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Just how much hotter will a black enclosure be?

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Hi, DS.
Sounds like it's time to end the conjecture and actually test the enclosure for heat transfer capabilities.

Find a cheap meter that allows you to plug in a K-type thermal sensor. Some even come with the sensor. If not, buy a sensor recommended by the meter manufacturer. The meter may be a dedicated thermal device or a DMM with the plug and a thermistor selection on the selector dial.

Install a power plug on the enclosure. Seal all other openings and holes. Install a power resistor rated for 3 watts or better (you say the SBC is rated for 2.8W, max) across the power plug inside. This resistor must be a value such that the voltage you apply will make it dissipate 2.8W (or so). For instance, if you have a 12V supply handy, you can apply this across a 51 Ohm resistor (or multiple resistors to arrive at that value) to get around 2.77W.

Drop the K-type sensor (this usually looks like a tiny bead at the end of two wires in a funky sheath of insulation) into the enclosure. Install the enclosure top as best you can, trying to seal it tight without crimping the sensor wires. Plug the sensor's bladed plug into the meter. Now turn on the meter and power supply and watch the temperature rise. If it stops rising and stabilizes under the max temperature for the SBC, you're golden. If it rises higher than max, you're toast. Do this in the shade and in the sun.

Either way, you've settled the question and can get on with it.
kenjj
 
Hero999 said:
Painting it white might not make any difference as the white paint might still absorb in the infrared range which is what really makes the difference.

You could try bolting a large heat sink to the shady side of the box.

Its easley tested , paint two peices of steel , one white, one black and put them in the sun on a summer day. You will notice a very large difference in temp.
The white one will be about 10 degrees above the ambeint temp., and the black one about 30 -50 degrees above ambeint temp.


Some materials heat up more or less in a given sun /temp, and I guess thats because some radiate heat back out more effectively.

Thats why you dont see composite(epoxy /fiberglass/ graphite) aircraft painted anything but white. Even painted yellow they can get hot enough soften and distort/sag/ and cause structural failures.

sam
 
The white one will be about 10 degrees above the ambeint temp., and the black one about 30 -50 degrees above ambeint temp.
Reason for this is that the energy which isn't reflected is converted to heat. Although some (but not necessarily all) white paints may not reflect infrared, it does reflect a lot of the visible spectrum, all of which would otherwise be converted to heat.
 
justDIY said:
the paint is only but a thin skin, the black box is still underneath, some of the visible light will be reflected by the paint, but most of it will still be absorbed by the box.
As far as reflectivity is concerned only the outer layer really counts.

white paint on a virgin color box will work better, but that sounds like a lot of trouble. how about searching out another enclosure vendor?
More energy reaches the earth in the infrared part of the spectrum than the visible so the colour makes little difference. A white box that absorbs infrared will actually get hotter than a black box that reflects infrared radiation.

The best solution is to cover it with a reflective coating so I would wrap it in aluminium foil if I were you.
 
Thanks everyone for the comments--I'm impressed with all the discussion this topic generated.

My solution is to always keep the unit out of direct sunlight on hot days and be very strict in the instructions for portable units when I am not mounting the equipment myself.

Unfortunately, because this is for a computer with a display (+70C or +158F max), keyboard, buttons, several cables, etc., I can not cover it up. I also cannot control the color of the paint because it would add too much time and expense to the manufacturing process.

Thanks,
Dale
 
What about my aluminium foil idea though?

sam2 said:
Its easley tested , paint two peices of steel , one white, one black and put them in the sun on a summer day. You will notice a very large difference in temp.
The white one will be about 10 degrees above the ambeint temp., and the black one about 30 -50 degrees above ambeint temp.
You're missing the point, it's infrared radiation that makes the difference not visible, it doesn't matter what colour the pieces of steel are, it's how much infrared radiation they reflect or absorb that makes the difference.
 
Good point.

However, I remember rightly the box is black. Also some aluminium alloys are more reflective than others and a rough surface is more likely to gather dirt than a smooth surface. I remember my dicast aluminium box being sigificantly less reflective than the matt side of a piece of aluminium foil.
 
Hero999 said:
As far as reflectivity is concerned only the outer layer really counts.

More energy reaches the earth in the infrared part of the spectrum than the visible so the colour makes little difference. A white box that absorbs infrared will actually get hotter than a black box that reflects infrared radiation.

The best solution is to cover it with a reflective coating so I would wrap it in aluminium foil if I were you.

With visable light yes ,with infrared no.

Not true,as far as heating goes its about 50/50, visable light/infrared

Foil would be redundant. Without the foil the infrared would pass through the paint and be reflected by the metal surface of the box anyway, especially if it was polished.
sam
 
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Hero999 said:
What about my aluminium foil idea though?


You're missing the point, it's infrared radiation that makes the difference not visible, it doesn't matter what colour the pieces of steel are, it's how much infrared radiation they reflect or absorb that makes the difference.


Again not true, I know from research, experimentation, and practical application that that the color of an object dramaticaly influences how much energy it absorbs from VISABLE light.

In fact even relatively rough metalic objects will reflect infrared specularly( like a mirror), because of infrareds longer wavelengths( longer than visable light).
polished stainless steel, alum, and gold are frequently used as reflectors in infrared industrial applications.


sam

Have you never gone out in your garden on a sunny day, and put your hand on two similar objects painted light and dark colors?!
 
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sam2 said:
Again not true, I know from research, experimentation, and practical application that that the color of an object dramaticaly influences how much energy it absorbs from VISABLE light.
I am not doubting that, all I'm saying is that I wouldn't judge how well something's going to absorb infrare, therefore how wrm it will get in the sun, based on soley its colour.

In fact even relatively rough metalic objects will reflect infrared specularly( like a mirror), because of infrareds longer wavelengths( longer than visable light).
polished stainless steel, alum, and gold are frequently used as reflectors in infrared industrial applications.
True.

Have you never gone out in your garden on a sunny day, and put your hand on two similar objects painted light and dark colors?!
No, but I've found that some white objects get very hot even though I wasn't expecting them to, like our white plastic garden furniture for example.
 
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