[time-nuts] Aging rate of crystals

[Rick Karlquist]

At the beginning of the E1938A project, I did some humidity
tests on the 10811.  It was fairly sensitive to humidity.
I think I remember being able to get parts in 10^8 shift.
The E1938A, having the bridge oscillator circuit, is
completely insensitive to humidity and nearly so to
temperature (10^-13 per degree C).  It's aging is basically
the same as the 10811, since the crystals are basically
the same (different package).

Rick Karlquist N6RK


Neville Michie wrote:
> 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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>>
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