Date: Thu, 24 Dec 2009 08:57:42 +1300
From: Bruce Griffiths <bruce.griffi...@xtra.co.nz>
Subject: Re: [time-nuts] Cheap Rubidium
To: Discussion of precise time and frequency measurement
<time-nuts@febo.com>
Bob Camp wrote:
Hi
So if I want to set up 4 uncorrelated systems, that would require 20 tons of
water split into 4 tubs. Each tub would be roughly 3' x 4' x 15'. Of course
> if they are all in the same basement, I still have a correlation
problem. My
guess is that no matter what I do, any system that controls all the systems
the same way will run into correlation.
Oils, silicon fluids, and the like mostly hold less heat than water so the
tubs would get bigger. Maybe a few tons of mercury...
Try about 145 tons of mercury per rubidium source as the specific heat
of mercury is about 1/29 that of water.
The redeeeming feature is that it will only occupy about 2.14x the volume.
The specific of some oils may be as large as 1/2 that of water however
the density is around 10-20% lower.
Active heat control and a rational heat sink is sounding like a better
approach...
Distributed heating using wire wound or printed heaters perhaps, but to
reduce the associated magnetic field bifilar winding should be considered.
Non-inductive power resistors, which are commercially available, have
very low magnetic fields.
The low-inductance resistors have Ayrton-Perry windings, which are bifilar.
<http://www.token.com.tw/resistor-pd/power-resistor-ah.htm>
The major limitation is that the 25W or so dissipated by the rubidium
source has to be transferred to ambient without raising the rubidium
temperature too much.
This limits the maximum thermal resistance between the baseplate and
ambient that can be safely used.
I would be tempted to regulate temperature by actively controlling
the speed of the fan (or pump) driving air (or oil) through the heat
sink, as has been suggested.
Joe Gwinn
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