heat sink for video camera in vacuum

We have some nice beefy grounding braid at work. It looks to be about 1 x

1/4" cross section (it's a tubular weave, rolled rectangular), which is probably more like 1/8" of meat once you squash out the airspace. Two feet of that will do, well, easy enough to figure out.. *+.125+inch+*+1+inch+%2F+2+feet+*+40+kelvin

But hey, heat pipes aren't terribly expensive. And they're sealed, right?

Doesn't matter, at least not very much.

I don't have proof to this effect (i.e., I haven't read it in a book anywhere), but it should be that the "effective area" of a radiant surface is proportional to its emissivity.

Suppose you have a flat black bowling ball inside a polished aluminum chamber. If the chamber's emissivity is only 0.01 (probably not unreasonable for a polished aluminum or copper surface), and the ball's,

1.00, and the chamber has 100 times the surface area as the ball, then it should be down by half.

Presumably, you can decompose a non-black surface into two components, the reflective and absoptive parts. A surface of emissivity E behaves the same as a perfectly absorbing surface with area A*E plus a perfectly reflective surface with area A*{1-E). Since the chamber is the only other surface in this problem, we can simply ignore its reflective component, because it doesn't participate in the exchange of radiation, only scattering it. Note that the scattering means, radiation from one surface will return to itself, at the rate of the ratio of its area to the total area.

Under these conditions, the chamber looks like it has the same area as the ball, with both being placed inside a perfectly reflective chamber, so the radiative transfer has dropped by half relative to a black body chamber. So the thermal conductivity drops by half, relative to a completely black chamber.

You can rephrase this problem as a thermal resistor divider: one heat source couples into the vacuum (the divider node) with a certain resistivity, while the sink couples out with another resistivity. Total thermal resistance is the sum of both: if the chamber is large and black, its resistance is miniscule and can be ignored; the source resistance dominates. If one is very reflective, its resistance per area will be much higher, with the corresponding effect on total resistance. Note this suggests the temperature of the vacuum itself is something midway between the two sinks, which is true -- pure vacuum has a temperature!

If you want to sleep better at night.... uhh, does Caswell sell a spot anodize and dye kit? Is there such a thing?...

Tim

--
Deep Friar: a very philosophical monk. 
Website: http://seventransistorlabs.com

I don't know. That's why I suggested testing it to 15 psi, the highest pressure it can see in a hard? vacuum. the case would give a much larger surface area to radiate the heat out to the chamber.

tm

It may be worth checking that the black anodised surface is truly black in the thermal IR band. I don't know for sure but I think the way they do it results in the visible dye locked under a clear layer of Al2O3. It is the properties of the latter that determine thermal emissivity.

It might look black in the visible but if it doesn't measure right using a non contact thermal IR thermometer then it isn't black in the right thermal IR waveband needed to lose waste heat. Black PVC tape as a temporary fix or black epoxy paint ought to be adequate.

Is use of silcone vacuum grease permissible inside it?

--
Regards, 
Martin Brown

Liberated is good.

Not in a position to do much concrete at the moment. I can certainly offer a decent lunch though and show you some 'stuff', and see what develops.

--sp

Best regards, Spehro Pefhany

--
"it's the network..."                          "The Journey is the reward" 
speff@interlog.com             Info for manufacturers: http://www.trexon.com 
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

1 x

ht?

I was thinking a strap lets him reposition the camera.

Yeah that was what I was thinking during my shower this morning. The internal modes of the chamber (black body) are filled up to kT. (maybe kT/2.. but whatever... factors of 2 to be ignored.) And the walls are at the same T, so radiating some amount. And it all has to balance out. Having more reflective walls just means the photons (energy) bounces around longer inside the chamber before being absorbed. But then the same bit of energy bounces around inside the metal of the chamber wall for a long time till it's emitted.

Thanks for 'playing',

George H.

e e

her

ace

e e l

his

Oh the job inquiry was mostly just a lark... but hey one never knows. (I'm sorta flaying around looking for some other project to throw myself at.)

But hey lunch might be fun... My wife mumbles about taking the kids up to the Toronto science center. Maybe I could combine trips. (Don?t hold your breathe though.)

George H.

"The Journey is the reward"

eff.com

I'm a research associate at a major university till next week. I've done it for 10 years. You are dependent on soft money (Grants) or your contract rolls over every 2 years at most places. Its filled with politics and is hard work, but beware that unless you are very lucky, it can be a short term job from the constantly changing funding situation. Very few of us ever make it to permanent staff levels.

Many professors cannot get along with technicians and engineers, because they can get very frustrated when they interact with the hardware world and it does not fit the theoretical model in their head. Sadly it is often the role of the technician or research associate to give bad news when the physics does not fit, or when the equipment breaks and you have to go tell them you need 5-10K$ to fix something.

I had been very lucky to work for a really good hardware guy for 8 years. That was a pleasure. Teaching students how to use machine tools, obtain a ultra high vacuum, make precision measurements, or align a laser system with 41 mirrors has been a absolute pleasure. I'll miss that part.

----------------------------------------------

Getting back to the camera in the vacuum, this is a job for a simple heat pipe.

Steve

It probably depends on which individual case you pick out of the general "they". Cases that are designed to seal up as they are squeezed will probably fail the test.

You might be able to _make_ a case with some acrylic and simple tools or glue, and have it be effective. The hardest part would be the window, and that could be salvaged from one of the underwater camera cases that failed the vacuum test.

--
My liberal friends think I'm a conservative kook. 
My conservative friends think I'm a liberal kook. 
Why am I not happy that they have found common ground? 

Tim Wescott, Communications, Control, Circuits & Software 
http://www.wescottdesign.com

Sounds good, let me know when you're thinking about it. The Science Centre is about 25 miles further East.

I'd be tempted to put in a five minute shut off timer to the power supply.

Perhaps a big, fat, robust mount - conduct the heat away rather than worry about radiating it away in vacuum? If using copper sheet to radiate, oxidize it black(-ish) (not difficult) for better emission. But a nice thick fitted aluminum clamp arm might be a way to go...

Two cameras (or three) with the 5 minute on timer might also be a way to go. If 5 on/5 off isn't adequate time to cool, 5 on, 10 off and use 3 in rotation if there's any need or want to keep fiddling with it continuously.

--
Cats, coffee, chocolate...vices to live by 
Please don't feed the trolls. Killfile and ignore them so they will go away.

The problem is the camera may need to be mounted a foot or more from the wall of the chamber. maybe not this time, but in other experimental runs the physical setup could be very different.

That is kind of a pain, and the technician would not have to adjust for each camera being in a different position. We have fabbed the heat spreader, and painted it black. I hope the paint has good emissivity. Still unknown is if the camera has good or bad coupling of internal heat sources to the case. We will find out next week.

Jon

You can get an estimate of emissivity with an IR thermometer. Look at the "reflection" of your hand bounced off the surface in question. Move it around maybe. If the temperature remains the temp of the paint, emissivity is high. The more of your hand's temp you can see, the lower the emissivity. Shiny copper is almost a perfect mirror at thermal wavelengths.

It's more fun with an imager.

Most paint is pretty good.

--

John Larkin         Highland Technology, Inc 

jlarkin at highlandtechnology dot com 
http://www.highlandtechnology.com 

Precision electronic instrumentation 
Picosecond-resolution Digital Delay and Pulse generators 
Custom laser drivers and controllers 
Photonics and fiberoptic TTL data links 
VME thermocouple, LVDT, synchro   acquisition and simulation

Jon Elson wrote: ^^^^^ A long long time ago, that was a good place to pick up the latest ZIP or ARJ or whatever. Now wuarchive is NXDOMAIN. Sic transit gloria mundi. (Tuesdis are usually worse!)

The one "bullet" camera I took apart didn't do that well in this department. The housing was an aluminum extrusion with slots on the inside wall for the board to slide into. The board was long and skinny, maybe 2/3 of the camera diameter by maybe 2 inches (50 mm) long. It was a chord of the housing, not a diameter. There wasn't anything filling the gap between the board and the housing. If yours is like this, it may help to fill the gap - the "solder" side may be easier to fill than the component side.

In a totally different application - a panel-mount PC+computer with about a 6" LCD - they used the sheet metal case as part of the heatsink. The hotter chips had some kind of thick foam (closed-cell?) stuck on them, which contacted the rear case when the thing was assembled. It didn't matter if this foam was also electrically conductive, because it only touched the plastic packages of the ICs. I don't know if it was a really thick version of the silicone pads you can get for mounting power transistors, or something else.

Matt Roberds

Two possibilities: put a thermal switch in contact with the powered parts, so it shuts down instead of destroying itself. Or, arrange cooling (maybe something as simple as copper capillary tubing, fed from some compressed airflow, with suitable feedthrough fittings). Silicone oil or silicone vacuum grease won't outgas much, and you can probably pack that around the circuitry, bed the capillary tube in it, and proceed.

Hopefully, you won't require welding to a conflat flange to accomplish the plumbing this will take. A blind capillary tube, with a teflon coaxial delivery tube, might be the easiest geometry.

Some 35 years ago there was a spot anodizing product called Iridite (tm). Wow, it is still around. Kind of a bummer about the hexavalent chromium of the older formulations.

?-)

Bummer that it's carcinogenic, or bummer that they don't use it anymore because it works so much better? :-)

Tim

--
Deep Friar: a very philosophical monk. 
Website: http://www.seventransistorlabs.com/

(tm).

chromium

The former, there are revised formulas that meet the same MilSpec today.

?-)

WOW! wuarchive was a LOOOONG time ago!

Thanks for the info, I wouldn't be surprised if this one was built like that, too.

We have some non-foam thermal pads that we use to couple chips to heat sinks, it works well in vacuum.

Jon