[time-nuts] backfill

[Bob kb8tq]

Hi

You do work pretty hard to turn the metal can into a getter. The other issue is that
the adsorption and desorption rates are quite different. Once you get the “stuff” 
into a layer state, it takes a long time to get it back out. Much better to not have it 
there in the first place. 

If the precision crystal is being used in a TCXO, water has the nasty tendency to 
freeze. When it does, you get a frequency bump. That’s a difficult thing to compensate
for. Does it occur at < -65 (you hope) or at -45 (you hope not) ? The practical approach
is to get rid of as much of it as you possibly can.

Bob

> On Jun 10, 2017, at 12:22 AM, Neville Michie <namichie at gmail.com> wrote:
> 
> It is possible that the ageing of a crystal is associated with the redistribution of the surface water
> monolayer, under the influence of the minute temperature gradient of an oscillating crystal.
> Some energy is dissipated in the quartz, so some gradient may exist.
> When a crystal is resting, the water may redistribute in the sealed package, but when run again, the water 
> redistributes due to the temperature differences.
> Adsorbed water is in equilibrium with its environment, and, given time, will migrate along temperature gradients.
> It could be just one more mechanism in frequency drift.
> I would try using a reactive metal getter in the package to pick up any mobile water molecules.
> 
> cheers, 
> 
> Neville Michie
> 
>> On 10 Jun 2017, at 9:52 AM, Bob kb8tq <kb8tq at n1k.org> wrote:
>> 
>> Hi
>> 
>> You can’t quite process a crystal at 300C, but you can get close.
>> 
>> Bob
>> 
>>> On Jun 9, 2017, at 7:38 PM, Neville Michie <namichie at gmail.com> wrote:
>>> 
>>> My memory of high vacuum work is that you need to pump for 4 hours 
>>> at 300C to remove the water monolayer from glass.
>>> On top of the that water monolayer is another water monolayer that comes off more easily,
>>> and on top of that another………..
>>> 
>>> cheers,
>>> Neville Michie
>>> 
>>> 
>>>> On 9 Jun 2017, at 10:57 AM, Richard (Rick) Karlquist <richard at karlquist.com> wrote:
>>>> 
>>>> 
>>>> 
>>>> On 6/8/2017 5:08 PM, Bob kb8tq wrote:
>>>>> Hi
>>>>> In this case hydrogen + oxygen (like from oxidized metal) goes to H20. You very much do
>>>>> not want water running around inside your crystal holder… Helium is inert.
>>>>> Bob
>>>> 
>>>> Exactly right Bob.  The 10811 guys used to go nuts
>>>> about keeping water out of their vacuum system.
>>>> There were certain temperatures known as "water
>>>> points" at which some water was released.
>>>> The retained water was in spite of the temperature
>>>> already being above 100 degrees C (boiling).
>>>> It has something to do with monolayers of
>>>> water molecules not boiling away.
>>>> 
>>>> Rick N6RK
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