On 17/04/2018 14:33, T i m wrote:
On Tue, 17 Apr 2018 14:06:56 +0100, Martin Brown
wrote:

On 17/04/2018 13:50, T i m wrote:
Hi all,

I have some neat extruded ally cases for my std Raspberry Pi's and I
was about to put my new 3B+ in one and wondered, because it seems to
get the warmest of all the Pi's so far ... if the interior finish
(shiny / matt) / colour (white to matt black) would make any
difference to how efficiently the case would re-radiate the (Pi) heat?

There is some difference notably shiny metallic surfaces are terrible
radiators of thermal IR at ambient temperatures.

I hadn't really considered how will the case would re-radiate the
heat.

Almost any colour of
matt paint is black in the thermal IR band.

Ok.

How much difference it makes
on a plastic box which is intrinsically a poor conductor I don't know

Sure, but this is ally so I thought it might be more predictable etc?

If you can connect the aluminium box to the heatsinks then that is the
best way to get the heat to the outside.

So, the case has a slightly shot blasted / gold anodised finish
throughout and I was wondering if painting the inside matt black (acid
etch spray primer and matt black spray) would have any cooling
advantage at all or would it in fact actually make matters worse
because the paint would act like a thermal barrier?

It will probably help improve things by painting it black.

So from what you said above that means inside and out?

I once built
a linear 12v PSU using the then new 78xx series linear regulators and it
went into thermal foldback mode when put into a shiny aluminium box.

(A bit like I have here etc).

Painting the shiny metal matt black solved the problem completely - it
still ran warm but nowhere near going into self protection mode.

Interesting.

I was surprised at the time that it made such a difference. When an
object becomes around 30C above ambient temperature the radiation losses
start to become significant (scale like T^4 in absolute temperature).

Anyone confirm the practical science behind it all please?

Arranging it so that there is a vertical path and warm air rises may
help to encourage it to stay cool.

That is my normal solution with PC's and the like but in this case I
was sorta interested to see if I could 'get away' with it by just
changing the colour etc.

I was sorta working from the idea that many laptop 'brick' type PSU's
are in sealed / plastic enclosures ... often get warm or even fairly
hot without much in the way of negative consequences (what feels 'hot'
to us isn't generally that hot for most electronics etc).

Forced air ventilation is the other way to keep things cool.

I was just trying to remind myself of the science behind the ability
of a surface to reflect or absorb heat and if it was a function of
it's colour or not, especially when the heat wasn't being radiated in
the visible light spectrum?

There are some very cute designer materials that are capable of cooling
when placing in sunlight under a clear blue sky. They reflect most of
the visible light incident from the sun whilst radiating incredibly well
in the thermal band IR.

http://newsroom.uts.edu.au/news/2015...being-hot-city

Priced at the unobtanium level at present but it is an interesting
technology if a cheap plastic film version can be manufactured.

--
Regards,
Martin Brown