[time-nuts] HP 10811 Response I Replies

[Rick Karlquist]

Perry Sandeen wrote:

> Wrote: Doing what you describe will result in a very sensitive humidity
> sensor, having eliminated the thermometer effect.
>
> I do not understand.  I believed that since the OCXO temperature will be
> substationally higher than the surrounding temps, any residual moisture
> would migrate to a lower temperature.  The fiberglass insulation inside

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