[time-nuts] Thunderbolt E failing

[Poul-Henning Kamp]

--------
Hal Murray writes:
> 
> phk at phk.freebsd.dk said:
> > I later talked to a geologist who knows the Danish underground well, and he
> > estimated I would have needed to drill to at least 25m depth to escape
> > seasonal temperature-changes and cited a research paper from the 1950'ies
> > where the did precisely that experiment. 
>
> I found a paper showing skin depth at 1 year of 3-4 meters for soil, a bit 
> over 1 meter for ice and rock.

Yes, as a general rule, the yearly temperature-swing attenuates a
lot in about a meter or two, and that's why you generally bury a
ground-heat loop buried a meter or so.

But if you measure the temperature with one decimal place or better,
you need to dig deeper, because now the difference between individual
year's weather matter, and in my case here in Denmark: Underground
water-transport and in particular where and when that water comes
from starts to matter.

Basically I would have had to drill to "the stable water table" and in my
case that is approx 25 meters down, which is also the typical depth of
atesian wells in Denmark.

> Does a pipe in the ground stay dry?  Or how do you keep in dry?  In summer, 
> I'd expect it to be cool and if the humidity in the outside air is above X% it 
> would condense in the pipe.  Do you just cover the top to avoid air mixing?  
> Does in need a desiccant?  ...

It would need to be closed to the atmosphere, which you can either
do properly or patch up the imperfection with desiccant.

More tricky:  Internal air-circulation.  Either guarantee that the
temperature increases all the way from the bottom to the top, which
here in Denmark would mean heating the top-end during winter, or
install baffles every 1-2 meters to resrict airflow.

It would have been fun, but I can get the same temperature stability cheaper.

-- 
Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
phk at 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.