Quote from Svartalf on October 13, 2011, 20:35
Quote from Bacan on October 13, 2011, 15:41
I would think that the Flash Card connector pads would get compromised by the liquid. Unless of course you cleaned those contact pads of fluid every Flash Card removal & insertion cycle.
Depends on the fluid. Novec wouldn't do that, for example.
Only problem with Novec is that it's thermal transport characteristics are such that it's only slightly poorer than air conduction- compelling reason for it is that you don't have to worry about condensation with it when you drag it deep below ambient.
I'd think you'd be "okay" with Mineral Oil, really- the only problem is that it wicks up through stranded wire bundles and it's messy overall.
Really, though... I doubt that this will NEED any of those sorts of cooling. It's TDP's not much more than a Watt, if that much.
This not-so-brief note is addressed to several here who may be thinking about liquid-cooling a RasPi, etc., perhaps just for the fun of it.
It's true that 3M's Novec fluids would not cause the problems and messiness you'd encounter with oils (e.g., mineral oil), but they have their own set of challenges. Novec is actually a family of engineered fluids with different chemistries and boiling points, and they're all solvents for *something* (like any liquid, including water). In the case of Novec fluids, such as variants of hydrofluorinated ether (HFE), they're solvents for plasticizer (think soft plastic), like what you'd find in the insulation on wires and cables. As a solvent, Novec fluids don't "eat away" the insulation (i.e., it doesn't act like acid), but rather, they make make it swell and get brittle (i.e., not soft). The plasticizer dissolves into the Novec fluid, and contaminates it (creating a need for a filtering mechanism). Also, the Novec fluids I know about are generally not compatible with the silicone that you might apply to seal capacitors if you were using oil, but fortunately, no silicone is needed for Novec fluids (with most types of capacitors). Novec fluids aren't compatible with thermal grease either, not that you'd need thermal grease on a RasPi.
On the other hand, I'd have no hesitation whatsoever about immersing a RasPi in Novec HFE-7000, which has a boiling point of 34°C (94°F) at atmospheric pressure, but that's more easily said than done. You would need to use teflon-coated wires (or coat the plastic insulation with epoxy), and use a very well sealed container. HFE-7000 is pumpable below -121°C (-186°F), and circulates at that temperature about as well as water does at room temperature. HFE-7000 is slippery indeed, so a water-tight joint that would easily hold back pressurized water at room temperature would likely not hold pressurized HFE-7000 at all; it could very well spray out like a lawn sprinkler. At more than $230 to $250 per gallon, that would be an expensive lesson. The boiling point is low enough that your skin (37°C or 98.6°F) will boil it on contact, so it would be quite the negative experience if you drank some (it's inert and not particularly toxic, but would expand more than 100:1).
That said, HFE-7000 is an excellent coolant for electronics, but the packaging needs to be designed as a pressure vessel (because the liquid expands by more than 100x as it boils). The liquid would easily flow through the gap between the RasPi's processor and piggybacked memory chip(s), and would boil on the surfaces. Since liquid at its boiling point does not change temperature, all the chips would be held to the boiling point temperature plus a small delta (say, 5°C to 10°C, to account for thermal resistance, although this could be reduced somewhat with a suitable "boiling enhancement" coating), thereby achieving isothermal (single-temperature) operation with no hot spots. This has all been done before, and it works well in an engineered system (I have lots of data).
Water can be a good, cheap, phase-change thermal working fluid. Although it should be intuitive that water cannot be used in this way for direct immersion of electronics, there are four primary reasons why this is so:
1) While *pure* water may be relatively dielectric (electrically non-conductive), contaminated water is not (think of being electrocuted in water).
2) While pure water may seem inert, it can be quite corrosive to various materials, and likewise contaminated water.
3) The normal boiling point of water (100°C) is well beyond the operating temperature of most electronics.
4) When boiled, water expands by a factor of 1700x, which is so dangerous that firefighters are specially trained to deal with it, and why special licenses are required to build and operate steam-based facilities.
Using HFE-7000 as an example working fluid (since it is inert and dielectric, and poses no problem for electronics immersed in it), and by operating at the normal boiling point of the working fluid (34°C), the fluid’s capacity to absorb heat is much better than water at the same temperature, by a factor of 47x. It's also better than cold air (0°C) by a factor of 150,000x, and better than warm air (40°C) by a factor of 175,000x. The key is to use the fluid at its boiling point, where phase-change effects dominate. At that temperature, what really matters is the LHV, or latent heat of vaporization. In our example, HFE-7000 has more than 100x as much heat-absorbing capacity at its boiling point as it does at lower temperatures, and similar characteristics may be observed with other phase-change fluids. The converse is also true, in that, once the fluid has acquired the heat, it is also very efficient at rejecting it back to the ambient environment (i.e., to any temperature lower than itself).
If I were going to use an engineered fluid with a low-cost device like the RasPi, I would also consider a fluorinated ketone (FK) like Novec 1230 fire protection fluid, which has a little higher boiling point (49°C), and is thus somewhat easier to contain (due to a lower vapor pressure at ambient temperatures). Accordingly, the chips would operate at a higher temperature (say, between 55°C and 60°C), but that would typically be just fine for embedded chips. The key would be to construct the chassis from aluminum or another thermally conductive material, and hermetically seal it (which is a bit of a magic trick) with electronics and fluid inside (adding the fluid later instead requires more tricks). Simplistically, the cooling cycle would be like this: The RasPi chips boil some of the liquid, creating local vapor bubbles which condense in the fluid or on the inner sides of the container (as long as the container is below the boiling point of the fluid). Given a low-heat-flux CPU like the RasPi's ARM11, and a container with sufficient surface area, no pumps or circulation systems would be necessary to reject the heat. To minimize fluid volume (cost), the electronics should occupy most of the space within the container. The key difficulty is with achieving the hermetic sealing, which gets more difficult as the ambient temperature goes up. If the container itself cannot be kept below the normal boiling point (i.e., below 49°C, which is higher than the ambient temperature anywhere on earth), then the internal pressure will continue to rise, thereby raising the boiling point, until one of four things happens: 1) the boiling point exceeds the container's temperature, allowing the vapor to condense, or 2) the boiling point temperature exceed the electronics limits such that that devices fail and cause shutdown that reduces the temperature, or 3) the container springs a link due to the increased pressure, or 4) the container bursts due to high pressure, having no leaks to relieve the pressure (which indicates that a good design should have a pressure relief valve, like a water heater). A good pressure relief valve might cost as much as a RasPi, so such designs may be difficult to do cost effectively.
More could be said, but I've already been too verbose (more than you probably wanted to read), and I'm out of time.