[time-nuts] Symmetricom CSAC is Cs acting like a Rb unit

Neville Michie namichie at gmail.com
Wed Jan 19 23:25:26 UTC 2011

Rubidium cells with a side arm run the side arm at a lower temperature.
Then the excess rubidium condenses in the side arm.
The vapour pressure in the cell then becomes that vapour pressure that
exists over the solid rubidium in the side arm. If the pressure  
rises, more will deposit in the side arm.
If some is lost/absorbed by walls, then some of the solid sublimes to  
make a more vapour.
By controlling the temperature of the side arm you control the  
pressure in the lamp.
Side arm temperature may be just due to design, i.e. half way between  
the heated cell
and the circuit board temperature.
I have only played with an LPRO which has no side arm and so no  
reserve rubidium.
It has only just enough to make the desired pressure. I think the  
cells with side arms are neat,
you should never run out of gas, and there is always the possibility  
if elevating/depressing
the pressure by by changing the side arm temperature.
cheers, Neville Michie

On 20/01/2011, at 6:55 AM, Magnus Danielson wrote:

> On 01/19/2011 06:44 PM, Poul-Henning Kamp wrote:
>> In message<DBB70463B9D04F6589FBF97B2E4BE131 at vectron.com>, "Bob  
>> Camp" writes:
>>> Conventional gas cells have a finite lifetime on the
>>> lamp.
>> It used to be that Rb's would fail lock because the bulb dimmed  
>> from Rb
>> absorption into the glass, but I think they got that fixed with a  
>> teflon
>> coating.
>> Thes days most of them ultimately fail from the high operating  
>> temperature,
>> through a variety of mechanisms.
>> For the CSAC, my bet would be that the laser is the limiting factor.
> I agree, for the CSAC I would expect that the laser would slowly  
> dim, especially as it sits within the oven. Not optimum to keep the  
> junction temperature down.
> For my older Rubidium, the Rubidium lamp had splattered the  
> Rubidium. When I heated it the first time I did get a dimmed glass,  
> but heating it again orienting it properly (so hot gas rising into  
> the back where I wanted it) I resolved the problem... within  
> minutes. So I really wonder if it absorbs into the glass.
> Exactly why splattered rubidium cause a problem for the rubidium  
> lamp I am not 100% sure about, but my guess is that maybe it will  
> expose too large area and thus causing too large gas-pressure to  
> let the RF-field light up the gas. If someone who actually know  
> could explain it, I would be happy to learn.
> Cheers,
> Magnus
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