Jim --

For the regular Lehman design, the amount of mass used does not affect the
sensitivity of the seismometer, per se.  However, a larger mass makes the
instrument less sensitive to convection currents and reduces the effect of
any friction in the boom hinge.  On the other hand, a larger mass means a
more heavy-duty suspension system and perhaps more friction in the hinge.
So it is a tradeoff, and there is really no right answer.

If you intend to put build this system into a feedback seismometer (such as
the VBB approach), there are other design considerations that have a direct
relation to the size of the mass.  If this is in your plans, do what those
design considerations dictate.

I'd go with a smaller mass (7 lb or less seems like a good amount) because
it is easier to work with and hurts less when you drop it on your foot.
Suspension methods (even for amateurs) have improved quite a bit since the
original articles were published, which allows the use of smaller masses
without sacrificing performance.  The use of flexures is generally
preferred in the suspension -- they provide a stable hinge with very little
friction.

In diagrams, the tilt toward the mass end is often exaggerated for the sake
of the illustration.  In practice it usually amounts to a millimeter or
less, and is most easily adjusted by turning the leveling screws in the
seismometer base.  This adjustment directly affects the period of the
seismometer.  The longer the period, the more you will be able to see
longer-period signals, and the less stable the mass centering becomes.  You
will probably that with longer periods, you will be re-centering the mass
fairly often.  Others probably have had more experience with this than I,
but periods longer than 20 seconds or so become significantly less stable.

A suspension wire is easy to use but can contribute parasitic vibrations
itself -- it can vibrate just like a guitar string.  Using a rod for the
top suspension can increase the frequency of these vibrations enough to be
above the frequencies of interest, and this can be filtered out of the signal.

Hope this helps.  Ask if you have more questions.

Karl Cunningham
La Mesa, CA.
PSN station #40
karlc@.......


At 10:40 PM 10/13/1999 EDT, you wrote:
>I am a bit confused regarding the construction of a Lehman (Garden Gate) 
>seismo.
>Text books at the local university library indicate Garden Gate seismographs 
>with mass of 25 kg or more.  However, simular seismographs made by PSN 
>members use masses of about 7 lbs.  Why the difference?  Is the larger mass 
>more or less sensitive?
>Additionally, in the text book examples reflecting the principle of the 
>Garden Gate seismograph, they all show the boom slightly tilted down towards 
>the mass end.  However, most instructions I have seen on the list I believe 
>states that one should "level" the boom.  Which is best for sensitivity -
and 
>if the boom should be tilted to what degree is appropriate?
>A number of pictures on PSN web sites show a long thin rod as part of the 
>boom suspension.   Others just show a turn-buckle.  What purpose does the 
>long thin rod serve?
>Any clarification would be appreciated.


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