## PSN-L Email List Message

Subject: Re: Vertical Seismometer with Honey Support
From: Brett Nordgren brett3nt@.............
Date: Sun, 16 Nov 2008 22:23:56 -0500

```Gary,

Making a vertical seismometer using an ordinary coil spring is going to be=
=20
difficult.  On the one hand it has to be strong enough to support the=20
seismic mass, but at the same time its lifting force should change very=20
little as the mass moves up and down if you want to get a long period.  It=
=20
may be possible to do that with a very long (several meters) spring, but=20
it's not likely to be practical due to the spring's likely tendency to=20
vibrate from side to side.  Normally, verticals tend to fall into two=20
types, the LaCoste 'zero length' coil spring design and the more modern=20
astatic leaf spring ones.  They both have the property that the spring's=20
lifting force on the mass changes very little, or not at all, as the mass=20
moves up and down.

For sketches of how they work, check Professor Wielandt's papers.

http://www.geophys.uni-stuttgart.de/lehre/skripte/old_skripte/seismometry/hb=
k_html/node11.html

and

http://www.geophys.uni-stuttgart.de/lehre/skripte/old_skripte/seismometry/ma=
n_html/node15.html

It is worth the effort to browse around elsewhere in these documents.=20
Although they are similar, they are both filled with really good=20
stuff.  Some of the math may be a bit advanced, but even without the math,=
=20
there is much there that should be useful.

As I recall, the period will be 2 Pi sqrt(M / K)  Where M is the mass and K=
=20
is the spring constant, defined by K =3D (force change / length change).=
This=20
should all work if you get the units right.  I'm doing this from memory, so=
=20
someone please let me know if I messed it up.

Regards,
Brett

At 03:25 PM 11/16/2008 -0800, you wrote:

>For the last few months I have been working on a vertical seismometer.=20
>Work to date has been mostly with the measurement of N/A or newtons per=20
>amp calibration factor for 3 different geometry versions. Results will be=
=20
>reported in the next several days. Last week I started to cut metal and=20
>put together the concept for my vertical. See=20
>http://gelindgren.=
>The jar of honey supports the vertical spring. The most interesting part=20
>is the bearing support. See=20
>http://ge=
>The black ball bearings are made of silicon carbide and =BC in diameter.=
The=20
>balls are epoxied into a partially drilled hole in the aluminum cross bar.=
=20
>The balls ride in two sets of two  1/8 diameter rods made from tungsten=20
>carbide and will be polished to a mirror finish. As configured the=20
>resonant period is very short, maybe =BD second. Is there an equation that=
=20
>defines a vertical seismometer resonant period. I assume the spring=20
>constant is part of the calculation. Is there a recommended source of=20
>compression springs with various constants. Comments are most welcome.=20
>BTW, the jar of honey will be replaced by a 4x4 lab jack.
>
>
>
>Gary
>
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>Gary Lindgren
>
>585 Lincoln Ave
>
>Palo Alto CA 94301
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>650-326-0655
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>
>
>www.blue-eagle-technologies.com=20
>
>cymonsplace.blogspot.com
>
>
>
>

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