[time-nuts] Better option than oil

Neville Michie namichie at gmail.com
Tue Sep 20 23:09:53 UTC 2011

At last, some one talking sense.
The property of dry sand that is so useful is called thermal  
The distribution of thermal mass and thermal conductivity gives an  
extremely high attenuation to high frequency ambient fluctuations.
It follows from some interesting physics and gives the opportunity to  
use Gauss's Error Function and Bessel functions.
The other great process is the reduction of radiant heat transfer,  
often contributing as much heat transfer as thermal conduction in  
"insulated" ovens etc.
Air is an excellent insulator, and there is nothing to be gained by  
using other gasses, particularly if they have higher dielectric  
constants that would
hardly help piston trimmers.
Thermal diffusivity is why deep cellars make such good clock vaults.  
15 metres of rock/soil attenuates even annual temperature swings to  
less than one degree, and daily swings by hundreds of dB.

cheers, Neville Michie

On 21/09/2011, at 4:19 AM, Murray Greenman wrote:

> Folk looking for alternatives to dunking things in oil might  
> consider trying a variant of the following technique which I have  
> used with success:
> The thermal stability of TCXOs and XOs can be improved considerably  
> for experimental purposes by slowing down the thermal time constant  
> between the device and ambient. A simple technique which I have  
> used consists of a recycled polystyrene box (picnic box, frozen  
> food or dry ice transporter) filled with dry sand. I place the  
> device under test in a plastic bag or other small container sealed  
> as far as possible to keep sand out, and bury it in the middle of  
> the sand. The sand has a high thermal mass, and that combined with  
> the polystyrene insulation gives the device a thermal time constant  
> to ambient of many HOURS.
> While I'd not suggest this is the right solution for an OCXO  
> intended for use in free air, it is a way of achieving impressive  
> short and medium term stability with simple unheated devices,  
> especially if operated in a modestly controlled (e.g. air- 
> conditioned) ambient environment, since the time constant is  
> significantly longer than the ambient temperature cycling. The  
> technique is almost good enough to remove diurnal ambient  
> temperature variation.
> 73,
> Murray ZL1BPU
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