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Long story short, I have an assassin robot that needs a backpack refrigerator unit to regulate its internal temperature, as well as the temperature of all the various systems attached to it - whatever its power source is, it runs hot. This robot is designed to kill people in pitch-black environments, and therefore uses echolocation as its means of direction-finding rather than sight/light emission. It can also see, meaning that if you turn on a light, it'll get you.

Normally, this robot is very, very quiet - if you were in a pitch-black room, it could sneak up to within feet of you without you noticing, since the sound waves it sends out to echolocate are past the bounds of human hearing. Moreover, it's very good at moving in ways that don't produce very noticeable air currents.

However, this robot can be detected and avoided in a pitch-black environment by one very specific mechanism which the designer either did not foresee or felt was an unavoidable one: the coils on its refrigerator unit are so hot that, when an airborne dust/lint particle drifts into them, it burns up with a small but noticeable snap-crackle noise - think this, but more sporadic. When in a dark environment without any light sources, this is the only way to determine where it is - listening to the little trail of snap-crackle noises as it moves.

How hot would this robot's refrigerator coils need to be in order to burn dust particles up like this?

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    $\begingroup$ my guess: hot enough to very visibly glow. $\endgroup$ Commented Jan 15, 2022 at 0:59
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    $\begingroup$ Fahrenheit 451. 233 °C. I wouldn't have thought that one day this would become a reference so obscure that somebody who wants to write science-fiction would not know about it. $\endgroup$
    – AlexP
    Commented Jan 15, 2022 at 1:03
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    $\begingroup$ Other thing is if its that hot, you would start noticing a burning smell when when dust contacted it. $\endgroup$ Commented Jan 15, 2022 at 2:06
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    $\begingroup$ If I'm reading en.wikipedia.org/wiki/Attenuation#Attenuation_coefficient correctly, the attenuation factor for a 100 kHz ultrasound signal in air at 10 meters is 164 dB. That is a reduction by 10 ^ -16.4 (0.00000000000000003) of signal strength going outbound to a target at 10 meters and then the same reduction again for the reflected echo. I think the assassin robot is going to need a guide dog to actually find its targets. $\endgroup$ Commented Jan 15, 2022 at 2:28
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    $\begingroup$ As for noise, a butane flame from an ordinary cigarette lighter reaches ~3500 F, well above the temperature at which a solid (e.g. an assassin robot) will glow visibly. Buy a lighter and sprinkle some dust/lint on it and see if you hear anything. My guess is that there will be no audible sound even at that temperature. $\endgroup$ Commented Jan 15, 2022 at 2:48

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Instead of dust, it is water.

I thought of this idea when I was out grilling just now. It is spitting rain out. When a raindrop hits the hot grill cover it makes a pop and fizz just as you describe. The grill was 350F.

Wherever your robot is, it is wet. Not pouring rain but damp. Maybe a humid cave where water drips from the ceiling. Maybe an abandoned building and the roof leaks. In any case, sometimes a drop of water lands on the cooling coils. That makes your giveway pop and fizz. 350 is not too hot for cooling coils.

If your robot wants to use its grill to make my turkey bacon burgers I will share the recipe. Although people might then locate it by the smell of deliciousness.

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the coils on its refrigerator unit are so hot that, when an airborne dust/lint particle drifts into them, it burns up with a small but noticeable snap-crackle noise

Unless the dust particles are very large and made of some material that will combust vigorously, what you'll get is the smell of burnt dust, not the sound. Obviously, not every household has easy access to things that can glow yellow or white-hot and can be exposed to air, so confirmatory experiments are difficult, but you can always just take my word for it.

Now, if there is a reasonably amount of large particles of something that combusts vigorously enough to produce a crackle, what you're risking is a dust explosion which will be very dangerous both for the robot and its would-be victims. So please don't hide in the gunpowder store room, or follow anyone into it.

the sound waves it sends out to echolocate are past the bounds of human hearing.

There are devices called "bat detectors" that down-convert the ultrasound emitted by bats to frequencies audible to adult humans. These devices have existed for decades and are not technologically complex to build. Making directional devices would be harder, but not by too much.

Also note that a system designed to jam ultrasonic echolocation woudl also be straightforward to make and could conceivably be run continuously without causing problems for adult humans... it might be unpleasant for dogs and children and other small hairy things with sensitive hearing, but it beats getting assassin-robotted, right?

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  • $\begingroup$ Literally any finely divided dust could explode. Lots of dust explosions happen in grain silos for example. $\endgroup$ Commented Jan 15, 2022 at 10:56
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    $\begingroup$ @JourneymanGeek yes, I'm aware of that, and the page I linked says as much. Not just any dust can cause crackling when heated, though. $\endgroup$ Commented Jan 15, 2022 at 11:03
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That seems like a design flaw. Another thing that makes sparks is electrical arcing - and damage would do that.

The original question seems like it was designed by the same school of thought that puts exposed thermal ports in world killing weapons.

So build a silent killer than can see in the dark. Give your protaganist a chance to damage it in some heroic manner. You hear a snap crackle noise after that, not cause the designer decided to add in a mandatory hotdog roaster. Something that hot will also likely glow, while sparks can be hidden or be visible in an appropriately dramatic moment.

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  • $\begingroup$ The OP isn't asking about electrical sparks. They're asking about dust burning on some kind of heat exchanger. $\endgroup$ Commented Jan 15, 2022 at 10:16
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    $\begingroup$ Its a frame challenge I guess? $\endgroup$ Commented Jan 15, 2022 at 10:55

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