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Best Pi 3 case with 40mm+ fan mounting?

In perfect convective heat transfer, the heat transfer coefficient will be constant and "Newton's law of cooling" applies, so then yes, with changing ambient temperature a new equilibrium will be reached where the temperature difference is the same as before; i.e. 1 degree hotter room = 1 degree hotter cpu.

BUT... to quote Wikipedia: "The heat transfer coefficient is often relatively independent of temperature in purely conduction-type cooling, but becomes a function of the temperature in classical natural convective heat transfer. In this case, Newton's law only approximates the result when the temperature changes are relatively small."

In open cases, Newton's law will almost certainly be a good approximation. Maybe the lack of circulation in druck's fridge explains the slight divergence there, and I don't know WHAT happened in the car.

Reply to
A. Dumas
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Argh! That was supposed to say conductive, convective is the OTHER case. Sorry.

Reply to
A. Dumas

I don't think what you quoted says what you think it says. Conduction or convection don't care what temperature things are at until the temperature gets so extreme that it causes a change in physical properties, i.e. liquefying or chemical changes.

I think what the text is saying is that for convection the heat transfer coefficient will depend on the *difference* in temperature instead of the ideal case where the temperature delta doesn't matter.

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Rick C
Reply to
rickman

Its horrendously complex. Radiation is to the 4th power of temperature, and empirical data shows that convection is massively dominated by the ability of air to get way from the heatsink either cost its blown, or because there is a good path for convection.

Strapping a huge heatsink on a chip in a closed box merely delays the onset of thermal issues, it doesn't solve them

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Dennis Miller
Reply to
The Natural Philosopher

No, while they aren't a necessity, but they still lower the temperature by a couple of degrees, which may be useful in some circumstances.

---druck

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Reply to
druck

Well here is the data:-

Chip temperature : 26C Inside car air : -5C On car seat : -4.5C On car roof : -7C On windscreen : -6C Under car : -1.5C On path : -3.5C

It certainly shows the radiative losses of the probes on conductive surfaces which could see the sky, as they read significantly less than the ambient air temperature. The one sheltered under the car and on the less conductive path were considerably warmer. The car hadn't been driven for 14 hours, so was otherwise cold.

So there could be some radiative warming to the Pi from the car's interior which eliminates some of the effect of the colder ambient temperature. Where as in the fridge the surroundings are at usually slightly lower than the air temperature, given people keep opening the door.

It's a long time since I did thermodynamics at university, and I may well have been asleep.

---druck

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Reply to
druck

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