[time-nuts] HP 10811 Response I Replies

[ws at Yahoo]

A problem I've seen when using the hermetic sealed soldered version of the 10811 from a dual oven unit,
 is when the case is sealed the osc makes a good barometer because of changes in its case due to barometric changes.
A 1 inch difference (such as 30 to 29) caused something on the order of 1e-11 freg change.
To see the effect just need to put a little pressure on  the case 
So now I'm trying to determine which is worse, leaving the freq adj screw tight and sealed or leaking a little, inside a box containing desiccant.

ws

******************
> 
> Heating up a space does not change the absolute humidity AFAIK.
> It only changes the relative humidity.  We did tests where we
> "sealed" a 10811 inside a box that was held together with so-called
> "hermetic" epoxy.  We put it in an environmental chamber at a
> constant temperature and constant low humidity and let it stabilize.
> We then increased the humidity to something like 80%,
> while holding the temperature constant.  Within
> minutes the frequency changed more than the spec for the entire
> temperature range.  Therefore, you should do your experiment with
> the hermetic version of the 10811.  The hermetic version is soldered
> shut, rather than using epoxy, which turns out not to be hermetic,
> no matter what they claim.
> 
> Rick Karlquist
>