rpiswag
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Does the Raspberry Pi work in a Vacuum?

Mon Mar 02, 2015 3:44 am

Does the Raspberry Pi model B,B+ or 2B work in a vacuum? I am worried about the caps exploding on the B. When I say vacuum I mean the definition about zero air pressure and not the thing you (or your robot) cleans your house with.
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OSIAS
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Re: Does the Raspberry Pi work in a Vacuum?

Mon Mar 02, 2015 4:28 am

the only response i can think of is why wouldnt it.

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Mandrewpi
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Re: Does the Raspberry Pi work in a Vacuum?

Mon Mar 02, 2015 4:31 am

If it doesn't work it isn't too expensive but I don't see why it wouldn't
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Re: Does the Raspberry Pi work in a Vacuum?

Mon Mar 02, 2015 4:39 am

rpiswag wrote:Does the Raspberry Pi model B,B+ or 2B work in a vacuum? I am worried about the caps exploding on the B. When I say vacuum I mean the definition about zero air pressure and not the thing you (or your robot) cleans your house with.
Dave Akerman has sent a number of Raspberry Pi's up to high altitude and I don't believe the Raspberry Pi has failed. So, I would say probably. What are your plans for the Raspberry Pi and a vacuum?

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Re: Does the Raspberry Pi work in a Vacuum?

Mon Mar 02, 2015 5:08 am

I wouldn't worry about a lack of pressure. I'd worry about a lack of a medium to convect heat away from the SoC and LAN chips.

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iinnovations
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Re: Does the Raspberry Pi work in a Vacuum?

Mon Mar 02, 2015 5:58 am

W. H. Heydt wrote:I wouldn't worry about a lack of pressure. I'd worry about a lack of a medium to convect heat away from the SoC and LAN chips.
Radiation works in vacuum.
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adlambert

Re: Does the Raspberry Pi work in a Vacuum?

Mon Mar 02, 2015 6:02 am

There is a B+ currently being tested for space flight certification. I know this particular application is for use inside a pressurised space craft but I think the testing would cover at least being safe during a depressurisation event, if not staying functional.

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Re: Does the Raspberry Pi work in a Vacuum?

Mon Mar 02, 2015 6:52 pm

Cooling would be the major problem in a vacuum. I do not think they would work just with radiant energy cooling!

An older version such as the B model might lose the capacitor can, but they can work without it anyway. Again cooling would be a significant issue.
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Re: Does the Raspberry Pi work in a Vacuum?

Mon Mar 02, 2015 7:13 pm

I can see that cooling in a vacuum would be an issue if the device was normally being cooled with the help of a large surface area heat sink with heat being carried away with convection or forced air movement.

It's not so clearly obvious in the case of the little Pi SoC.

There must be somebody out there in school or uni or whatever with a vacuum chamber they could borrow to measure this.
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Re: Does the Raspberry Pi work in a Vacuum?

Mon Mar 02, 2015 9:24 pm

There have been enough Pi's been taken to the edge of space to determine that the little blighters don't over heat unduly when they don't have air moving around. There have been Pi's up to around 40km where the air pressure is so near a vacuum as to make no difference. Certainly space suit territory.
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Re: Does the Raspberry Pi work in a Vacuum?

Mon Mar 02, 2015 9:26 pm

I believe there are heatsink designs for better radiative cooling instead of convection, if in doubt something like that could be used to help with cooling in low-pressure environments.

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Re: Does the Raspberry Pi work in a Vacuum?

Mon Mar 02, 2015 9:30 pm

Trust me, guys. Radiation will work fine.
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Re: Does the Raspberry Pi work in a Vacuum?

Mon Mar 02, 2015 11:32 pm

iinnovations wrote:Trust me, guys. Radiation will work fine.
Have you tested it? For how long?

As regards 40Km being, essentially, space, try moving at orbital velocity at that altitude.

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Re: Does the Raspberry Pi work in a Vacuum?

Tue Mar 03, 2015 2:11 am

At an altitude of 40Km the air density is between 100 and 1000 times lower than in is on the ground. I forget exactly.

That is low enough to be confident that convective cooling is not having any significant effect up there. If the Pi has worked up there without any significant increase in operating temperature then we can assume radiative cooling is sufficient.

We could imagine adding a nice shiny heat sink might make it run hotter!

Moving up to higher vacuums is not going to make any difference worth mentioning.

So, the question is, did the guys doing the Pi in high altitude balloons experiments log the temperature of the Pi? If so what did they see?
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iinnovations
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Re: Does the Raspberry Pi work in a Vacuum?

Tue Mar 03, 2015 3:24 am

W. H. Heydt wrote:
iinnovations wrote:Trust me, guys. Radiation will work fine.
Have you tested it? For how long?

As regards 40Km being, essentially, space, try moving at orbital velocity at that altitude.
If natural convection is sufficient for a Pi at the relatively cool temps it runs at, and it ok to much hotter temps (it is), radiative cooling will be fine. It also goes as T^4, so even if it's necessary to heat up a bit, that's fine. It's also radiating into a vacuum, which is ideal.

How long isn't important. It will reach steady-state.
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adlambert

Re: Does the Raspberry Pi work in a Vacuum?

Tue Mar 03, 2015 5:50 am

I believe temperatures in the upper stratosphere are pretty cold, though there may be reactions to the UV solar radiation picking temperatures up in daylight hours it will still be below zero centigrade at its warmest.

So could vacuum behaviour at ground level ambient be different from 40km AMSL?

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Re: Does the Raspberry Pi work in a Vacuum?

Tue Mar 03, 2015 6:00 am

adlambert wrote:I believe temperatures in the upper stratosphere are pretty cold, though there may be reactions to the UV solar radiation picking temperatures up in daylight hours it will still be below zero centigrade at its warmest.

So could vacuum behaviour at ground level ambient be different from 40km AMSL?
Yes. Radiation will be less efficient with background radiation and reflection from the ambient.

Strictly speaking, in vacuum "ambient temperature" is irrelevant/meaningless for a body with a finite mass, as are conduction and convection. You need molecules of something (e.g. air, water) to transfer heat to/from a body. A good analogy is touching 50F water vs. 50F air. One feels much colder because the medium has a different capacity to hold and transfer heat.

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Re: Does the Raspberry Pi work in a Vacuum?

Tue Mar 03, 2015 9:46 am

adlambert wrote:There is a B+ currently being tested for space flight certification. I know this particular application is for use inside a pressurised space craft but I think the testing would cover at least being safe during a depressurisation event, if not staying functional.
It is indeed being tested for depressurisation, along with loads of other stuff, ready for space certification and the trip to the ISS for AstroPi. Note it's the B+ not the B2 being tested. The list of stuff to test keep getting longer apparently! It's also getting a nice milled aluminium case.
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Re: Does the Raspberry Pi work in a Vacuum?

Tue Mar 03, 2015 11:45 am

You cannot count on radiative cooling of electronics in a hard vacuum.

At 85C the RPi's SoC radiates just 145mW, and gets 65mW back from the walls of the vacuum chamber (at 20C). Compare that with 300-600mW power consumption. In order to shed 600mW the chip must reach 250C!

A conductive path is needed to cool the chip. In the case of the RPi conduction to the PCB may be enough, but I wouldn't count on it.

(I'm assuming an emissivity close to 1. The real emissivity will be slightly lower.)

adlambert

Re: Does the Raspberry Pi work in a Vacuum?

Tue Mar 03, 2015 2:23 pm

This is an interesting thread, something I hadn't considered.

I looked this Stefan-Boltzmann law.

http://hyperphysics.phy-astr.gsu.edu/hb ... tefan.html

Seems to apply.

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Re: Does the Raspberry Pi work in a Vacuum?

Tue Mar 03, 2015 5:50 pm

TimG wrote:You cannot count on radiative cooling of electronics in a hard vacuum.

At 85C the RPi's SoC radiates just 145mW, and gets 65mW back from the walls of the vacuum chamber (at 20C). Compare that with 300-600mW power consumption. In order to shed 600mW the chip must reach 250C!

A conductive path is needed to cool the chip. In the case of the RPi conduction to the PCB may be enough, but I wouldn't count on it.

(I'm assuming an emissivity close to 1. The real emissivity will be slightly lower.)
There will be significant thermal conduction to the PCB, mostly by way of the power and ground pins spread via the power and ground plane, so pure Stefan-Boltzmann on the SOC is going to be far more pessimistic than reality. That said, rigorous thermal analysis of this is non-trivial. For what it's worth there are some thermal images of the Pi on the web, e.g.
http://www.tooms.dk/?page=http%3A//www. ... C_ID%3D286
One thing that is apparent from this is that the LAN chip and the Regulator are running hotter than the SOC in that particular case. I believe this has changed with the B+.

As a back of the envelope estimate, one could do Stefan-Boltzmann using the area of the board and total power dissipation (say 0.004 m^2, 2 W, 0.85 emissivity) which gives 320 K for "average board temperature". Assuming the temperature gradients stay the same as in the linked thermal image, the SOC is about 10 C higher and the LAN chip is about 16 C higher. If I haven't bungled the calculation, that puts us remarkably close to normal room temperature operating conditions. That people have done high altitude work with the Pi gives some credence to this estimate.

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Re: Does the Raspberry Pi work in a Vacuum?

Tue Mar 03, 2015 6:03 pm

TimG wrote:You cannot count on radiative cooling of electronics in a hard vacuum.

(I'm assuming an emissivity close to 1. The real emissivity will be slightly lower.)
I don't think this is true (the basic assumption that you cannot count on radiation to cool electronics). While in this case I underestimated the Pis power consumption in my mental back of envelope, the math still shows radiation sufficient. A more realistic consumption figure is closer to 1W and a good SOC max target temp is 85C, since it will begin to take cooling measures at this point.

I think the issue with your approximation is probably that you are assuming an emission area that is only the SOC -- that it is floating. In reality, the inefficiencies that consume power are spread across the board, and hence the area is larger. More important, the surrounding PCB will conduct heat away and also radiate. You'll find that as you make successively larger areas radiate at elevated temperatures, you will very rapidly eclipse 1W at well below 85.

Now, if we're actually in vacuum, there is zero conduction away from the board, and the entire thing will be isothermal, except for minor differences in emissivity and the gradient across which the conduction takes place. Sure, the thermal conductivity of FR4 is bad, and you would need to take into account the traces, so it gets complicated. But at first approximation, the entire board is emitting at an elevated temperature, whose temperatures depend on the distribution of consumption and how it is conducted across it. Under these conditions (radiation from surroundings notwithstanding), you will find that the board doesn't have to go far from room temperature to emit 1W. Now to bring our Pi back from a black hole and assume some background radiation (from your vacuum chamber, the sky, whatever), it needs to be some degrees over the ambient temperature of the surroundings to effect any radiation. I find that if the whole board is 45C it will radiate about 1.5W. Obviously the SOC and other components will need to be hotter to accomplish this, but then they'll radiate more .. you get the picture. It's a complicated problem that would need a numerically integrated solution.

As for conduction, unless there is a path off the board, the heat has nowhere to go. A good first-order approximation is that it doesn't.

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Re: Does the Raspberry Pi work in a Vacuum?

Tue Mar 03, 2015 6:49 pm

I don't see why it would not.. Look at all the electronics in space, the station and etc.. Matter of fact it would probably run quite faster being in such a cold enviorment.. Put ur computer in a freezer sometime and leave it running and see just how much faster it will run.. Same principal.. So Military's opp main frames are stored way below zero for speed.

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Re: Does the Raspberry Pi work in a Vacuum?

Tue Mar 03, 2015 6:57 pm

Ask me this question after we go through the non-convective environment and explosive decompression tests as part of our functional certification for upload to the ISS.
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Re: Does the Raspberry Pi work in a Vacuum?

Tue Mar 03, 2015 7:01 pm

If the RasPi were in actual space, rather than just a vacuum as in the OP question, there will be other thermal events to consider. Direct sunlight, shaded etc. Temperatures vary widely in space.

So the question is whether it will operate in a vacuum. From the answers it appears the answer is yes, under controlled conditions.

Whether it would operate in open space? It would not operate in open space without an enclosure to help control thermal events. It would either be to cold to operate or to hot to operate. The operable range being somewhere in -20C to 85C. That range is narrowed somewhat by the fact that if it is already at 85C then there will be little or no headroom for the heat generated by the board itself. I am not sure what temp the clock crystals will start having trouble at either.

It would be interesting to see the results of the testing when they are complete!
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