Why does hot air cool?

Submitted by AspGuy25 t3_10k4ov8 in askscience

My coworker has a part (the part is a PCB in a metal housing with the cover off) in a thermal chamber. It is like a convection oven. He set the chamber to 105C. When I view my part with a thermal camera, I can see the electrical bits are all at the temperature that I would expect. But the metal housing for the part is closer to 85C. The unit was in the chamber for over an hour, so it should have been “soaked” by that time.

I was talking to a coworker and he said that the hot air in the chamber was cooling the part because it was flowing. But that didn’t make sense to me because I would think that air at 105C would make everything about 105C. Why does hot air cool metal and reduce the temperature of the metal to below the temperature of the hot air?

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Appaulingly t1_j5ojqgw wrote

Thermal cameras aren't that great at accurately measuring temperature particularly when comparing different materials. The emissivity of the metal with be very different to that of the other materials and so a different temperature will be measured.

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dumb_password_loser t1_j5opqcs wrote

Yes, he just sees a colder environment (assuming door is open) reflected on the metal.

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Detectorbloke t1_j5oq3uv wrote

In addition: There's special paint to measure the temperature of normally reflective surfaces.

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AbnormalMapStudio t1_j5pl9jf wrote

Even easier, you can buy emissivity labels (also called IR labels) and just slap a sticker on.

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dr_reverend t1_j5rb0ol wrote

Even easier. Just use some masking tape.

Recommended up to 100c but I doubt 105c would be a problem.

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Lampshader t1_j5rzle5 wrote

The FLIR sales rep recommended black electrical tape when I asked about this.

I dunno what its melting point is but 98% of electronics workbenches will have it within arm's reach already so it's got that going for it

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bella_68 t1_j5s8tpz wrote

Idk how hot a car gets when sitting in the sun but it was hot enough to melt the glue on the electrical tape covering my steering wheel. Interestingly, the glue was liquid and the liquid was everywhere but the tape was still on the steering wheel because the way it was wrapped and the fact that the glue was also still there to some extent.

Unfortunately for me, I get a rash anytime I touch glue

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ArchitectOfFate t1_j5u5knj wrote

The tape itself has NEC standards regarding when it melts and burns, and is designed primarily to melt unless exposed to really extreme temperatures. There’s usually a rubberized or vinyl part of the tape that can turn it into a sticky mess. Good electrical tape shouldn’t have problems at “normal” temps, but on a hot day in the south/southwest a car can get upwards of 150 degrees F, which is pushing it even for the good stuff.

Humidity also doesn’t help and can cause some glues to break down faster.

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VoilaVoilaWashington t1_j5vaa02 wrote

> The tape itself has NEC standards

Which is probably not true of random black tape people buy at the dollar store.

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ArchitectOfFate t1_j5vfsry wrote

Exactly. I didn’t want to accuse OP of buying cheap tape, but proper electrical tape will be clearly labeled as electrical or electrician’s tape, will have the NEC standard on it, and costs $3-$5 a roll unless you’re buying in bulk.

And even then the minimum is 170 degrees. 3M and a couple other companies certify their vinyl tapes above that, but one that’s barely compliant could still get hot enough in a hot car for the glue to start to soften.

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BeetsMe666 t1_j5ug11a wrote

I am a refrigeration mechanic and have had to inform far too many restaurant operators about this fact.

They log the temps in the equipment taken by an ir gun, and they are all over the map. The most consistent recording is done with a probe thermometer in a bottle of water.

Too many employees don't question the reading and just log the number that is far out of range.

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SXTY82 t1_j5p8cb0 wrote

The first time I took a look at a window with my thermal camera I could see the reflection of myself in the thermal image but not in the window itself.

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ThisTooWillEnd t1_j5ppwaq wrote

I've been playing with a thermal camera and not only do I observe what you're describing, I also clearly saw my reflection in a tile wall with the thermal camera.

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Dusty923 t1_j5pj1j7 wrote

Glass reflects different wavelengths of light differently. House windows may also have an infrared-reflective composition or coating (for more efficient cooling in warm sunny weather).

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Denamic t1_j5rocnx wrote

Thermal cameras use IR. IR, being light, bounces off reflective surfaces.

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zebediah49 t1_j5s0zx3 wrote

The relevant point is that visible light will happily go through a normal glass window, whereas long-IR will not. Windows are opaque (and pretty reflective) to thermal cameras.

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SXTY82 t1_j5p84u4 wrote

yes. Also the reflectivity will affect non-touch thermal measurements. I use them at work now and then and anything that is silver colored, even non-shiny metals, I have to paint black to get an accurate reading from them.

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VulfSki t1_j5r0tb4 wrote

This is the answer.

I have used thermal cameras in my line of work work a part in an oven.

And I have literally been able to see the reflection of my own body when I pointed the thermal camera at the metal walls. That's likely the issue here. An emissivity issue.

The coworker's explanation sounds like a misunderstanding of how convection works. Just because air is flowing, doesn't mean it's cooling down the metal. Flow doesn't equal cooler. You need to remove the thermal energy somehow. If it's a closed system the heat isn't going anywhere.

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chillaxinbball t1_j5rd007 wrote

The best way to think about thermal cameras is to think about cameras.

Imagine you are trying to measure how hot something is by how much it's glowing. For example you go to resistive coils and see how bright they get. You can get a decent measurement just by seeing how bright it gets. Now try to measure a mirror reflecting an image of the coils. You'll get a very similar measurement. Is this because the mirror is just as hot? No, you are mostly measuring the reflected image of the coils and not the mirror itself.

It's the same thing with thermal cameras. Metal is very reflective at the wavelengths used to measure thermal radiation. You are essentially looking at a mirror and will not get accurate readings from it in much the same way.

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Glasnerven t1_j5rhfa4 wrote

> Just because air is flowing, doesn't mean it's cooling down the metal. Flow doesn't equal cooler.

Roughly speaking, flow equals better heat transfer. We're used to thinking that flowing air = cooler because we--our bodies--are usually warmer than the air around us. Even if it's hot enough outside that the air is warmer than we are, then we're probably sweating, so we have evaporative cooling going on--and flowing air makes that happen faster.

So, in our personal experiences, we almost always find that it feels cooler when air flows over us. It's really easy to over-generalize that and assume that it's a universal law.

You can easily test it at home, though. Get two little cups of ice, make sure there's the same amount of ice in each one, and then put a fan on one and let the other sit in still air. You should notice that the ice with the fan melts faster--the flowing air is heating it faster than the still air is heating the other ice.

You might also already have a device built around this effect: a convection oven or air fryer.

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VulfSki t1_j5rqk0a wrote

Yes, I understand how all that functions. Humans get cooked down from perspiration, which is because the energy to evaporate sweat is partially comes form the heat on our body, and air flow helps with that.

But airflow itself doesn't equal cooking in the general sense, because it only works if you are removing heat by taking it to something that is at a lower temperature, can't violate newton's law of cooling.

You did provide a great explanation, even though I was already aware of all that.

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orange-cake t1_j5pkl19 wrote

I've seen laser thermometers that can measure emissivity, are they able to accurately compensate or do you need to know the material?

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MasterPatricko t1_j5pzgt2 wrote

A "laser thermometer" as commonly sold is just a small FoV infrared thermometer with a laser pointer strapped on top. (The exception is very advanced thermometers which use calibrated lasers and optical ranging to measure reflectivity.)

Usually these thermometers accept emissivity as a setting. They don't measure it.

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InfintySquared t1_j5q3tvm wrote

> Usually these thermometers accept emissivity as a setting

That little nugget said more than most of the in-depth answers above. Thank you, you done good.

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aspheric_cow t1_j5ostme wrote

Infrared cameras measure the temperature by measuring the amount of infrared radiation emitted by a surface. But metal surfaces inherently emit less (it had low emissivity) so the infrared camera reads low. Try putting a temperature sensor (RTD, thermistor or thermocouple) on the metal and it should read 105C. Or put a dab of paint or piece of tape on the metal and aim the IR camera at that.

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shlepky t1_j5p2k9q wrote

Infrared thermometers usually have to be calibrated. When you get them, they measure radiation as if the surface they're measuring has emissivity ratio of 1 (black body radiation - which means all of the bodies heat is radiated out). If you know what the actual temperature is, you iteratively change the emissivity until you get the correct measurement. When you measure a different surface, you'll have to repeat the same process though. Cc: /u/AspGuy25

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LitLitten t1_j5pufhq wrote

Iirc this is also why they can’t really “see” through glass.

Blocking the frequency range of infrared light or long-wave infrared typically detected by cameras. One reason cars get ungodly hot is due to a bunch of light being absorbed, infrared being emitted, but unable to pass through the windows.

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Craqueleur t1_j5ou01s wrote

Yeah, as previously said, you should put tape or paint on the metallic surface to measure its temperature with your thermal camera.
In my engineering school we did an experiment to measure the emissivity of different type of surfaces .

We could see that without entering the emissivity values, the IR camera would show 64°c for a polished metal plate even if the real temperature (measured with a thermo-couple sticked to the surface) was 74°c .

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DiaperBatteries t1_j5r5o7w wrote

Capton/polyamide tape works great! It has a very high emissivity, is super thin and can take high temperatures.

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KenKaniff- t1_j5pglb8 wrote

Hot air cannot cool something that's at a lower temperature. Convective heat transfer can only occur when the object and the air flowing around it are at different temperatures. And heat will always flow from the hot thing to the cold thing. It will never flow the other direction. Also you should use a thermocouple for this, it will be more accurate. Just gotta make sure you dont short out the pcb with the metal thermocouple tho.

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pjgf t1_j5pm6t4 wrote

The only exception to this of course is evaporative cooling. Moving air can cool a wet surface even if the temperature of the air is above the surface temperature.

That originally what I though this question was asking but it turns out it’s just an instrument problem.

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jermdizzle t1_j5pvjww wrote

I first experienced this while deployed to Afghanistan. Some Brit showed me how to put my bottle of water into a sock and soak the sock with water from a nearby stream. I'd then swing the sock around for a few minutes just to get the water temperature below the 120+ degrees F it was likely at so that it didn't raise our body temp when we drank it. It would often hit 120+ F ambient air temp and sometimes the water would either be in the sun or on top of some surface that was even hotter than the air temp.

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a_cute_epic_axis t1_j5qn7mx wrote

This is how a sling psychrometer works. It has two thermometers, one which has a cloth soaked in distilled water wrapped around the bulb. This produces a "wet bulb" and "dry bulb" temperature, from which you can determine humidity, enthalpy, dew point, etc. The wet bulb will always read the same or lower than the dry bulb. If they read the same, the RH% is 100%. If they are far apart, it's closer to 0%.

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jermdizzle t1_j5riwi5 wrote

Very cool. So many early (and even more modern types, like inertial and star-based navigational systems) measurement and navigational tools rely on extremely simple and primitive concepts or operations, but they produce such useful data.

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a_cute_epic_axis t1_j5rl0d1 wrote

> like inertial

One method simply measures how long it takes light to go through a coil, and if the coil is rotating it will take slightly longer or shorter than if it isn't. Three coils perpendicular (orthogonal?) to each other and you know how you're moving. Run the long term results through a filter and you can determine your latitude as well.

(Technically it's two beams in each fiber going opposite directions and they compare the phasing).

https://en.wikipedia.org/wiki/Ring_laser_gyroscope

https://en.wikipedia.org/wiki/Fibre-optic_gyroscope

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uiucengineer t1_j5pzrh5 wrote

That’s not an exception, it’s a different thing from convective heat transfer

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minimal_gainz t1_j5q5n6c wrote

Yeah, but that's usually what people are actually thinking of when they say "moving air cools better". It's not really the air moving, it's the evaporation, but the average person doesn't necessarily know that.

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uiucengineer t1_j5q64oe wrote

Moving air does actually cool better without evaporation. The air just has to be colder than the thing. That’s why your computer has fans.

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Zarathustrategy t1_j5puy6z wrote

Yeah but the guy who answered incorrectly was definitely thinking of evaporative cooling

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Bruzote t1_j5pp7hu wrote

Yours is a very important comment, even if it doesn't propose the explanation for the "cooler" metal. Your comment reveals how important it is to understand the problem. The lack of understanding is a bane to the engineering field! (Though, it is a gift to some physicists who love an excuse to bust the chops of engineers, a la Sheldon Cooper.)The foul temptress of expediency of thought occasionally seduces engineers into spouting off solution methods that are associated with the problem. They can do this without actually confirming that the solution method is truly applicable. As you pointed out in this case, the solution of "explain the cooling by conductive loss to the air" could not apply since the air was hotter than the metal. (Also, eEven if the air were cooler, it would be in order to do a scale analysis to see if it was the primary issue.)

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TheoryOfSomething t1_j5qmn7j wrote

I know you were probably not trying to be completely rigorous in your answer, but I would add an important qualifier to the gist of your answer. All other things being equal (AKA ceterus paribus), heat will always flow from the hot thing to the cold thing.

Heat can and sometimes without (human) intervention does flow from a cold thing to a hot thing, but it requires some other change. That change could be doing work on the system ala refrigeration. The change could be that a chemical potential gradient is causing a certain type of particle to move preferentially from a cold region to a hot one which leaves the cold region even colder (something like this is the principle behind Helium dilution refrigerators). I'm sure there are other example, although I can't think of more off the top of my head.

So to be more general we might say that closed systems tend toward maximum entropy (or equivalently minimum grand thermodynamic potential), which usually means heat flows from hot to cold, and all else being equal guarantees that heat will flow from hot to cold.

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Chemomechanics t1_j5pk1w8 wrote

>I was talking to a coworker and he said that the hot air in the chamber was cooling the part because it was flowing.

This can be true only for a part that's perspiring.

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0sted t1_j5pd9q2 wrote

Reflectivity. The metal housing is reflecting the cooler temperatures behind the thermal camera and including it in the imaging.

Relatable similar occurance: I was working on an offshore oil platform once using a thermal gun to try to identify a leaking valve and found out pretty quickly that the polished steel gasket between the valve and pipe would reflect the temperature of open sky. I thought I found a leak when the gun registered a chilly 32F but was actually at like 100F on contact.

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Nvenom8 t1_j5pltdt wrote

That shouldn't be true unless there's something being evaporated from the surface, in which case flowing air might speed evaporative cooling. By itself, there's no way for warm air to cool an object to below its own temperature.

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Prestigious_Carpet29 t1_j5qap0a wrote

As others have said, the metal part isn't cooler - it's an artifact of the thermal camera.

It comes down to emissivity, and reflections.

A "thermal camera" or "(non-contact) IR thermometer" measures the radiated heat (long-wavelength thermal energy) emitted by the object you're pointing it at.

All objects emit the same spectrum (strictly spectral distribution) of radiation, depending only on their temperature - look up Blackbody radiator and colour-temperature.

The absolute amount of energy ("brightness") also varies strongly with temperature, but depends also on a property of the material, known as its emissivity.

For simplicity, IR cameras typically only measure the strength of emission at one wavelength (usually somewhere around 3-12µm), and determine the temperature by the "brightness" at that wavelength.

For most common matt/dielectric (non-metal) materials, the emissivity is 97-99% - which is the default calibration for an IR thermometer.... but metals, especially polished shiny ones, and especially gold has a lower emissivity, so the IR thermometer will under-read.

You can look up the emissivity for different materials and set the IR thermometer calibration accordingly, to get correct readings ... but be aware that metals can also look like mirrors, and you may "read" the temperature of the thing in the reflection in the metal, rather than the metal itself - or somewhere between the two.

If you wear a metal ring (especially a gold one) on your finger an point the thermal camera at your hand, you'll see the ring is darker (and reads "colder") - even though its true temperature is likely to be close to that of your fingers.

This is something that not enough people know about IR thermometers and IR cameras.

It's a physics thing! :-)

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IKnowWhoYouAreGuy t1_j5px33f wrote

Answer: Your question is a bit of a misnomer in itself and your coworker doesn't seem to understand thermodynamics - The real question is why the center of the oven is warmer than the outsides of the oven, which yes, is partially due to convection (the movement of the fluid in the oven circulating away heat as it travels) and the design of the oven (where latent 'pools' of fluid allow heat to build up), but is mostly due to the fact that the fluid being circulated is AIR which is an insulator due to the nature of the gas composition and it being in gaseous form. Air insulates regardless of its velocity, but as father Bernoulli taught us, the faster it moves under constant pressure, the less entropy is able to pool into measurable heat. For this oven, it makes sense that the outsides would be cooler than the insides, since the whole point of an oven is to concentrate high heat within a container that won't combust. There are other issues at play such as the difference conductivity and radiation constants of different elements, but that's another reason why oven's are constructed with materials that melt far above the safety limits, such as sand/acrylics. You can measure this yourself by going into a temperature controlled environment (read: "in-doors") and touch a wooden surface, then touch a metal surface, noting that the metal surface will always seem cooler to the touch because metal is a poor insulator and will emit heat calories to entropy at a much greater rate than wood, which is carbon-based and has evolved to literally be pockets of air at the molecular level.

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Joe30174 t1_j5qpjyh wrote

If all the air is the same temp, such as in your explanation, air flowing would do the opposite from cooling. The kinetic energy of the flowing air would actually lose kinetic energy to heat stuff up more. Now it is very negligible, but it certainly isn't cooling anything down.

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Dashkins t1_j5r4hxr wrote

Your coworker is wrong. The thermal camera measures temperature based on the amount of radiation (I) it senses:

I = s e T^4

Where T is temperature in Kelvin, s is a constant (5.67E-8), and e is emissivity, a value between 0 and 1. For most objects, emissivity is between 0.9 and 1, so the camera assumes an emissivity in that range and solves for T using that equation (T = (I/(s e))^1/4 ). However, metals have low emissivity, often in the range of 0.3 or lower!

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PineappleLemur t1_j5rx08g wrote

Thermal cameras see IR, metals, especially bare metals have a really low emissivity, meaning that they radiate very little IR and reflect the rest. (what the camera sees)

Think of it as a mirror, aluminum foil for example has emissivity of under 0.1 (it's a scale of 0-1, 1 being 100% radiating/absorbing) so only 10% of the total possible energy is being radiated and 90% are being reflected.

What you're seeing is a combination of the reflected IR + the radiated IR.

So the surface reflected is likely what you're seeing + the housing.

Take an aluminum foil and literal look at yourself with the camera. Foil is basically a mirror for IR as a glass mirror is for white light (what we see)

Emissivity is affected by material/color/surface. To get a better reading you want to paint it with a black body color, something black with a rough surface and very little visible shine (matte black)

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TexasPop t1_j5pgppo wrote

Temperature is one of the most difficult things to measure. For example it is not just to put a thermometer inside an oven an do readings if you want a correct value. But if you are satisfied with an approximate value, this is ok.

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pr1m0pyr0 t1_j5qiivi wrote

materials have different emissivity. Plastic on wire is ~1, metals are way different.

Metals also reflect heat from different sources. You can tell if you're measuring a reflection by moving yourself around and measuring from different angles. reflections will change but the emited ir wont

You need to put a bit of black insulation tape on the metal and use that for temperature. Or use it to work out the emissivity of the metal.

Theres another setting for specular reflectance. You crumple up some tinfoil and take a reading from that for background reflectance

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Willbilly1221 t1_j5uzwrf wrote

What you are describing is called thermal dynamics. We all know hot air rises, and cool air sinks. As long as the air is moving you have an exchange of energy. As the hot air rises, it creates a vacuum below it that pulls in colder air from below. The cooler air saps energy from the hot parts and gains heat. It then takes the heat and rises with the hot air above creating another vacuum below. That also pulls in fresh new cooler air to surround the hot parts. This continuous movement creates a convection or cycle if you will that keeps air from becoming stagnant. When you have stagnant air the air can only absorb so much heat. Circulating air can absorb more heat away from hot parts do to it being in motion. Thats how fans typically work is to force a convection of cool air to push stagnated hot air out of the way, so cool low energy air can absorb heat energy and travel away from the hot surface.

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[deleted] t1_j5p3p7u wrote

[removed]

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StunningScientist267 t1_j5p44uw wrote

Much like a flame of a fire or heat from an oven, as particles accelerate w heat they expand. This expansion makes them eventually condense back down due to temperature loss over distance. Hence why we get rain systems over mountains. If you want it to rain, build something big. Move mountains. Just ask permission first.

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Bruzote t1_j5pmhhr wrote

The other comments basically reveal that every physical problem we try to understand requires approaching the solution using a mental model with built in assumptions and limitations. So, perhaps when we have a question or face a challenge when something doesn't make sense, it is good to look deep down through our pyramid of assumptions upon which we place our understanding. Unless we solve the problem, then all assumptions are suspect. For example, while others point out the probable main issue here (the metal's emissivity), there are other assumptions. There is the assumption that the metal, the other parts and air should all be at the same temperature. In fact, the problem is a bit complicated when you get into the slight differences. The objects each lose energy at a different rate by emitting (mostly) IR photons at different rates (paramaterized as "emissivity"). Additionally, the objects conduct heat at different rates. Since they all lose heat via IR to the cooler environment outside the chamber (which also irradiates the same parts, but with fewer IR photons), the parts will never purely reach the temperature of the air in the chamber. What equilibrium temperature each one reaches, down to the hundredths of a degree perhaps, depends on how quickly the parts can conductively and radiatively absorb heat compared to how quickly they lose heat via the same processes to the outer environment (via) IR, the inside air (via conduction and IR), and the parts they touch (via conduction) and the parts within line-of-sight (via IR). So, the problem is actually quite complex and only simplified if we don't care about small temperature differences. Plus, if we did care about such small differences, the measuring device would have to be highly accurate.

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Historical-Offer3075 t1_j5prxx3 wrote

Yeah, if you blow hot air over metal that is much hotter than that air, you will cool the metal. If you blow hot air over metal that is colder than that air, you will heat up the metal. There's no getting around this, ha ha.

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jermdizzle t1_j5pvrnz wrote

Unless some form of phase change surface cooling (evaporation in this case) is in play.

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bikerlegs t1_j5q9frf wrote

Everyone has already mentioned the flaws with the temperature gun so I won't delve into that. Instead I'll explain "wind chill". Temperatures feel colder when there is wind for 2 main reasons.

One of them being evaporation on a wet surface. This is why licking your finger and holding it to the wind helps you tell the wind direction by which side the cold part of your finger is facing. Wind accelerates evaporation which is an endothermic process (absorbs heat).

The second reason is that heat transfered through conduction happens when two molecules bump into each other and exchange their individual kinetic energy. Much like two balls in billiards hitting each other. When a while material is subjected to this the kinetic energy of each molecule is represented as heat. So wind allows more molecules to bump into each other and thus more heat transfer.

Now, it is incredibly important to note that this process of heat transfer through conduction works in a specific way to AVERAGE the temperatures between two materials. So cold air cools and hot hair heats up. Don't believe me? Hold your hand in front of a hair dryer. 😆 Given your material isn't wet evaporation can be dismissed and temperatures will only average. So that means your hot oven is definitely heating any material you throw in it that is at a lesser temperature until they reach a state of equilibrium and the convection (wind) accelerates this process.

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a_cute_epic_axis t1_j5qnz9n wrote

Wind chill can never reduce the temperature of an object below the dry bulb temperature of the air, unless that object is wet. In that case evaporation can occur and will result in cooler-than-dry-bulb temps until the water evaporates or until the air reaches 100% humidity.

The only time wind chill has any other effect is if the item in question is producing heat...e.g. a person or a house with the heat on inside. In this case the object will experience greater heat loss as the windspeed goes up. If the object doesn't have an internal heater, then the wind chill will speed cooling until the object reaches the dry bulb temp, and then have no impact.

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TommyTuttle t1_j5qa6nf wrote

But I’m confused about one thing: wind chill does not reduce actual temperatures. If water is in a chamber at one degree above freezing with a wind chill of 20 below, that water will not freeze. Because the temperature is still above freezing. The only freezing you’d get is due to evaporative cooling, ie an actual drop in the real local temperature of the water.

Metal is not subject to evaporative cooling. So I’m confused as to how wind chill might lower temperatures in a hot chamber. You’d think the wind would bring everything rapidly to the air temperature.

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jawshoeaw t1_j5qcc6n wrote

That's correct. Windchill annoys me. It's often misused and confusing. There are Reddit posts saying things like "Here I am in Michigan in a T shirt, it's -60F today". but it's actually 0F with windchill of -60F. And yes, wind does not cool things below ambient unless they are wet (or unless the wind itself is bringing in colder air)

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jawshoeaw t1_j5qd1il wrote

Windchill is a human experience and does not apply to physical objects like a computer. No matter how fast the wind is blowing over your dry computer, it cannot reduce the temperature below the temperature of the air making up the "wind". What it can do is reach an equilibrium temperature faster. At least in my experience (and wikipedia) this phenomenon is called "air cooling your computer" and not wind chill.

edit for clarity: wind definitely cools things off faster than no wind :)

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bikerlegs t1_j5qdk4w wrote

This isn't true. I also just looked on Wikipedia and it explains the same thing I explained. Wind chill is the additional cooling effect that wind brings. Not necessarily from evaporation, it still included conduction. So you're dry computer is still experiencing wind chill by definition.

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jawshoeaw t1_j5qdu0v wrote

From the very beginning of the article on Wikipedia:

"Wind chill or windchill (popularly wind chill factor) is the lowering of body temperature due to the passing-flow of lower-temperature air.

It's a non-scientific term with no agreed exact formula and has nothing to do with inanimate objects.

Edit: Of course wind cools things - didn't mean to be nit-picking, I just don't like "windchill" because it's poorly defined. I mean there could be moisture on inanimate objects, that could be removed by dry air and cool further.. but there are good terms for those phenomenon such as evaporative cooling, convective, etc.

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a_cute_epic_axis t1_j5qoaif wrote

> Windchill is a human experience and does not apply to physical objects like a computer.

It does if the computer is on and is exposed to wind. It has nothing to do with being human or alive, and everything to do with having an internal source of heat. Even in things that have no source of heat, wind chill will accelerate cooling until the object reaches the dry bulb temp (assuming there's no evaporation), as you mention... which rather obviously means it is not limited to "human experience".

You can argue that the term windchill refers to something typically used to describe human/animal outdoor comfort, but the concept is in no way limited to that.

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TheJasonKientz t1_j5qx95a wrote

Is the PCB in use or is it off?

The operating temperature of any PCB is going to be higher than it’s environment because the electrical currents create heat through the part. So if you’re measuring 105C at a powered on PCB, then I would argue the chamber itself is not actually 105C, but lower.

When you say the chamber was set to 105C is it regulating the temperature with a thermocouple that is actually measuring the temperature? If so where is that thermocouple? And where is the heat source? Is there one heat source or multiple? Is the chamber being heated by driving heated air through it? Also how insulated is the chamber?

It seems likely that the air in the chamber isn’t actually 105C. This could be for a lot of reasons, like if the chamber isn’t insulated very well or if the heating element is too small or the heat transfer from the element to the air isn’t very efficient or if the feedback loop that targets a temperature setting has a bias from poor temp sensor placement or calibration. Any or all of these things could lead to inaccuracies in the chamber temperature. And it’s very likely that you don’t actually reach thermal equilibrium no matter how much you “soak” if the chamber is losing heat energy, which it almost certainly is.

Think about a 3D printer, the bed temperature is regulated with a thermocouple that is attached to the bed. If you set that to 105C bed temperature, the air in the enclosure is absolutely not going to be 105C no matter how long you soak for. Because the thermal losses from the chamber are too great.

My guess is that this is what’s happening to your chamber. I’d put an old fashioned thermometer in there with no PCB and see how close the air temperature is to the setting after a 30 minute to one hour “soak”

Edit: if you want to tell me more about the chamber (ie answer some of my questions about how it’s heated) I can help get more specific. Also knowing what the chamber exterior is made of would be helpful.

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Gorillaman1991 t1_j5rb6j3 wrote

Your coworker is not correct, unless the item is evaporating a liquid like water or something. If it's completely dry, 105c air is going to bring the object up to 105c whether it's blowing or not, but if it's blowing it will actually be quicker because the cooled air from the object is being removed and replaced by new hot air

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TheWillRogers t1_j5rfj1g wrote

Heat cannot of itself pass from one body to a hotter body. If two bodies of different temperatures are in contact then heat will flow from the hotter body to the cooler body until the thermal energy is equalized. This will continue until the universe stops functioning.

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rankedscalper t1_j5sikni wrote

They way I think about it, is that it loses energy density and gains electrons. There’s someone known as Renzo who did plasma electrolysis and synthesized oxygen from sulphur at low current on YouTube. I think that usually we describe this as solely an electron exchange but that the electron configuration can be broken down by photo-disintegration and measured by decay of any atom into helium concentrations. Imagine electron configurations with like 2[He] and think about an electron shell being a cloud of probability with resistance being relative to the radiant energy. Radiant energy is primarily responsible for black holes

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Slappy_McJones t1_j5szpoe wrote

If you are scanning through a port window, it might be a reflectivity issue, like others have stated. Pop-open the chamber door (briefly) and take a scan or cross-check with a quick responding contact thermocouple probe.

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drthip4peace t1_j5pmgxc wrote

Heat=energy.

How much energy is required to raise the temp of silver is less than steel because one is more conductive than the other. Properties of the material being heated play a role but heat is lost over time. Assuming you are not putting any more energy (heat) into the system.

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jermdizzle t1_j5pw2s0 wrote

I don't think this applies as this is a nominally steady state environment, i.e. it's temperature is stabilized regardless of input heat required to stabilize the internal ambient fluid temperature. It's just because they're reading a reflective surface with an IR camera/thermometer, thus giving incorrect readings.

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drthip4peace t1_j5qmi6i wrote

why not? Is this metal the outer layer of the system. One side at 105 the other at some other temp, such as room temp. Conduction of the material is important especially if the question is why is one thing one temp and this other thing another temp. How thick is this metal? Why would it not act as a "heat sink?" I think it applies, it is thermodynamics but the last time I thought I was wrong I was mistaken.

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jermdizzle t1_j5rhyl2 wrote

The metal housing wasn't the oven's housing, it was the housing of the part being heated, or at least that's what the OP typed grammatically. They may have meant something different. Either way the temperature delta wouldn't have anything to do with the heated air, it would just be thermal conduction if it was the interior of the oven wall that was cooler than the internal ambient air. Also, you can test this on your own very easily. Go try to use a thermal camera or IR thermometer on a shiny piece of metal. You'll receive obviously inaccurate readings. Place a piece of dark tape or paint on said metal and watch as the temperature is more accurately represented.

Edit: I'd also like to point out that this is inside of an oven. There is more heat being introduced to the system on a consistent basis to maintain a constant-ish temperature. If there was a hunk of metal inside that was still below the target temperature, it'll keep introducing heat. Preheat your kitchen oven empty and then with a cast iron skillet inside. The heat capacity is many times higher with the skillet vs just atmosphere.

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drthip4peace t1_j5ud622 wrote

ok well how is the camera accessing the part inside a container, inside a thermal chamber and why is a camera being used to measure temp and not a temp probe? If it is inside an oven and the question is why would one material in the oven not be the same temperature as other materials in the oven the conductivity of the materials is a logical answer especially since the material in question is a metal housing. Yes the reflection of light will influence measurements of light but there is no mention of light being reflected by the user I am assuming is familiar with the instrument they have chosen. Granted matter is limited but it surely within the realm of possibility that the metal housing is acting as heat sink. This is even more possible when the notion of moving air is added to the equation. Air moving over metal... what is the housing protecting the part from? The conditions should not be the same inside the housing as outside the housing because it would longer be a housing but just another part being heated returning me to my original point that the amount of energy required to raise the temp of the material will vary based upon the properties of the material. It just so happens that in this case known properties of the known materials suggest heat can and should be lost.

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jermdizzle t1_j5ueeie wrote

Hot air doesn't cool. Thermal cameras and ir thermometers give invalid readings of reflective surfaces. These are two facts that answer the op's question and they are both relevant to the scenario. I don't really have anything else to say because there isn't any more to say about the scenario.

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drthip4peace t1_j5uhfkp wrote

Hot air does cool according to the laws of thermodynamics, but sure ok the laws of the universe are suspended in this thermal chamber because of this mystical housing, that must be what it is protecting the part from, and it is a reflection that the OP fails to mention, why would it give a consistent reading of 85? SO this reflection is also amazingly consistent? The entire part on all sides has the same reflection? Wouldn't it or shouldn't it vary significantly based on anything that would impact the reflection? You are assuming that not only that this reflection exists, but that all of the variables that influence this reflection remain constant and unchanging because the result is unchanging. How improbable is that? You are welcome to make as many assumptions as you like but there is not reason that I must make the same assumptions or agree that your assumptions are correct.

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Sprinklypoo t1_j5punwl wrote

Barring fans and other means of heat transfer, your answer is flow. Heat flows. The PCB is hot, and the room ambient air is probably around 70F. The casing is heated by the PCB but cooled by the ambient air as the heat flows outward.

This can actually be quite accurately modeled, but I'd probably want a better picture of your solid state system before attempting to do so.

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