[time-nuts] HP 10811 Response I Replies

[Neville Michie]

Humidity is a confusing subject to many engineers and scientists.
Unlike most parameters it is a quantity with two input variables,
concentration and temperature. There are many ways to combine
these to give different units.
As a research scientist I spent most of my career working with
"composite materials" which exhibit great sensitivity to humidity.
Most composite materials respond to the Relative Humidity with
only a small temperature dependance. So 80% RH has the same
effect at any temperature. Note that the Absolute Humidity varies
exponentially with temperature.
For a fixed Absolute Humidity (say 10gms /m3) and at 70% RH the
relative humidity changes about 10% per degree celsius. So if you
have a sealed container with some water vapour in it the RH will vary
about 10% per C*.
If you have a fixed ambient humidity, a heated enclosure will have a
humidity that falls 10%/K as the temperature rises.
Now there are many grandiose "environmental chambers" sold to
scientists and engineers that perform poorly. They have internal
temperature gradients, so even if the concentration of water vapour
is uniform the distribution of relative humidity is not.
If a chamber set to 80% RH has a 2 degree gradient it could have  
internal
condensation.
The problems are made worse by the plethora of nearly useless humidity
probes made by manufacturers who are having a bidding war based on
claimed specs.
Since there are very few facilities to calibrate humidity sensors,  
and no
company can make a dollar by having their humidity measured more  
accurately,
there is no pressure to improve instrument quality and the situation
remains that there is a lot of misunderstanding about humidity.

The instruments I have built have an inaccuracy of less than 0.1% RH,
and I have built isothermal chambers that can be programmed to 0.1% RH.
They are based on calculable processes for calibration, and so have
absolute calibration.

In the case of quartz crystals in ovens, when the oven is 30K above  
ambient
the relative humidity is very low, so you would expect there to be very
little absorbed or adsorbed water to interfere with stability. The  
main effects are
surface leakage on hydrophilic surfaces and dielectric absorption in  
composite
material insulators. There is a second order effect that the  
dielectric constant
of air changes with absolute humidity. Humidity sensitivity would  
seem to me
to be a problem of the measurement system rather than the item being  
tested.

Cheers, Neville Michie





On 23/09/2011, at 9:07 AM, Rick Karlquist wrote:

> 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
>
>
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