On 02/02/2017 09:09, A. Dumas wrote:
> 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.

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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