Hi Chris,

As you suggest pressure variations are an issue with verticals.  Most 
of our builders are using some form of pressure case to maintain 
constant atmospheric pressure (actually density) around the 
instrument.   The designs are many and varied.

Gary Lindgren's case design (scroll down)
http://sites.google.com/site/seismicsensorinfo/fbv-design/

Dave Nelson's original commercial plastic case which worked well but 
flexed too much.
http://bnordgren.org/seismo/FBV/IMG_1426.jpg

Recent Dave Nelson case designs using short sections of large 
rectangular structural steel tube.
http://bnordgren.org/seismo/FBV/IMG_1666.jpg
http://bnordgren.org/seismo/FBV/IMG_1670.jpg

New Barry Lotz case design
http://www.seismicvault.com/

Dave built one instrument with a compensating float made of foam 
polystyrene, which significantly reduced the buoyancy noise, but not 
to the degree you would obtain with a good case which can also 
provide some thermal and possibly magnetic isolation.

Mathematical correction using measured barometric pressure, as I 
understand it, can also be used to solve another problem.  The 
magnitude and distribution of the total atmospheric air mass over the 
seismometer causes small long-period variations in the local value of 
'g' which translates into long-period noise, which still exists even 
if you have a pressure case.  It's my understanding that you have to 
mathematically compensate for that effect if you want to make good 
whole-earth-mode measurements.  We might try that in the future if we 
could figure out how to keep the instruments' temperature 
sufficiently stable. (micro-degrees per minute)

Regards,
Brett

At 08:06 AM 11/30/2010, you wrote:
>
>  Hi Gary,
>
>     There are three approaches to correcting for signals produced 
> by atmospheric pressure
>  changes. 1 enclose the seismometer in a hermetic chamber. 2 
> Measure the air pressure changes and compensate using a digital 
> correction signal 3 Fit a compensation chamber to the other side of 
> the seismometer hinge. This can give perfect balancing and was used 
> in1960-70 old type seismometers. Try using Al drinks cans? It may 
> be easy to set up and it is cheap!
>
>     Regards,
>
>     Chris Chapman
>


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