Chris,

In regard to your several questions. (the first address was OK, btw)

The pressure containment for the vertical seis is not a necessity,
for amateur work especially, that requires expensive connectors.
In fact, an absolutely sealed case is not needed, only one that extends
the time of external pressure variations to several hours. So a 
hermetic seal (I used many in building a cyclotron in grad school)
for high vacuum isn't needed; the "potted" DB-25 works well. The only
consideration for a "leaky" containment is that wherever the pressure
equalization seepage is, it must not create a jet or draft that blows
the mass around. Some of the earliest pressure cases over the large S5000
long period seismometers were made from washing machine drums (tubs).

Another way to reduce pressure noise is to not use a real long
operating period. THe buoyancy effect varies as the square of the
period, so a 40 second operating period has 1/4th the pressure noise
as when operating at 160 seconds.

THe use of sealed volumes on the opposite side of the hinges to 
compensate for the mass buoyancy has been experimented with. If only
the mass volume was real, and everything else (coil, boom, parts,etc)
had no volume, the compensation could be calculated: Volume(mass) x
boom(length) = volume(compensator) x extension(length). As it is, it
is a trial/error effort. I have used glass xmas ornaments as well as
sealed pill jars, etc, and have realized some improvement. Unfortunately,
the compensator adds to the thermal problem. 

I haven't used any active (constant power or thermostatically controlled)
thermal heating because it makes an instrument almost impossible to
adjust if the cover has to be removed, which causes an hours to days
long transient if the internal temperature is even slightly warmer.
I think that good thick insulation is the best solution. THis leaves
the hardware at the ambient temperature, so only the radiant heat from
the person adjusting the seis is a temporary problem. 

I have experimented with thermal compensation via an auxillary coil
(25 turns wound above the main coil), using precision micro-thermometer
ICs; this requires considerable electronics, since everything has to
have 0.01% stability, and still adds electronic noise. The $24 solution
is the small gearmotor mounted at the hinge with a weight on a 6" threaded
(6-32) shaft extension parallel to the boom. Operating the motor simply 
translates the weight (which has an oversize (10-32) threaded hole for 
easy gross adjustment) parallel to the boom. Being a physical adjustment,
it is completely passive to electronic transients like power outages.

Regarding capacitive transducers: the usual configuration is to have
the outer plates fixed to the frame and the moving center plate pivoting
at the hinge or suspension between them. The SG seis is such a configuration.
A more compact horizontal can be made by inverting the pendulum, with
the hinges at the bottom, so the capacitive plates are vertical. For
high sensitivity, the gap is small, less than 0.5mm. Volumes have been
written on the treatment of edge effects vs linearity. Plates with large
areas and round edges seem to work with a circular shield around the outer
circumference. Often the outer plates are perforated to reduce air damping. 
Another trade-off is the excitation/detection frequency; higher is better,
of course, but when all is said and done, frequencies from 20khz to 100 khz
are used. The first consideration seems to be the gain-bandwidth figure
vs noise of the amplifier connected to the center moving plate. Naturally,
alignment IS critical, and lateral movement changes the sensitivity.

I don't use capacitive sensors because of all these difficulties. THe 
VRDT is a very forgiving configuration.

Regards,
Sean-Thomas

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