[time-nuts] Double ovened 10811-60158 on ebay

[Magnus Danielson]

From: "Ulrich Bangert" <df6jb at ulrich-bangert.de>
Subject: Re: [time-nuts] Double ovened 10811-60158 on ebay
Date: Sat, 12 Jul 2008 13:09:55 +0200
Message-ID: <769FE70BC9AD430CA6506EEEC90E3B8B at athlon>

Ulrich,

> > I keep wondering if not a passive oven (metal box, 
> > insulation, metal box) would be sufficient. Worst case 
> > temperature change rates would be significantly reduced such 
> > that the oven loop can track it better. The remaining 
> > temperature shift will be less. Basically acts like a lowpass filter.
> 
> I have been thinking pretty much the same and therefore I built me an
> "outer" box for my 10811 that has abt. 2-3 cm of free air between the
> outer surface of the 10811 and the inner surface of the box to make the
> 10811's temperature regulation "look" into the same "thermal leakage
> impedance" that the designers may have in mind when they decided for the
> controller's parameters. 
> 
> Then comes 2 cm of massive aluminium which's heat capacity - in
> conjunction with the surrounding air's thermal resistance - is expected
> to give the desired "lowpass filter" effect.
> 
> Due to the nature of my disciplining system I have been able to measure
> the overall effect of this shielding quite precise: The lowpass effect
> is well defined by 
> 
> a) an increase of "phase delay" between temperature changes outside and
> changes of the oscillator's frequency that is now in the order of some
> hours.

Which does not as a big supprice but rather confirms the expected.

> b) the frequency changes (as far as they seem to be related to
> temperature) appear to be "smoother" and more predictable

Which also matches expected and other measurements done.

> Nevertheless I have been a bit disappointed that the overall frequency
> changes along the diurnal changes of the surrounding temperature in my
> flat have been pretty much the same in terms of amplitude with the above
> mentioned phase delay.

Your time-constant for the lowpass filter does not do much to the diurnal
frequency. The temperature changes still go through. The point with the
passive oven is that it does smooth things out, and could potentially make
the oven design inside it an easier task. If temperature changes is the main
problem then it may be all that is needed to get the extra margin. It is
certainly not a perfect solution, but an interesting exercise which may be
of help for some cases.

> I have then started to compute what thermal time
> constant may be expected from this arrangement. This is not trivial
> stuff because in contrast to the aluminium's thermal properties their
> counterparts for the surrounding air are not well defined. As far as I
> remember, a BEST case assumption gave a result of 2-3 hours that seems
> to match the empirical results. If you are out for time constants that
> are able to smooth diurnal changes I guess you will have to throw in
> MUCH more material.

You would also like to build multiple stages to form a higher degree filter.
The outermost box is really there to create the chamber around the isolating
air and in there your heavy mass (big cap) is gets heated (charged) and chilled
(discharged) through the air and black-body radiation.

Having the heavy mass fairly directly exposed to the surrounding air will
provide better conductivity through drag...

Oh, humidity will change the heat conductivity of the air noticeably.

Cheers,
Magnus