[time-nuts] backfill
Bob kb8tq
kb8tq at n1k.org
Sat Jun 10 09:20:05 EDT 2017
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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