Questions about measuring copper coil temperature from a complete noob - Page 1
Forum section: electronics / general

author: AndrejaKo
author_rank: Frequent Contributor
timestamp: February 08, 2022, 08:45:28 pm
content: Hello everyone, I've gotten my hands on a Bosch GTC 400 C thermal camera (160*120), which seems to be mostly designed for civil engineering work, and I'm trying to use it to measure temperature of a coil which heats up due to strong RF current going through it.So the coil is usual lacquered copper, and it heats up, and when I measure it with thermal couple, it's some 50 degrees Celsius, and my "calibrated" finger agrees.On the other hand, when I measure it with the thermal camera, I get around 130 degrees Celsius. So my first thoughts were reflections and emissivity. The outside temperature is around 5 degrees, and I'm not sure what to use for emissivity, but I got the 130 degrees with 0.95, and as I decrease it, the temperature goes up.So what am I missing here? Any links to any literature on this topic would be very helpful.

author: srb1954
author_rank: Super Contributor
timestamp: February 09, 2022, 05:34:09 am
content: Quote from: AndrejaKo on February 08, 2022, 08:45:28 pmIt is difficult to guess the emissivity of such an object as it will vary considerably depending on the surface material and surface finish.The easiest way to take the emissivity variation out the equation is to paint the item under test with matt black paint. This may alter the thermal properties of your item very slightly but this variation should be much less than the potential errors in trying to determine the emissivity and overall you should be able to get a much more accurate temperature measurement.An alternative to paint is to wrap the item in a matt paper tape such as masking tape. The thermal connection from the item to the tape is more tenuous than with the paint option so you may need to allow additional time for the tape temperature to stabilise before taking an IR measurement

author: Ian.M
author_rank: Super Contributor
timestamp: February 09, 2022, 06:28:12 am
content: https://www.flir.com/discover/rd-science/use-low-cost-materials-to-increase-target-emissivity/N.B. contrary to rumors you may have heard, black sharpie (or other permanent marker ink) isn't very effective for increasing IR emissivity.https://www.w0qe.com/IR_temperature_msmts.html

author: Bill W
author_rank: Super Contributor
timestamp: February 09, 2022, 01:30:10 pm
content: Quote from: AndrejaKo on February 08, 2022, 08:45:28 pmThis cannot be an emissivity problem, it is out in the wrong direction.It might though be reflections of something around 150°C, in which case it is black paint / masking tape time as a first test.To explain the emissivity, and other possible causes.....Set emissivity to 1, or 0.99The camera is telling you that the energy coming in to those pixels is that of a black body at 130°C[I am ignoring Planck curves, energy integration over 8-14um etc from here on. Assume temperature is linear to energy (it isn't)].... or more precisely that the energy coming in to those pixels is of a black body 100°C above the 30°C internal calibration flag it has just measured.With emissivity set to 0.5, it would assume that the incoming energy is 50% object and 50% 'scene'''Scene' may be the image average or maybe an internal or an air thermistor. It is never more than a guess.Either way that would assess the same energy input w.r.t the 30°C shutter as being from an object at 230°C (50% of a 230°C object plus 50% of a 30°C 'scene' if 'scene' was 30°C)You'd get a higher temperature still if it used 'scene' = 5°C, ie 255°CSo the emissivity 'correction' is doing what it should, but cannot make a real 50°C look like 130°C with emissivity set to 0.95.Things to try after the tape / no reflections test:Set emissivity to 1.Eliminates any misguided 'correction' effectsIt may be getting a very wrong 'scene' temperatureHowever it is impossible to push 50°C up to 130°C at emissivity 0.95 (using accurate energy calculations)Does it read your hand temperature OK, even with the RF on ?Does it read other known items OK ?It may be getting a very wrong shutter temperature due to RF interferenceBill

author: AndrejaKo
author_rank: Frequent Contributor
timestamp: February 09, 2022, 09:35:48 pm
content: Hello Bill, I think that your point here is the right one and thank you for your explanations.The specific measurement of the coils which I'm referring to were done with RF transmission off (however, my end-goal is to do measurements with RF running), so there shouldn't be any RF interference (unless RF did some damage to the camera already).The camera does show expected temperatures for boiling water or for ice bath. For some extra checks, I measured the temperature of my skin and the temperature of my central heating radiator, and for them too, I'm within a couple of degrees away from a type K thermocouple. So, at least in some cases, it does show correct temperature.

author: Rydda
author_rank: Contributor
timestamp: February 10, 2022, 01:21:43 am
content: Hi.I have no experience of heat generated by RF, but is it possible that what the camera is detecting is extremely uneven power dissipation in the coil? The resolution of the camera may be insufficient to pinpoint the location. So some part of the coil may actually be at 130 C.

author: thermalengineer
author_rank: Contributor
timestamp: February 10, 2022, 01:33:34 am
content: Maybe you should choose the thermal camera with a higher object temperature range

author: IR_Geek
author_rank: Regular Contributor
timestamp: February 10, 2022, 07:42:06 pm
content: Any visible pictures of the coils? Just a guess, but you might be seeing a cavity blackbody effect? Your apparent temperature appears higher because of all the reflections. Top that off with low resolution. Your single point thermal couple is only reading from a small area.

author: AndrejaKo
author_rank: Frequent Contributor
timestamp: February 23, 2022, 02:26:37 pm
content: Quote from: IR_Geek on February 10, 2022, 07:42:06 pmSorry for the delay, I had the device in a difficult to access place.Attached is a visible-light image of the device. It's an antenna tuner, and the coils which are interesting are on the right side, number 3 and 4 when counting from top to bottom.

author: IR_Geek
author_rank: Regular Contributor
timestamp: February 25, 2022, 03:22:18 pm
content: Thanks for the visible picture. doesn't appear to be a cavity effect. Very interesting and odd problem. Taking cue from srb1954, try placing a piece of black electrical tape to see if the temp is close.It just might be at that temp. If you can get close enough to resolve each wire, it might give you a better indication if there is a local hot spot.Good luck.

author: srb1954
author_rank: Super Contributor
timestamp: February 25, 2022, 09:39:14 pm
content: Quote from: IR_Geek on February 25, 2022, 03:22:18 pmMake sure to use a tape with a matt surface finish. Normal electrical tape has a glossy surface and will give anomalous IR readings.

author: IR_Geek
author_rank: Regular Contributor
timestamp: February 26, 2022, 04:34:45 pm
content: Quote from: srb1954 on February 25, 2022, 09:39:14 pmVery good point. FLIR has a couple of good articles.https://www.flir.com/discover/rd-science/use-low-cost-materials-to-increase-target-emissivity/"Scotch™ Brand 88 black vinyl electrical tape has an emissivity of 0.96 in both the short wavelength (3-5 μm) and long wavelength (8-12 μm) regions,"

author: AndrejaKo
author_rank: Frequent Contributor
timestamp: February 26, 2022, 04:37:40 pm
content: Well, I just ordered some Scotch 88 tape, and I'll report the results when it arrives. Much of the electrical tapes available locally seem to be quite shiny.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 27, 2022, 01:10:24 am
content: BTW, it's not the screws heating up, is it..? I see a few, fairly close to some coils. (Could try plastic screws there.)Yeah, just have something reasonably thick and emissive on top.In the mean time, you can peek around at angles, you're likely to see some hot spots, either where something emissive is stuck on (e.g. the, what looks like a bead or strip of plastic inside the larger ones; maybe the smaller ones don't have that, though I see a bead of adhesive along the side of some?), or see a few spots which, due to a lucky series of reflections, happen to be dark and therefore reasonably emissive.To explain that last one better: consider the stack-of-razor-blades optical black structure. The blades aren't very emissive by themselves, but due to reflections between the steep angled faces, light is eventually absorbed -- and radiated in turn. You'll occasionally see spots like this on screw heads, between heatsink fins, inside corners of various things; probably the troughs between wires in the coils do too, though with such low resolution it may be hard to resolve the actual emissions -- when everything gets blurred together with the cold-looking surface, it's anyone's guess. If you have a magnifying lens or can adjust focus, see if you can resolve it.Put another way, it's kind of the inverse problem of specular reflection. Whereas you can have some random glossy/chromed/etc. surfaces, like, ever seen when a beam of light shines in through the window from an unnatural angle, and it's just reflecting off a car parked outside or something? Or even secondary (two cars, or more)? Like that, but imagine the light path, instead of bouncing off a few shiny surfaces to a point source, instead bounces around at just such an angle, that it bounces into bounces into... and ends up taking a really long, circuitous path around all the shiny surfaces. Those spots will look visually dark (because, like the stack of razor blades, all the reflections end up absorbing light, even if each individual reflection absorbs very little), but at IR will have significant emission. But just like specular reflections from point sources, the spot size will be very small, so you may need better resolution to properly resolve them.Practically speaking, that's to say: look around from all angles, and try and find a spot on the thing that shines brighter than anything else. And of course, check that it's not a reflection from something else hot nearby -- easy enough to confirm by shadowing those other things.As for thickness and emissivity, the substance needs to be at least a few wavelengths thick (at the speed of light in the material*) to have much effect. So, as mentioned, permanent markers don't work very well: the mark is too thin. The ink itself has good emissivity, but there's not enough of it. You can see this yourself if you keep drawing over an area (assuming the pen doesn't soak up the existing residue, have to do it carefully). So, grease pencil or paint pen is a good alternative -- goes on thick.Similarly, wire enamel might not have great emission at IR, or is too thin. May have to paint or tape over it to get good visibility.*Metals, being conductive, have a rather low speed of light inside them, and also steep losses. So, waves tend to decay in them within a fraction of a wavelength -- skin effect. Thus, even quite thin foils can still be quite reflective. And, the index of refraction is the ratio of velocities, so metals have effectively an extremely high index, and that's another way of saying, they're very reflective. (Put yet another way: they're lossy, but the loss is so poorly matched to the impedance of free space, that most incident energy just bounces off it.)Tim

author: agiorgitis
author_rank: Regular Contributor
timestamp: February 28, 2022, 08:21:41 am
content: you can stick a thermometer's probe on the coils themselves to get the exact temp. (Some multimeters have very tiny temp probes)Then calibrate the camera (via emissivity) to match your thermometer's readings.You can also paint them black with a non reflective marker and set emissivity around 0.95 or so

author: Ian.M
author_rank: Super Contributor
timestamp: February 28, 2022, 09:30:29 am
content: Quote from: agiorgitis on February 28, 2022, 08:21:41 amWhen dealing with heated parts that are small and low power, the problem with using any type of 'thermometer probe' is getting good thermal contact and the amount of heat the probe shunts away via its wires. Its certainly *NOT* going to give you the 'exact temp'. The *ONLY* thing a typical multimeter type K thermocouple probe is going to give you is a lower threshold for the actual temperature of the part as its certain to under-read. Its a different matter using one to measure the temperature of large metal parts that you can get good thermal contact to, with a heat flux that makes losses along the probe wires negligible.I initially noted and T3sl4co1l has explained in some detail that *most* black markers are useless for providing a high IR emissivity surface. Typical permanent markers (aka: spirit markers) e.g. the well-known 'Sharpie' brand leave far too thin a coating to be black in the mid to far ir spectrum. If its not totally opaque on glass or acetate its just not going to work as a high IR emissivity coating. A flat blackPAINTmarker is likely to provide much higher IR emissivity due to the much thicker layer of pigment it deposits.I strongly recommend reading the FLIR "Use Low-Cost Materials to Increase Target Emissivity" article that both myself and IR_Geek have already linked.

author: AndrejaKo
author_rank: Frequent Contributor
timestamp: February 28, 2022, 11:12:17 am
content: Quote from: agiorgitis on February 28, 2022, 08:21:41 amAbout the red part... Well, I can't!I really don't have much experience with this field (that's why I have noob in title), but at least on my camera, the lowest temperature I can measure is with emissivity set to 1. I can't go above one. On the other hand, the temperatures being read are higher than what the thermocouple and my hand are saying. Sure, if I had a few degrees of difference, I'd write it off to some minor effect I'm not familiar with, but 80 degrees Celsius is actually a big difference.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 28, 2022, 11:51:12 am
content: Quote from: Ian.M on February 28, 2022, 09:30:29 amI find okay results with thermal paste, and an average probe (namely the one that comes with the BM235). Thinner  wires are definitely a plus, and it's not like you have a whole lot of thermal mass here (just copper wires!). Mind also, the meter may go bonkers from the RF. (I've had trouble reading low-side switching transistors -- the drain tab going at 400V or so, for example.) Finally, the probe may throw off the tuner's tuning a bit; this is probably the least important effect, but of course it matters how wide of a ratio the tuner is set for. I would guess on the order of 0.2pF for the probe touching enameled wire? Also with high ratios -- if high voltage is present, the enamel may breakdown entirely, a further hazard of zapping the meter.So, nothing unintuitive, I mean it's just bits of metal. Don't have a lot of thermal mass to start with? Not gonna get a great reading, or it takes relatively long to stabilize; etc.QuoteI assume they're talking about such a product ("non reflective", matte?), though I'm not too deep into the marker market (hah?) and maybe that's referring to something else; or just poor word choice, or lost in translation.Quote from: AndrejaKo on February 28, 2022, 11:12:17 amKeep in mind, mostly all the camera can do, is convert received energy to equivalent temperature.If a surface isn't giving off much energy (emitted or reflected), it's probably cold. Note that, between emission and reflection,it can't tell the difference. It's making the assumption that 1. received energy is at a corresponding temperature, and 2. it's all coming from the surface itself, not reflections.So you can get situations where, like, you've got something hot nearby something shiny and cool, and the cool thing looks blazing hot; but you move around a bit, and realize what you're actually seeing is a mirrored perspective from that nearby thing! So, try to get a feel for the scene, figure if this effect is skewing your measurements. It's not hard to see in IR, definitely weird (look at faces!), but still very much your basic geometric optics / camera sight at work here.If you set a lower emissivity in the camera, it's assuming that the surface simply isn't emitting as much, and so raises its calibration factor -- everything reads higher. And if you figure you're looking at some uniform surface, that isn't reflecting anything else hot nearby (ideally, reflecting just the cold of absolute zero, perhaps!), then this will work fine. But if you get into any of the situations above -- the surface is nonuniform (more emission in spots), it's reflecting nearby heat sources, it's an uneven (rough or creviced) shape, etc. -- those can all increase the apparent temperature.So it's really best, if you can, to either stick something onto the surface -- maximizing its emissivity so you don't have to ask the above questions -- or hunt around for spots of high emissivity, if it's shaped in such a way to be able to find such spots (so, not flat surfaces, but lots of folds, crevices, inside corners, etc., maybe).Finally -- if this is in actual operation, then your finger may read extra high, because it's being microwaved (effectively)! I'm sure you're aware of the hazard of RF burns; skin is also prone to heating by electric (dielectric heating) and magnetic (diathermy) induction. You might literally be feeling it hotter than it is, because your very skin is heating up. Now, I've no idea what kind of power level, tuning ratio, frequency, etc. you're testing this thing at, I don't know if this effect is actually relevant, I might be making this up -- but it's another possible error, besides that fingers just aren't very accurate to begin with.My gut feeling is, if the camera itself is calibrated, and the surface is painted for high emissivity, it will give the most accurate results, also interfering with the circuit the least.Tim

author: Bill W
author_rank: Super Contributor
timestamp: February 28, 2022, 03:32:41 pm
content: Quote from: T3sl4co1l on February 28, 2022, 11:51:12 amNot quite, or hopefully not quite.With emissivity below 1 the camera should account for the assumed reflected energy, based on a guess (or input) of the 'surroundings'.In the screengrab from the OP it is set 5°C as 'surroundings'.Some cameras will just use an internal temperature sensor.If you have a low emissivity object setting and a hot 'surroundings' then the reading will go down. So not always up as your example which presumes absolute zero surroundings.Bill

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 28, 2022, 03:50:25 pm
content: Ah indeed, so there is some compensation for that, nice.Oh, I forgot to say something about temperature itself -- the emission spectrum tells us something about the temperature of the object as well, independent of the intensity of that radiation -- and, assuming the emissivity is equal across frequencies (which also need not be the case -- again, using the example of thin films, a permanent (ink) marker looks black at optical frequencies, but gets ever "grayer" as you go deeper into the IR). We can fit a black-body curve to the spectrum and determine the temperature that way; and if we have multiple temperatures in the scene, we'll know because multiple curves will be needed to fit the smeared-out spectrum.This gives a full, physical picture of what's going on, but what the camera observes, again isn't that full picture. Namely, it's sensitive to a broad range of frequencies, and doesn't resolve any spectrum within that span -- it's a B&W camera, not color as it were. So, this is why these cameras cannot resolve temperature this way, and must rely on emissivity.(You could reconstruct such a scene, using a series of IR filters to block all but a much narrower part of the spectrum, and then comparing the relative brightnesses in each image. The filters act as sampling points on the black-body curve, so that for, probably two or three filters, a temperature can be determined, and with four or more, perhaps even multiple temperatures per point. So -- interesting to think about, but hardly practical; short of an actual multi-spectral camera that can do it already, maybe, but good luck budgeting for that.)Tim

author: mawyatt
author_rank: Super Contributor
timestamp: February 28, 2022, 06:31:10 pm
content: Back in 1980 we developed a battlefield instrument (XM21) capable of remotely and passively detecting chemical warfare agents at a distance of over 5 miles. The controversial technique was based upon passive spectral radiometry in the 8~12 micron region, and employed a tiny moving mirror interferometer, and ultra sensitive cooled (~70K) HgCdTe detector. The general idea was to use the finite but small temperature differences between air, clouds, terrain and such to help identify the potential chemical agents. So controversial was this passive technique that the program was halted because the presidental science advisor claimed the technique theoretically impossible!! A demonstration at the US Army Chemical Proving Grounds amongst congressional personnel and staffers proved the science advisor wrong!! The program continued and went into production protecting Allied Battlefield Soldiers and made the cover of News Week a few decades ago!Fitting a complex multi-source & emissivity black body radiation (Stefan-Boltzmanns T^4) at 8~12 microns was not trivial, and required the XM21 to have a set of known BB emissivity calibration factors that included the instruments self-radiation effects. A technique was developed (later patented) where a known BB source (treated alumina substrate) was servoed to ambient temperature, then various delta temperatures above and below ambient "measured" for calibration. Also a known plastic with specific chemical "signatures" was also measured using the interferometer to create various calibration interferograms.These calibration interferrorgrams allowed a complex curve fit to the input scene black body effects which was subtracted out leaving the net residue of the various emitters which included the net absorption and radiation of the scene. This residue also contained the various chemical "signatures" of the chemicals contained within the scene, including any potential agents which could be identified and alerts/alarms created.Anyway, creating BB fit for a single emissive source is straightforward, however a multi-emissive source may prove more difficult, especially when one needs to separate the sources to help identify features.Best,