Rex,
re. The pressure enclosure using the aquarium:

Yes, the idea is to have no pressure differential across the
1/4" aluminum plate that the seismometer is standing on. The
aquarium above and the 1/2" polycarbonate plate under the 
aluminum support plate are clamped together with flat 1/4" 
gaskets to form the seal. In fact, there are large openings
in the aluminum plate. The 25-conductor cable is fed through a
sealed DB-25 adapter in the polycarbonate plate. This plate
also has some large (1"dia) holes that are taped over with
clear 3M tape to prevent any large pressure difference from
inside to outside that might cause an implosion of the aquarium.

This idea still has problems. The main one is that the top
edges of the $9 aquarium that become the seal to the aluminum plate
when it is inverted are not straight. I'm not sure that this is
typical;  both of the ones I bought have the problem of a gap of
about 1/32". I recommend taking a straight edge into the pet store.
I have had to use very expensive "super-soft" neoprene from McMaster
Carr to get the seal to work. I am also using much stronger springs
in the latches (27lbs/in), and am considering using 8 latches
rather than 6.

I put a dial barometer inside to test the seal by sucking on a
tube through a valve in the base. The case will maintain a 1"(Hg)
vacuum for several hours. I haven't tried it on the "live" STM-8
here. So I don't have any data on how well it works; the pressure
noise has periods from tens of seconds to 20 minutes or more.

If the barometric noise gets serious here on a windy day, I can
significantly reduce it by switching the VBB operating period
down to 40 seconds, since the barometric buoyancy varies directly
with the square of the period.

Regards,
Sean-Thomas

BY the way, I hope to send out a bunch of SASE envelopes soon; I
have brought most of the drawings and schematics up to date. Unfortunately,
improvements are always in progress, like a revamp of the oscillator
to use a MAX voltage reference and a CD4060 crystal oscillator.

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