PSN-L Email List Message

Subject: Re: Horizontal boom length, for stability and period lengthening
From: meredithlamb meredithlamb@.............
Date: Sun, 24 Nov 2002 16:05:30 -0700


Hi Chris and all,

Excellent educational note you wrote below.  Its probably a much
better answer over the question of stability/tilt/drift/period  in some
recent emails "debates", than I've realized before.

I've never been a fan of long booms myself, but perhaps the
ones with such, do operate with more stability, and longer possible
periods, than those with the shorter booms.  The original email
"debate" involved the "ball bearing pivot" v/s the "Zollner" pivot
suspension, and went from there into stability and long period
aspects.

There is alot of photographed web amateur "coil/magnet"
horizontals ("old Betsys"), ( ha) with seemingly excessively long
and thin diameter (weak?) booms using various boom/mast
"contact" pivots.   I do think alot of these were constructed in
the "hope" that the "mechanical length or amplification"
would somehow yield a better signal; rather than for any prior
known stability/period lengthing building thought.

About all I can say for the only "Zollner" seismo pivot I have
(Sprengnether), is that its boom length is perhaps quite short,
some ~ 15" between the coil sensor and the pivot area.  It
does have a stability problem much over 15-20 seconds.
Anyway.....I'd expect a Zollner pivot seismo might (?) also
improve in this stability/long period aspect "if" the boom were
longer....but exactly how much it could improve is unknown.
On the other hand, I'am not about to tear it apart...ha.

Take care, Meredith

ChrisAtUpw@....... wrote:

> The major difference between a large instrument and a small one lies
> in the period lengthening required. A 1 metre boom has a natural
> pendulum period of 2 sec. To get it to give a 20 second period (1/10),
> you have to suspend it at an angle to give a restoring force of g/100
> -> 0.573 degrees.
>      A 25 cm boom has a natural pendulum period of 1 sec. To get it to
> give a 20 second period (1/20), you have to suspend it at an angle to
> give a restoring force of g/400 -> 0.143 degrees.
>      The smaller boom will be more difficult to set up and will give
> four times the deflection for any given ground tilt. It will also be
> proportionately more sensitive to any springyness in the suspension
> and to any frictional effects.
> Chris Chapman


Hi Chris and all,

Excellent educational note you wrote below.  Its probably a much
better answer over the question of stability/tilt/drift/period  in some
recent emails "debates", than I've realized before.

I've never been a fan of long booms myself, but perhaps the
ones with such, do operate with more stability, and longer possible
periods, than those with the shorter booms.  The original email
"debate" involved the "ball bearing pivot" v/s the "Zollner" pivot
suspension, and went from there into stability and long period
aspects.

There is alot of photographed web amateur "coil/magnet"
horizontals ("old Betsys"), ( ha) with seemingly excessively long
and thin diameter (weak?) booms using various boom/mast
"contact" pivots.   I do think alot of these were constructed in
the "hope" that the "mechanical length or amplification"
would somehow yield a better signal; rather than for any prior
known stability/period lengthing building thought.

About all I can say for the only "Zollner" seismo pivot I have
(Sprengnether), is that its boom length is perhaps quite short,
some ~ 15" between the coil sensor and the pivot area.  It
does have a stability problem much over 15-20 seconds.
Anyway.....I'd expect a Zollner pivot seismo might (?) also
improve in this stability/long period aspect "if" the boom were
longer....but exactly how much it could improve is unknown.
On the other hand, I'am not about to tear it apart...ha.

Take care, Meredith

ChrisAtUpw@....... wrote:

The major difference between a large instrument and a small one lies in the period lengthening required. A 1 metre boom has a natural pendulum period of 2 sec. To get it to give a 20 second period (1/10), you have to suspend it at an angle to give a restoring force of g/100 -> 0.573 degrees.
     A 25 cm boom has a natural pendulum period of 1 sec. To get it to give a 20 second period (1/20), you have to suspend it at an angle to give a restoring force of g/400 -> 0.143 degrees.
     The smaller boom will be more difficult to set up and will give four times the deflection for any given ground tilt. It will also be proportionately more sensitive to any springyness in the suspension and to any frictional effects.
Chris Chapman

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