Marchal van Lare wrote:

> Hi All,
>
> I think my main frame looks very unstable, so I've decided
> to start 'all over again'.
>
> I'm thinking of making the main frame of one big upright metal
> bar (20-30 centimeters wide), not like the two upright pipes
> from the article. I think this will improve the stability of the
> device.

Making the mast more massive will help, but it will likely still
need bracing (say two bars running to the side of forward,
allowing space for the boom movement) to prevent the mass
from influencing the tilt.

>
>
> What do you suggest, a aluminium frame or can any metal be used?
> How big should the base plate be?

For the seismometer you have, it should be thick enough to not
bend from mass movement (braces on top or underneath are
a help, if the thickness is like less than 1/2 inch/12.5cm).  The
length and width will likely be determined by your wallet.  I
don't exactly remember the original design but I think it could
be roughly 1 foot/40cm wide by maybe 2&1/2 foot/90cm
in length.....which is maybe roughly like the Lehman design.
Aluminum is usually preferred over iron for its non-magnetic
propertys and easier to work with.  Again, basically whatever
you can find or afford and/or fit into your design.  The above
is a rectangular design....but a triangle shape if available might
be even better, especially for stability and the 3 setscrews you
will need.  Don't know what you use at the present for setscrews,
but  a fine thread makes adjustments quicker for tweaking.
Overall, build it as strong or non-flexible as possible....you will
have less mechanical stress tilt in the long run.

Building a good stable seismometer is not a quick operation.
The only seismometers I ever built are the S-G's I use....but
lacking alot of mechanical metal working equipment, it took
some 4 months to "find" the parts....but with an idea of
longevity and stability.

Aluminum where I live is not cheap ($1.00 a pound scrapyard)
and the larger sizes are kind of rare...but suitable material is
probably out there....one has to call/search for it.

Building a seismometer IS a project for sure....but once you
get it constructed and in operation, it all becomes worthwhile.

Should the project bog down, you might even check area
geophysical equipment outfits, or universitys or seismic stations
for unused older equipment....who knows?  The newer and
smaller seismometers are rapidly replacing the older bigger stuff.

Meredith Lamb






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