At last, some one talking sense.
The property of dry sand that is so useful is called thermal
diffusivity.
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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