[time-nuts] Thermal Time Constant

[Bruce Griffiths]

Thermal resistance is measured in degrees (C or K or ..) per watt.
Its inversely proportional to area and proportional to thickness.

I think the clueless clown who created that table means that for a 1 
square meter panel of the specified thickness the thermal resistance is 
the tabulated value measured in Kelvin/Watt.

Bruce

Brooke Clarke wrote:
> Hi Bruce:
>
> What does m2K/W mean?  See:
> http://building.dow.com/europe/uk/proddata/styrofoam/thermal.htm
>
> 50 mm it's about 1.5 and for 100 mm it's about 3.
>
> Have Fun,
>
> Brooke Clarke
> http://www.PRC68.com
>
>
> Bruce Griffiths wrote:
>> Bruce Griffiths wrote:
>>> Brooke Clarke wrote:
>>>> Hi:
>>>>
>>>> For some time I've considered surrounding a free running 32678 Hz 
>>>> oscillator (like a Dallas 32khz, or one of the newer Maxim units) 
>>>> with thermal mass and insulation in order to get the time constant 
>>>> into the range of some days.  To get a feel for it a simple 
>>>> experiment shows that a half inch diameter brass rod 3.75" long 
>>>> (102 grams) has a thermal time constant of about 6 min 35 seconds 
>>>> when wrapped lightly in a towel.
>>>>
>>>> Is there a way to calculate the amount of aluminum and Styrofoam 
>>>> needed to get a time constant of say 100 hours?
>>>>
>>>> This came up in relation to WWVB clocks that free run for most of 
>>>> the time.  When you compare WWVB clocks it's not uncommon to see 
>>>> tens of seconds difference between them.  
>>>> http://www.prc68.com/I/Loop.shtml#TC
>>>>
>>> Start with the maximum thermal resistance the application can 
>>> withstand (determined by internal dissipation and acceptable 
>>> temperature rise above ambient).
>>>
>>> If for example the dissipation is 10mW and acceptable temperature 
>>> rise 10C then thermal resistance will be about 1000C/W.
>>>
>>> The thermal capacity required can then be calculated from the time 
>>> constant:
>>>
>>> C= 3.6E5/1E3 = 360 J/C requires about 7.2 kg of aluminium.
>>>
>>> The required thickness of styrofoam can then be calculated from the 
>>> surface area of the aluminium block.
>>>
>>> Achieving a thermal resistance of 1000C/W may be a little difficult 
>>> without using radiation shields.
>>>
>>> Bruce
>>>
>>>
>> Oops, the specific heat of Alum9inium is about 0.2Cal/gm/C or 0.8371 
>> J/gm/C so the mass of Aluminium required would be 430gm if the 
>> thermal resistance to ambient were 1000K/W. With a thermal resistance 
>> of 100C/W you need 4.3Kg of aluminium ....
>>
>> Bruce
>>
>>
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