene are still available at my gas supplier and are not that expensive. I typically get a larger size, though for my shop.

author: jonpaul
author_rank: Super Contributor
timestamp: March 15, 2022, 10:29:16 am
content: The torches will burn the wire insulation and the high temp may damage the TC accuracy. Have never seen this used.100W Tesla Coil is not the type of device for TC welding.Most TC welders are tacking with a cap discharge from HV DC source, and heavy electrodes with tungsten tips.You may get a bond but damage the wire insulation or TC accuracy.For just a few buy ready made form USA mfg like Omega or Fluke, $5..25 on epay with connectorsJon

author: jpanhalt
author_rank: Super Contributor
timestamp: March 15, 2022, 11:03:07 am
content: 1) Oxy-hydrogen torch was a standard method up until the 70's. Don't know about later. The (non)effect of intermediary metals is well described. That's why solder and brazing also work, if you can get a decent joint.2) I agree one avoids overheating as it affects insulation. That's why a very hot flame/torch is used.3) Image attached:Top = Fluke TC (Teflon is darkened by use over many years in roasts)Bottom = DIY with TiGBoth work and read the same within accepted error. You can see the heat-affected zone is only halfway to the insulation in the DIY probe.4) There is no debate that buying high quality probes is an option. Buying instead of making or fixing is always an option.

author: nfmax
author_rank: Super Contributor
timestamp: March 15, 2022, 11:52:54 am
content: You know you can just buy ready-made, mineral-insulated K type thermocouples with ready-fitted connector off the shelf, no?https://www.labfacility.com/mineral-insulated-thermocouple-with-miniature-plug-iec-types-k-j-t-n.html

author: jonpaul
author_rank: Super Contributor
timestamp: March 15, 2022, 12:07:15 pm
content: Omega Engineering, Fluke, Fischer all stock wire, finished TC, connectorshttps://www.omega.com/en-us/temperature-measurement/c/temperature-wire-sensorsepay $5..30'eaj

author: bostonman
author_rank: Super Contributor
timestamp: June 23, 2022, 03:00:17 am
content: Due to being in need of a few basic thermocouples, I ordered two typical temperature rating types off Amazon. The same blue thin wire that came with the meter I own, and a braided one that is suppose to be around 900 degrees C; both with the yellow blade terminal connectors.My meter is four-channel and the original probes, when all plugged in simultaneously, would read the same to within 0.5 degrees F (I attribute this to meter error because it's not a top line meter, probes picking up slightly different temps, etc...). Also, if I plugged one in backwards, the temperature would be obviously wrong.Now that I got these and tested one of each. The plus and negative sign are tiny and stamped into a small divot. Having not initially seen the signs, I plugged them in both ways and the temp didn't seem too far off when compared to the (only) original good one I have left. Also, when plugged in correctly, the temperature is quite a bit off when compared to the (only) original one; maybe about 4 degrees F.With my original probes, I compared them to a wall clock hanging on my wall that shows the temperature, and all four channels matched the wall clock, so I know my original probes were good and the meter is good.Does a possibility exist that a k-type probe can be made wrong/cheap and show wrong temperatures? I think these thermocouples are 1m longer than the original, does length cause issues? Looking at the manual, I can 'calibrate' the meter, however, I need a known source and I'm reluctant to tinker with the calibration.Also, I'm baffled why a noticeable difference doesn't exist when plugged in backwards.

author: magic
author_rank: Super Contributor
timestamp: June 23, 2022, 06:10:57 am
content: A thermocouple is a voltage source proportional to temperature difference between its two ends, with some series resistance.If the meter has high bias current (one crappy Chinese meter I tried did) then an offset error may occur with long and thin wires, which doesn't depend on insertion polarity.If the two wires are not at the same temperature in the plug an additional error will occur, which depends on polarity. The connectors must reach thermal equilibrium on both sides of the connection.If wire materials are wrong, a gain error may exist.Otherwise, I don't think there is much else that can go wrong.You should be testing with significant thermal gradient across the wire, like boiling water or ice.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 23, 2022, 11:23:07 am
content: Easy polarity test: grab the end in your hand, if it's rising, it's the right way around.Unless it's stupid hot out and ambient is same as hand temps, but, you get the idea.Also needs to be the same type as the meter is calibrated for, which is a good place for ice/boiling water to come in. All TCs will read near ambient, for [what should be] obvious reasons [now?], but the cold junction compensation (meter temperature) and scale will only track when matched.Tim

author: bostonman
author_rank: Super Contributor
timestamp: June 23, 2022, 01:06:22 pm
content: I'll do more testing later today.At first I wanted to confirm the connectors were correct and assumed they'd read fine. The first obstacle was not seeing a polarity indicator (I don't know why the manufacture doesn't make the plus sign a bit more obvious - even the old ones have a tiny plus sign - anyone who doesn't know better wouldn't catch on).I'll try the suggestions later today and update. As for the terminals being equal temperature, this could have been an issue since I went from outside to the basement. Also, I did want to submerge the ends in water so to provide a "constant" temperature.What happens if the wire end is submerged and I hold the thermocouple wire at some mid point, does holding it cause it to sense a different voltage due to the heat from my hand or is dependent on the wire ends (I assume the answer is that I'll change the temperature by holding it at some mid point).

author: mag_therm
author_rank: Frequent Contributor
timestamp: June 23, 2022, 01:19:10 pm
content: Type K welding recipe.Oxy Acetylene Size 00 tipAcet pressure ~ 4 psi, 50 kPaCut back t/c insulation to expose bright clean wires, and twist for about 1 turn.Clamp so that the twist is exposed, protruding horizontally ready to weld.Set flame as normal, then reduce oxy for excess actet feather about 10mm (3/8") long.(That is to get the flame temperature down)For the weld, don't allow the t/c to get in the feather, use the edge of outer cone (*) about 25mm away.The 2 wires will melt into 2 blobs, follow them with the flame,which will then hopefully fuse them into one as the blobs melt back into the twist.Then immediately remove the flame as there is no possibility to work the shape further.Starting with 0.2mm wires, I get an oval blob about 0.7mm by 1mm(*) The outer flame cone contains mostly CO2 and H2O. The inner flame has C and H which can cause metallurgical problems.

author: LaserSteve
author_rank: Super Contributor
timestamp: June 24, 2022, 01:10:31 am
content: McMaster Carr sells beautiful stainless steel jacketed probes with MgO or other ceramic insulation.That Said, Our commercial TC welder at work consists of 24,000 uF of low ESR capacitor, an internal 1-24v adjustable cap charger, and a metal can power scr. Triggering is provided by a string of closely spaced gate pulses to .reduce dud welds. Wiring to the electrodes is perhaps #8 in a very flexible welding grade flex cable.One discharge wire (Like Litz inside) is welded to a pair of pliers, the other to some sort of moderately porous graphite composition block that is almost ceramic like in composition.When I opened it up, my first thought was why didn't I just build this.However whatever is mixed with the graphite and sintered is worth having. Especially if your making 100 identical TCS a year like I need to do in one batch.You can get a good weld with a big copper stud as "side B" instead of the composition. You just have to clean it fairly often.I usually tightly wrap a few turns of 18 ga chromel tightly around the copper or alumel, snip the inner wire flush to the outermost coil turn, grab both conductors with the pliers. Take the exposed final turn with inner, press it firmly onto the block and hit the fire button. I'll then hit it with a second pulse. On good welds you do not hear the audible snap on the second , but you will see the EMF shake the welding cable.If set correctly I end up melting and fusing 1/2 turn to the inner.Beading my old way.Hf start TIG welder set for short pulse after the Argon comes on will nicely bead type T and Type K. Have not tried it with other pairings. Clamp the twisted pair into a vise.. Make the bench vise one side of the tig circuit. If needed line the vise jaws with copper plates.. Hold the Tig head about 1/16 inch above the new junction. Hit the foot pedal. Flash Bang Click puff of oxidedone. Hf in this case means the welder has the little Tesla coil built in to start the arc without contact.You will never see me use the cheap plated connectors.Steve

author: bostonman
author_rank: Super Contributor
timestamp: June 24, 2022, 02:33:14 am
content: Interestingly enough, after I stretched the new thermocouple wires, and compared them to my original, all were within about 0.5 degrees F.Initially I kept the wire twist on the themocouple before should I need to return them due to the connector not fitting or whatever the case. Either (as previously stated) the leads that plug into the meter need to settle to equal room temperature, or, leaving the wires in a twisted state causes a difference in voltage.QuoteI actually have a bench top mini welder on my wish list to tackle small projects. Some time in the future I plan to research a good one (and not too costly) to purchase.On a side note, my original thermocouple wires were failing and I thought it was a poor connection, however, the connector was glued. Eventually I got frustrated and cracked open one connector. What I discovered was due to 'turning' the thermocouple wire over time, the exposed metal wrapped around each other causing a short at the connector junction.These are the two types I bought:https://www.amazon.com/dp/B00OLNZ6XI?psc=1&ref=https://www.amazon.com/dp/B0967ZS74R?ref=ppx_yo2