[time-nuts] Thunderbolt stability and ambient temperature

Joseph M Gwinn gwinn at RAYTHEON.COM
Wed Jun 10 17:23:43 EDT 2009


time-nuts-bounces at febo.com wrote on 06/10/2009 03:13:19 AM:

> 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).

I read somewhere the suggestion to take the cast iron block from an old 
automobile engine and put it in a heavy insulated wooden cabinet: ~250 Kg 
of iron in an insulated box.  Cracked blocks are useless in an engine, and 
so are available in junkyards quite cheap.

Joe Gwinn




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