sensor questions

[jludwig]

On Saturday 07 January 2006 10:50, Tim wrote:
> jludwig:
> > It takes some time for the temperature sensors to respond and some time
> > for the mass of the heat sink to warm.
> >
> > This being the case,  from a "cold" (like say a half hour or more) start,
> > one can get an idea on the accuracy of the temperature sensor(s).
>
> In general, heat sensors are going to be close to the hot device, and
> measure the temperature at the point where heat will be a problem.
>
> Measuring the temperature of a heatsink, itself, is next to useless.
> There's no point in having a sensor further along the metal.  Quite
> apart from the delays involved in heat transfer, there'd be heat loss,
> too (you'd be reading colder temperatures than at the device that's
> getting too hot for comfort).
>
> I'd expect sensor readings to react fairly quickly; mere moments, not
> minutes.  Of course, they could be too late to react to something
> getting too hot to shut it down, that sort of thing is best handled in
> another way.  But you should have a useful gauge of whether the device
> is cold, warm, hot, or overheating, on average.
>
> You can see the sort of times involved between sensors registering
> changes and devices changing temperature if you stop the fan on
> something.  Depending on the device, the temperature climbs at a modest,
> and predictable rate, with the sensors showing it accordingly.
>
> NB:  Anybody thinking of testing that, do so with care.  Be it on your
> own head if you fry your CPU.  I did my testing while everything was
> still well under maximum operating temperatures.
>
> --
> Don't send private replies to my address, the mailbox is ignored.
> I read messages from the public lists.
Actually, (having been in the aerospace business thermal cycling satellites 
etc.) the thermal 'inertia' is much greater than you would expect.