[time-nuts] Improving the stability of crystal oscillators

[Neville Michie]

Enough speculation on ground temperatures.
The attenuation of cyclic variations of surface temperature die away
at an exponential rate with depth.
The thermal diffusivity of soils does vary but not so you can not  
generalise.
For daily changes, the temperature variation is attenuated to 1% of  
the surface swing between
0.3 and 0.6 metres.
For the annual swing, the distances are about 20 times greater.
I once was investigating a limestone cave climate, and I was  
delighted by
a pure sine wave of temperature that showed up on a data logger after  
a year
of measureing the temperature of the cave roof. It would not have  
been noticed except
that the logger was a high resolution model.
It was about 15 metres underground.
Higher frequencies have much higher attenuation, so the harmonics of  
daily and
annual  cycles are quickly lost.
Thermal transients propagate into the ground on a different law,  
Gauss' Error Function.
If the ground is cleared of trees, one hundred years later at a depth  
of 100 metres or
so there will be a record of the change in average soil temperature.
The Cave was Saltpetre Cave in Carter Caves Park.
Just thought you might like to know.....
Neville Michie





On 15/10/2007, at 1:18 PM, Hal Murray wrote:

>
>> I've heard there is some depth that building foundations need to  
>> be so
>> they  don't get winter frost heave that might be on the order of a
>> couple of feet,  but that's far different from an constant  
>> temperature
>> depth.
>
> That depends on where you live.
>
>> I'm guessing that 10 or 15 feet may be required to get fractional
>> degree temp  stability where I am.  The Wisconsin data was in loam
>> which I think means a  fairly good insulator.  They had the widest
>> temperature variation (15 c delta @  120 cm down).  I've got sand and
>> clay after the first foot or so which may be a  better thermal
>> conductor implying less variation.
>
> The math is the same as for skin depth on RF on metal.  The  
> temperature
> fluctuation decays exponentially with depth.  Lower frequencies  
> (years vs
> days) go deeper.
>
>
>
>
> -- 
> These are my opinions, not necessarily my employer's.  I hate spam.
>
>
>
>
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