Re: [time-nuts] Practical considerations making a lab standard with an LTE lite
Hi Jim:
It turns out that ground water that's being pumped is very similar to pumping oil. It's a limited resource.
There's a web page showing the GRACE satellite maps of California and that we are running out of ground water.
This isn't the page, but gives the idea:
http://www.cnyo.org/2014/08/19/nasa-space-place-droughts-floods-and-the-earths-gravity-by-the-grace-of-nasa/
So depending on ground water as a stable heat sink may no longer be an option as wells go dry.
It's been many years since the local water company has quit installing new meters.
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Jim Lux wrote:
On 11/23/14, 11:15 AM, Alex Pummer wrote:
by us in central California, we get 1kW/h square meter average around
the year, the south even more, el Cajon will have today +29C° in the
afternoon as of 23 of November 2014
I suspect more like the insolation peaks at 1kW/square meter or a bit more, the average over a day is somewhat less.
At JPL we have a weather station on line that displays this and I don't recall seeing significantly more than 1000 W/m2.
The nominal average 1.362 kW/sq meter at solar max is at the top of the atmosphere, and is normal to the incidence.
The surface insolation at the equator when the sun is directly overhead is about 1.04 kW/sq meter.
I think you'd get pretty close to that at solar noon in the Summer in Southern California, which is 32-34 degrees
latitude, so at the solstice, the zenith angle is 10 degrees, and cos(10) is pretty close to 1.
You do pick up some additional insolation from diffuse and scattered radiation from clouds or haze, but I'm not sure
that makes up for the attenuation due to the same haze.
Some time ago, I calculated that in Los Angeles (34 degrees latitude), a horizontal flat plate gets about 8-9
kWh/m2/day in summer and about 1-2 kWh/m2/day in winter.. Tilting the collector would help a lot in the winter
(Zenith angle is 56 degrees instead of 10), but there's no making up for the short days.
Getting back to the time-nuts aspects, there are some charts around that show the temperature variation as a function
of depth, latitude, soil and season. I know that for DSN, they went through all kinds of gyrations to calculate (and
measure) this for the optical fiber timing links between the antennas and the masers. For small dissipated power (I
doubt your oscillator is going to be putting kilowatts into the soil) you don't have to go very deep (single digit
meters) before the diurnal variation is down in the 0.1 degree or smaller. Annual variations are bigger.
http://www.builditsolar.com/Projects/Cooling/EarthTemperatures.htm
has a bunch of charts for some unknown latitude (probably mid Atlantic states, since the data is from Virginia Tech).
They appear to use well water temperatures as the measurement technique.
A bit more googling found a paper by one G. Florides that refers to the Kasuda formula.. (the link is hard to cut and
paste.. I'm sure if you google "Florides soil temperature" you'll find it)
and gives this reference
Kasuda, T., and Archenbach, P.R. "Earth Temperature and Thermal Diffusivity at Selected Stations in the United
States", ASHRAE Transactions, Vol. 71, Part 1, 1965.
Horizontal ground heat means that you are harvesting sunshine
accumulated in the top one meter of the soil. Much of the energy
is harvested from freezing the water around the pipe thus pulling
out the relatively high melting energy of water.
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On 11/23/14, 5:46 PM, Brooke Clarke wrote:
Hi Jim:
It turns out that ground water that's being pumped is very similar to
pumping oil. It's a limited resource.
There's a web page showing the GRACE satellite maps of California and
that we are running out of ground water.
Back east where that data is from, I suspect that's not a big an issue.
Water is many tens of meters down in most of California.
In message 54723237.7070707@pcscons.com, Alex Pummer writes:
by us in central California, we get 1kW/h square meter average around
the year, the south even more, el Cajon will have today +29C° in the
afternoon as of 23 of November 2014
Yes, the latitude means a lot for ground heating, both in terms of Sun
radiation angle and length of winter.
--
Poul-Henning Kamp | UNIX since Zilog Zeus 3.20
phk@FreeBSD.ORG | TCP/IP since RFC 956
FreeBSD committer | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by incompetence.
We should not forget that the LTE is not a free standing frequency source:
it is steered to GPS signals and, in the long term, will reflect the
accuracy of GPS and the stability of the as-received signal.
From the ADEV plot in the user manual, it looks like the time constant for
steering of the TCXO is about one minute; constructing an enclosure with a
time constant significant longer than a few minutes has diminishing returns.
Mike
-----Original Message-----
From: time-nuts [mailto:time-nuts-bounces@febo.com] On Behalf Of
Poul-Henning Kamp
Sent: Sunday, November 23, 2014 8:33 AM
To: Discussion of precise time and frequency measurement; Charles Steinmetz
Subject: Re: [time-nuts] Practical considerations making a lab standard with
an LTE lite
In message 20141123153744.bioKftA5@smtp16.mail.yandex.net, Charles
Steinmetz
writes:
First, mount the LTE in a cast aluminum box (not thin sheet metal,
something with some heft). [...]
Charles' design has some good points, but I don't agree with it.
What you are trying to do is to low-pass filter any thermal signals before
they reach the LTE or OCXO.
Charles' design works great from the outside, but doesn't do anything with
respect to the thermal energy expended by the encapsulated device
themselves, which will cause convection in the inner box.
(For LTE and OCXO it is probably less of a problem that changing
power-disipation will have a outsized effect on the central
temperature.)
Here is a much simpler and likely cheaper way to do it:
Put the LTE or OCXO in a small box of your choice. Even a cardboard box is
fine. A little thermal insulation in the box is OK, but not too much, the
heat must be able to get out.
Find a medium sized cardboard box, something like a cubic feet or so.
Place it where you want your house-standard, with some kind of thermal
insulation under it, two layers of old rug will do fine.
Lay a floor of bricks inside the box.
Build a "wall" of bricks along the outside of the box.
Place the smaller box in the hole in the middle, cut the corner of a brick
to run the cables without too much leakage.
Use a floortile as roof, possibly with a layer of bricks on top.
Close the outher cardboard box with tape to minimize convection.
Congratulations, you now have a cheap and incredibly efficient thermal
low-pas filter, which will allow thermal energy to move in both directions
-- eventually.
The outher cardboard box is not optional, unless you replace it with some
other "mostly air-tight" barrier.
The little bit of insulation the outher cardboard adds are not a bad idea
either, for instance it reduces the effect of sunlight hits the box at
certain times of the day/year.
But you can substitute any geological building material you have at hand for
the bricks, because the trick is that geological building materials have
just the right thermal properties we are looking
for: Decent but not too good thermal conductivity with healthy dose of
thermal mass.
Cinderblocks comes with convenient interior holes premade.
Aerated concrete blocks are also a candidate material but don't make it too
thick since it insulates quite well, and paint the surface to bind the dust.
--
Poul-Henning Kamp | UNIX since Zilog Zeus 3.20
phk@FreeBSD.ORG | TCP/IP since RFC 956
FreeBSD committer | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by incompetence.
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