[time-nuts] 10811 Extra Lagging etc

Neon John jgd at johngsbbq.com
Tue Jun 20 21:15:32 EDT 2006

On Tue, 20 Jun 2006 17:54:46 -0700 (PDT), "Rick Karlquist"
<richard at karlquist.com> wrote:

>You must be kidding.  No way are Zip Lok bags hermetic.
>Water vapor will diffuse right through.  (Note that waterproof
>does not equal hermetic).

I have some empirical data to back this up.  I recently bought a combo
"atomic clock"/weather station.  The remote temperature sensor also
contains a humidity sensor.  Without thinking, I put the transmitter
in a (heavy) freezer zip-lock bag and installed it in the NWS-style
shaded enclosure that also houses more conventional temperature and
humidity transmitters.

I was scratching my head for a couple of days for why the new humidity
reading almost exactly tracked the more expensive unit but with a
delay of about 4 hours.  Then it dawned on me about the bag.  Ambient
water vapor can diffuse through the plastic and reach equilibrium with
the outside in about 4 hours.  Out of the bag, the cheap sensor trails
the chilled mirror unit by 30 mins or so.

Actual water can't penetrate the bag so when the humidity is
condensing, the inside-the-bag unit peaks at about 95%.

FWIW, to put good moisture hermetic sealing in perspective, many moons
ago I worked as a design engineer for M&M Mars, the candy company.  I
was involved in the Summit bar (remember that one?) production line
packaging machinery design.  The sugar wafer used in that product was
highly hygroscopic.  The main reason that product failed was that we
weren't able to achieve sufficient shelf life despite trying all sorts
of quite expensive multi-layer metallized wrapping "paper".  Even the
multi-layer metallized stuff was moisture-permeable enough that the
wafer would go soggy after a couple of weeks.

>Rick N6RK
>Brooke Clarke wrote:
>> HI Rick:
>> For humidity control in other applications I have used Zip-Lok bags.
>> How about putting the 10811 in a Zip-Lok bag and run the wires through a
>> hole that gets epoxy sealed?  Some desiccant would lower the humidity if
>> that is an advantage.
>> Have Fun,
>> Brooke Clarke
>> --
>> w/Java http://www.PRC68.com
>> w/o Java http://www.pacificsites.com/~brooke/PRC68COM.shtml
>> http://www.precisionclock.com
>> Richard (Rick) Karlquist wrote:
>>>>Given that HP's engineers have done such a good job with
>>>>the oven and oscillator design, is there anything to be gained
>>>>by adding extra thermal insulation (lagging)?  How much
>>>Not a good idea.  The best thing you could do to improve
>>>temp stability of a 10811 is to tweak the resistors that
>>>proportion the power between the two heater transistors.
>>>At some ratio, the thermal gain will peak at over 1000
>>>typically.  If you just take pot luck, you would be lucky
>>>to do 100.  You are still limited by the tempco of the
>>>electronics, no matter what thermal gain you achieve at
>>>the crystal.
>>>You should also be aware that the 10811 is fairly humidity
>>>sensitive, which can seem like temperature sensitivity
>>>if the humidity and temperature change together.
>>>The HP E1938 was a much better design in terms of environmental
>>>insensitivity, but that didn't help much because the
>>>ultimate stability was limited by crystal frequency jumps,
>>>which didn't seem so bad with the 10811 due to the large
>>>environment errors in it.  In the E1938 they stuck out like
>>>a sore thumb.
>>>Rick Karlquist N6RK
>>>time-nuts mailing list
>>>time-nuts at febo.com
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