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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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