kb8tq@n1k.org said:

How does that compare with conduction through the mounting?

--
These are my opinions.  I hate spam.

Hi

That depends a bit on the package (HC-6 vs TO-5 etc).  The number of blank
mount points gets into it (2 point vs 3 vs 4 vs who knows). The thickness of the
mount is the last thing I can think of on the list.

A tab mounted rectangular blank will have a lot of mount conduction. A 2 point
spring mounted HC-6, not so much.

The simple answer is that the backfill is done because it does matter in a lot of
cases.

Bob

On Thu, 8 Jun 2017 06:55:07 -0400
Bob kb8tq kb8tq@n1k.org wrote:

This raises the question, why there is backfill (just for thermal conductivity?)
and how much it affects the Q of the crystal.

		Attila Kinali

--
You know, the very powerful and the very stupid have one thing in common.
They don't alters their views to fit the facts, they alter the facts to
fit the views, which can be uncomfortable if you happen to be one of the
facts that needs altering.  -- The Doctor

Hi

If you look at the thermal conductivity vs very low pressures, the conductivity
comes up pretty quickly from a hard vacuum. There is essentially no impact
on Q.

Bob

Hi Bob, it also depends on what you allow to leak into the vacuum. Hydrogen
is a pretty effective remover of heat :-))
Alan
G3NYK

----- Original Message -----
From: "Bob kb8tq" kb8tq@n1k.org
To: "Discussion of precise time and frequency measurement"
time-nuts@febo.com
Sent: Thursday, June 08, 2017 9:19 PM
Subject: Re: [time-nuts] backfill (was: Poor man's oven)

Hi

You really want to use Helium. Hydrogen is a bit reactive.

Bob

Bob wrote:

I don't think it's a problem.  The alternators on mains powerplants are
filled with Hydrogen under fair pressure, and the internal materials and
conditions are not that different from a crystal oscillator inside an
oven.  (IH was chosen both for its heat conduction and its low windage.
Helium is enough worse on both counts that it was a non-starter.  Of
course, windage isn't important in a crystal oscillator.)

In any case, it can't be any worse than filling space capsules with
Oxygen....

Charles

Bob is exactly right.  Read up on "mean free path" physics.
Just a little air will take care of conduction.
Full atmospheric pressure would drop the Q something like
a factor of 2.

In any event, conduction through the crystal mounts is
plenty adequate for the tiny thermal mass of the crystal
itself.  The mounts are embedded in the ceramic header,
which is fairly conductive.

Rick N6RK

On 6/8/2017 1:19 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