You shouldn't need to try to keep the gradients within the box low as
long as they are stable despite changes in ambient temperature,
pressure, humidity etc.
The mulitlayer technique just ensures that the distribution of heat flow
to ambient is invariant thereby ensuring that the inevitable gradients
within the rubidium package are relatively invariant.
Bruce
Bob Camp wrote:
Hi
If I could figure out a way to put the rubidium and a fan inside a box, that
would keep the gradients down to a pretty low level. Then do the layer thing
around that box. I could make it work with a set of fan blades on a plastic
shaft, but that could get a little crazy.
Bob
On Dec 23, 2009, at 5:46 PM, Bruce Griffiths wrote:
The stability of the temperature distribution within the baseplate can be
improved with respect to changes in the speed of the cooling fluid (gas, or
liquid) by connecting the heat exchanger (finned heatsink or equivalent) to the
rubidium base using alternating layers of good thermal conductor and poor
thermal conductor).
As the number of layers increases the inner high thermal conductivity layers
will tend to become isothermal even if the heat exchanger or the base of the
rubidium source is not.
This stabilises the temperature gradients seen at the base of the rubidium
source.
Bob Camp wrote:
Hi
The idea of locating the fan far removed from the heat sink and then using some
kind of a duct might work pretty well.
Bob
On Dec 23, 2009, at 3:31 PM, Joe Gwinn wrote:
Date: Thu, 24 Dec 2009 08:57:42 +1300
From: Bruce Griffiths<[email protected]>
Subject: Re: [time-nuts] Cheap Rubidium
To: Discussion of precise time and frequency measurement
<[email protected]>
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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