CyberTruck SS Heat

xtreembob

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This has probably been covered but mildly concerned about the skin getting really hot in direct sunlight. Luckily there won’t be any door handles to access the vehicle but wanted to get others thoughts about this. Boondocking in the desert sun of ~100 degrees would make touching the vehicle unlikely. I know there has been talk of wraps for the truck but I like the SS look and want to keep it simple.
This will not be quite as big a problem in the cold of winter but if cold enough your skin could stick to the surface when touching I suppose. Of course I’m talking below zero temps.
Thanks!
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ajdelange

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So a white car would have the same temp profile touching it as a black car in the sun?
Definitely not the same. For paints the black one would be hotter than the white because the black paint absorbs more than the white and both reradiate about as much as they absorb. Stainless being darker in color (unless it's mirror polished) than white paint absorbs more than white paint but it does not reradiate nearly as much so it could quite possibly be hotter to the touch than a white painted piece of stainless and perhaps even than stainless painted some other color. It all depends on the actual absorptions and emissivities of the materials.

While we are on the subject I'll mention that should you decide you want to experiment by putting a piece of stainless in the sun you won't be able to use an IR (contactless) thermometer to check its temperature unless you dial in the correct emissivity for the piece you are experimenting on. As emissivities are all over the map and even if you have the right one setting it in probably involves finding some obscure menu it is much easier to just stick a piece of electricians tape on it somewhere and point the gun at that.
 
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Delusional

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Thermal Conductivity is a measure of how fast a material transfers heat within itself, and how fast it gives up it's heat to another object in contact with it. Conductivity plays a dominant role in the equation of how hot something feels to the touch.
Put a piece of plastic and a piece of steel in the sun, they might both be the same temperature but the plastic will be much easier to hold in your hand. Plastic has much lower thermal conductivity... and density, which also helps.

I don't know why, but stainless has a much lower thermal conductivity than other steels.
Thermal Conductivity of Steel 300M57.739 W/m*K
Thermal Conductivity of Stainless Steel 30414.644 W/m*K
Source. https://thermtest.com/materials-database

Meaning that at the same temperature, the stainless will not give it's heat to your hand as quickly, and it's easier to touch.

This works for cold too... I wonder if you could safely touch your tongue to a frozen CYBRTRK.
 

Dids

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Don‘t know. So a white car would have the same temp profile touching it as a black car in the sun?
No. Black gets hotter. But silver and white tend to be the coolest. So I imagine the ss will be similar or better than a silver car
 

ajdelange

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Thermal Conductivity is a measure of how fast a material transfers heat within itself,
Yes.

and how fast it gives up it's heat to another object in contact with it.
No. That's determined mostly by the thermal conductivity of whatever separates the two bodies and their temperature differential. Thus, for example, there will be very little transferrance of heat between two blocks of silver (both with very high conductivity) separated by even a fraction of a mm of vacuum even though the blocks are at widely different temperatures. In the case of interest here what's between the metal and you is your skin which has conductivity of 0.2 - 0.7 W/m•K

Conductivity plays a dominant role in the equation of how hot something feels to the touch.
Yes, but it's the conductivity of your skin - not the thing you are touching. The internal conductivity of what you are touching does have an influence but what really matters WRT the thing you are touching is its thermal mass and its temperature.


Put a piece of plastic and a piece of steel in the sun, they might both be the same temperature but the plastic will be much easier to hold in your hand. Plastic has much lower thermal conductivity... and density, which also helps.
Its the density and specific heat i.e. the thermal mass of the plastic (which is less than that of an equal volume of steel) that is the driver more than the plastic's conductivity. But the most important factor of all is the emissivity of plastic is much higher than that of stainless so it wont get as hot - even if painted black. Probably - depends on the actual material properties as I indicated in my previous post.


I don't know why, but stainless has a much lower thermal conductivity than other steels.
Thermal Conductivity of Steel 300M57.739 W/m*K
Thermal Conductivity of Stainless Steel 30414.644 W/m*K
Source. https://thermtest.com/materials-database
The crystal structure.

It's a bit disingenuous to pick a high thermal conductivity steel and compare that to stainless when in fact steel conductivities can vary quite a bit:

Steels.jpg


Within this histogram there are 2 stainless samples, both in the second bar.

Meaning that at the same temperature, the stainless will not give it's heat to your hand as quickly, and it's easier to touch.
I've been wishywashing about here trying get across that conductivity of the material is not the main driver here while at the same time recognizing that it is a factor. Electrical engineers use an electrical circuit to model problems like this because the math is the same. So I'm going to show that model (but not do any math) in the hopes that it might make my reasoning clear.

Net.jpeg


The skin of the truck, on the left, is made up of separate bits all interconnected each represented by a capacitor whose capacitance is proportional to the bits' thermal mass. Conductivity between the pieces is represented by the thermal impedances Rs. The skin is assumed to be initially at uniform temperature so all the capacitors are equally charged to some temperature higher than human body temperture. You are similarly composed of thermal masses interconnected by thermal impedances. You are essentially a bag of water at 37 °C. Water has thermal conductivity of a bit less than 1 W/m•K (i.e. about the same as human skin and plastic). Steel has conductivity of 10 - 60 W/m•K (with stainless at the low end).

Water has specific heat of about 4000 J/kg•K and density of about 1 g/cm^3. Steel has specific heat of about 500 J/kg•K and density of about 7.9 g/cm^3. Thus the relative sizes thermal masses of the same volume in the steel and you are about the same and so the capacitors in the model are all about the same size with steel. But the interconnecting impedances are 10 to 60 times higher in you. When the switch is closed Cs discharges through Rk (your skin resistance) to charge Cn (the thermal mass that contains your nerve ending) so its temperature rises as the temperature of the steel to the left of the switch falls. But as soon as that happens the next capacitor to the left discharges to charge the capacitor next to the switch. This happens much faster, whether the conductivity be 10 or 60, than Cn, in you, discharges to charge the first Cu in you because the conductivity is at least 10 times higher in the steel. Cn charges much faster than Cs adjacent to the switch discharges.

Now with plastic the Rs are bigger so the caps distal from the contact point take longer to charge the cap right next to the switch and that cap discharges faster meaning the plastic cools faster at the contact point. Thus, I suppose, we conclude that that if the conductivity of the material is small it does effect how hot or cold it feels but if the conductivity in the material is 10 or more times that of the body, it doesn't make much difference.

This works for cold too... I wonder if you could safely touch your tongue to a frozen CYBRTRK.
Yes it does and by the same mechanism with the difference being that in the heating situation as soon as Cn gets sufficiently charged you say "ouch" and take your finger away. With your tongue on the swing set you can't do that. Not to take that bet one of the many pieces of good advice my dear father gave me over the course of his life. Apparently my grandfather never passed it along to him.
 
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Mini2nut

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Another side benefit of not having door handles. No searing hot stainless steel door handles to touch when entering the vehicle on a scorching hot summer day.
 

ReddykwRun

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Well there is the old low tech standby if temps are a big concern in your area.

1626551510246.png


Or you can upgrade to.

Prodex 10mm - Insulation/Radiant Barrier (reinforced foil/closed-cell polyethylene/reinforced foil) and make your own cover, tape is extra :p
83' x 4' rolls
Like a beer coozey for you truck.
 

ajdelange

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As you may have noticed I tend to get a little obsessed with this geek stuff so I had to go check out what would really happen. The sun isn't very cooperative today but even through some cloud the stainless of my barbecue pretty quickly rose to 10 °C above ambient temperature whereas the ordinary painted steel rim of a table in the same vicinity only rose 1 °C. Emissivity of the stainless is obviously low as without the tape trick it's temperature reads 15 ° below ambient. And, of course, it's rise above ambient confirms this.

Obviously the stainless feels hotter than the regular iron because it is 9 ° higher in temp.

Ignoring all the jargon etc in No. 7 (which I'm sure most of you did) and trying to put it in a nutshell I'll just say that the conductivity of the steel doesn't have much effect on how it feels to you because whatever steel you have it's conductivity is always at least 10 times yours. You can't draw heat from it fast enough to cool it.
 
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fritter63

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So it's 84 degrees here right now (3:40 pm). Just went over and touched the hood of my SS BBQ grill that has been in the direct sun all day. Didn't burn me, but not comfortable to keep my hand. on it.

Then I went over and touched the cross bar of my "Santa Maria style" grill, it's black painted steel. It was NOTICEBLY cooler to the touch.

Color me surprised, but I never had to take thermal dynamics in college.

IMG_1022.JPG
 

Dids

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Wow reading this one would get the impression that stainless which has low conductivity would get very hot in the sun. But this could be false. The measure of how hot a material gets is its emissivity and stainless ranges from low 0.07 for shiny polished stainless to a fairly high 0.8 for weathered stainless. Will the stainless stay hot for a longer time once it's moved from the sun. Certainly! Will it get hotter in the sun than painted steel? Nope, depends on color of paint, finish on the stainless etc.....
Chrome bumpers get even hotter.
 
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