[time-nuts] Thunderbolt stability and ambient temperature

Poul-Henning Kamp phk at phk.freebsd.dk
Wed Jun 10 03:13:19 EDT 2009


In message <4A2EFC6D.4020205 at xtra.co.nz>, Bruce Griffiths writes:

Bruce,

>>> The thermal time constant (not the thermal impedance per se) is what
>>> matters [...]

That is pretty much exactly the (mis-)definition of thermal impedance.

Thermal timeconstant or thermal corner-frequency had been much better names.

>>> It is possible to construct an enclosure with a long thermal time
>>> constant together with relatively low thermal resistance so that the
>>> temperature of a GPSDO or similar device within the enclosure only
>>> increases by a relatively small amount.
>>
>> Nope. This is essentially a thermal low pass filter.

Well, yes you can, but it is not very useful:

A really huge block of metal will do that:  It can transfer a lot
of heat (=low resistance), but will take a long time doing so (=high
impedance).

>Adding just a little insulation to the added thermal mass can
>dramatically increase the thermal time constant combined with a modest
>increase in operating temperature.

Isn't that exactly what I explained initially ?

A huge block of aluminium, encased in 1" of styrofoam ?

>>> Readily available inexpensive aluminium foil is a cheaper alternative to
>>> expensive noble metal foils.

But it does not stay as reflective.  By the time you add this layer to
the construction I mentioned, you care about the difference between
0.98 and 0.99.

Poul-Henning

-- 
Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
phk at FreeBSD.ORG         | TCP/IP since RFC 956
FreeBSD committer       | BSD since 4.3-tahoe    
Never attribute to malice what can adequately be explained by incompetence.




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