PSN-L Email List Message
Subject: Re: Another horizontal boom/mast pivot to consider
From: meredithlamb meredithlamb@.............
Date: Sat, 23 Nov 2002 01:04:18 -0700
Excuse my prior incomplete email....I hit the send button by mistake.
The original message only suggested a torsion/tension pivot to
.....consider......in a Lehman/garden gate/etc., seismometer.
David Close....I believe you are right, the Sprengnether
suspension (torsion/tension) could be called a "Zollner"
pendulum....its actually like the old Wood/Andersion torsion
seismometers of yesteryear. They were also period limited.
Thanks for the undate David!
Ball bearing v/s torsion/tension (Zollner) pivots
I read plus's and minus's into this topic. A "Sprengnether",
"torsion/tension pivot", is actually a type of more susceptible
dual tilt pendulum if one observes the upper pivot suspension
wire and the lower (boom/mass) pivot; and imagines a line drawn
vertically and centralized down between the two. You offset
one side (the mass) and their is a lesser movement laterally on
the other end of the boom (the torsion/tension pivot) by its own
nature. I just can't for the moment remember the name of such
a pendulum....a web "name" search comes up with nothing relivant.
Essentially the lower torsion/tension pivot "point" in itself
contributes a enhanced degree of instability, by its own
flexible nature and various influences of time, temperature, etc.
Being somewhat influenced by this in operation, their is a limit
to the period of a torsion/tension pivot in practice...perhaps a max
of up to ~20 seconds. I suppose one could say its not as
stable for longer periods and be correct....I've seen it myself.
Thats not all terrible of course.....up to 20 seconds is fine for
most amateurs. Of course, to add to that, one has to
recognize that a inertia based/operated seismic mass is also
susceptible to tilt induced phases by quakes....where the mass
will "follow" the troughs and peaks of passing seismic phases;
and one can see much longer "period waves" on their grams.
For the topic...I've left out "feed-back" seismos....which can
register much longer periods. On the other hand, its not the
long period surface waves thats really understood nor wanted;
(except for magnitude), its the shorter period waves that derive
the "p" & "s" wave, that is used to determine distance. Of course
the longer the period of the seismo, and the "looser" the pivot,
one should be able to observe more of the seismic phases beneath
the set period it has.
I've never used a "ball bearing" seismo. I don't know what they
are like. I can understand though, that perhaps with that rock
solid lower pivot, that its quite possible to have a longer period
stability.....as because now....one only has, the upper hinge wire
support/hinge angle wire to contend with, instead of the overall
"double hinge flexure" of a "torsion/tension" pivot.....hence a
"Lehman" seismo should have a lesser stability problem for longer
periods of operation.
I'd "think" the ball bearing contact approach is much better
than some of the past approaches we've seen; but even there,
the bearing idea is still within the old concept goal of using a
more hardened surface/s contact to prevent material failure, reduce
friction etc; was rather ongoing. I've heard of razors, rounded metal
tips various metals), carbide etc., on hardened tool steel, glass
etc. One very nice thing about ball bearings is that they are
fairly readily available and the very idea/approach itself is
nice to read about, as it adds some potential construction
material one could comtemplate using.
So...theres plus's and minus's with either pivot. I "think"
the ball bearing pivot can yes exhibit a longer period (read
as stability), but, it also will be abit less sensitive too
actual earth movement than a torsion/tension pivot with its
essentially zero friction design. For a amateur it may come
down to only what they believe will work and/or what material
they are able to get/find for their project....either one has
Theres other approachs with horizontal seismometers and
their own various hinges/pivot designs of course.
All these tilts, drift, instability all have a cause and effect.
For myself; its usually the water/moisture in the ground and its
variations of content which change the shape/tilt of the piers.
All outside changes in temperature affects the ground moisture
evaporation along with natural drainage, as does added rain/snow
storms of course.
Personally I like the "S-G" (hanging pendulum), as the "bulk" of the
normal seismo can be greatly reduced in volume, and the results
are fairly good.....and of course gravity itself kind of forces
the thin "pivot" flexure hinge, to not deviate laterally where its
usual sensor pickup reads. Its ultimate sensitivity comes down to
using the thinnest material available on the hinge/s...too stiff a
hinge/s and you lose some of the free potential pendulum inertia
"movement". A "S-G" normally points straight down, and doesn't
depend on any certain angle to derive its "period". One can of
course, add other displacement sensors and circuits to a "S-G",
to go beyond its normal or natural oscillation period. In
essence all horizontal seismometers are also tiltmeters.
Take care, Meredith
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