[time-nuts] Thermal time constant

[Bob Camp]

Hi

Sounds cheaper than gold plating.

I'd put a couple layers of foil in the 8" of foam as well. Maybe a layer
every 2". 

Bob


-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Brooke Clarke
Sent: Thursday, March 11, 2010 9:48 AM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Thermal time constant

Hi Bob:

How about just wrapping the aluminum block in aluminum foil with the 
shiny side out.  Then wrap the Styrofoam cube with the shiny side in 
before putting it in the aluminum box?

Have Fun,

Brooke Clarke
http://www.PRC68.com


Bob Camp wrote:
> Hi
>
> Since styrofoam is being rated as a building insulation, it's reasonable
to believe that the material on both sides is up around 1. I highly doubt
that somebody tossing styrofoam in walls is going to add a radiation factor
...
>
> In our application we're talking about a metal block inside a metal
enclosure, polishing the surfaces could drop the emissivity by>10X.  If the
budget allows, you could gold plate the surfaces in addition to polishing
them .....
>
> Bob
>
>
> On Mar 10, 2010, at 8:01 PM, Bruce Griffiths wrote:
>
>    
>> Oops forgot a factor of 4
>> Radiative heat transfer for surface with an emissivity of 1 at 300K is
about 612uW/square cm/degree C
>> which is equivalent to about 25mm of styrofoam.
>>
>> Which raises the question what's the emissivity of the isothermal surface
used when measuring the thermal resistance of a slab of styrofoam?
>>
>> Bruce
>>
>> Bruce Griffiths wrote:
>>      
>>> Radiative heat transfer (for a surface with an emissivity of 1) is
around 150uW/square cm /degreeC at 300K.
>>> That's equivalent to about 10cm of styrofoam.
>>>
>>> It seems unlikely that the radiative heat transfer component is included
in the thermal resistance rating for Styrofoam.
>>>
>>> The radiative component is independent of insulation thickness where the
insulation doesnt absorb in the 10-30um infrared region.
>>>
>>> Adding carbon black to the foam appears to increase the thermal
resistance of 25mm thick foam by about 10%.
>>>
>>> Bruce
>>>
>>> Bob Camp wrote:
>>>        
>>>> Hi
>>>>
>>>> That raises the interesting question of weather radiant transfer is
already included in the rated thermal resistance. My guess is that the 8" of
foam is enough to cover any radiation issues and still get you above 20 C /
W.
>>>>
>>>> Since you are probably starting with 1 or 2" slabs, including the
aluminum foil would be pretty easy. It can't hurt and it might help.
>>>>
>>>> Bob
>>>>
>>>> On Mar 10, 2010, at 7:29 PM, Neville Michie wrote:
>>>>
>>>>          
>>>>> I have been wondering about achieving the rated thermal resistance
from plastic foam,
>>>>> the problem being that radiant transmission may be very strong through
the foam.
>>>>> What happens if you interleave concentric sheets of foam plastic with
aluminium foil? (taking care to keep the foil
>>>>> on isothermal surfaces)
>>>>> Will this stop the radiant transfer and leave only the thermal
conduction of the plastic foam?
>>>>> cheers, Neville Michie
>>>>>
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