[time-nuts] Aging rate of crystals

[Neville Michie]

This is a theory that I have been thinking about for some time.
A possible physical process for crystal ageing is the desorption of  
water from
surfaces in the oscillator circuit.
Most solid surfaces, particularly those with oxygen in the molecular  
structure,
have at least a monolayer of water over the surface.
In high vacuum laboratory work you bake glassware at 300*C for 4  
hours in a
hard vacuum to remove this water to prevent future degassing.

When you have an oven with air interchange to the ambient, the
humidity in the oven becomes very low. For example air at 20*C and  
65% RH
drops to 3.2% RH if heated to 80*C. (the vapour pressure of water is  
still 1520 Pascals)

Relative humidity is a good predictor of the amount of moisture  
absorbed and adsorbed
by a solid.
The heating of the oven reduces the relative humidity, so much water  
would leave a surface
on warm-up but the last of the water is tightly bound and to reach a  
thermodynamic equilibrium under
these conditions is very slow.

If the air at room temperature had the humidity reduced to 32.5%,  
then the RH in the oven
would reduce to 1.6%, so the oscillator would still respond to the  
change in humidity.

The main evidence to support this theory is that it is a plausible  
physical process with
slow enough time constants and known hysteresis that could explain  
the days it takes
for my HP 10811A to return to the control voltage it formerly  
required to be on frequency
after it has been allowed to cool down for 2 days.
The best experiment I can think of to prove this is to run the  
oscillator in a paper bag until it is stable,
then trickle a flow of dry nitrogen  into the bag for a day or two  
and watch for oscillator drift as the humidity
in the oven drops to extremely low values.
It is a pity that I do not have bottled gas on tap any more.
cheers, Neville Michie

On 16/02/2008, at 7:24 AM, Rick Karlquist wrote:

> For ovenized crystals, any oven temperature change will
> cause aging to temporarily increase (and may change the
> direction as well).  Restabilization may take an hour
> to a day, depending on how much the temperature was changed.
> We haven't noticed that powering up the oscillator or not
> makes a big difference.  (This refers to leaving the oven
> on, and turning off the oscillator circuitry.  This experiment
> This is a theory that I have been thinking about for some time.is  
> easy to do on a 10811 since the supplies are separate.)
> Does that answer your question?
>
> Rick Karlquist N6RK
>
>
> iovane\@inwind\.it wrote:
>> Hi all,
>> the aging rate of crystals is known to be not constant.
>> Is there any known external cause affecting the rate, or it is only a
>> matter of casuality?
>> Thanks,
>> Antonio I8IOV
>>
>>
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