PSN-L Email List Message

Subject: Re: "EPICS" seismo suggestion
From: meredithlamb meredithlamb@.............
Date: Tue, 17 Sep 2002 22:24:52 -0600

Hi Charles and all,

Actually the A3515 Hall sensor, exhibits no gauss or magnetic
attraction (with no power on) that I could see either with a 
strong magnet or a gaussmeter set on its most sensitive scale here. 
However, some other Hall brand varieties do, as do probably a heavy
majority of other use, IC's....especially their tinned iron leads.
I've no idea of what a Hall with "power on" could significant 
influence; but I'd guess its very minor.

"Charles R. Patton" wrote:

> Aa a sensor solution for the SG seimo, it was proposed to use "…a $1.83
> A3515LUA sensor…"
> So this brings up this thought:  Most standard IC frames are magnetic -
> consisting of a Kovar frame.  And magnetic sensors in particular are
> often constructed with "magnetic concentrators" to focus magnetic flux
> on to the sensitive area of the chip.  Both make the chip physically
> attracted to ambient fields.  If the chip is not mounted on the SG arm,
> then the magnets must be, which would be even worse about ambient
> sensitivity.  So if one goes for the higher sensitivity of the SG,
> doesn't this create problems in terms of an actual physical sensitivity
> to the surrounding changing magnetic fields?

The Hall would have to be mounted on the boom/mass "normally" of course.
It would help to have some Hall shielding (mainly for immediate area
"noise") in any S-G with a Hall sensor yes....but I don't think its
absolutely essential even there.  Years ago, Robert showed me the
results of shielding versus no shielding with a small old oscilloscope
shield cut the noise about in half.

> I'm thinking back to some
> experiments that Roger Baker did on magnetometers in which his
> magnetometer is actually a magnet on a torsion balance.  He said he
> could detect a small magnet many feet away and I know he could sense
> geomagnetic changes.  It would seem to me that would turn the SG seismo
> into a geomagnetic field detector.    Roger's detectors were based on
> photodetectors and LED or Laser LED sources.

Yes, having a suspended magnet mass does react to external magnetic
changes....even solar origin flares which can make for magnetic
pole/s or earth area variations.  I have a separate such instrument
of this nature (diamagnetically levitated magnet)....its better
for some stronger solar field changes detection...or passing
buses/trucks (ha) than the occasional seismic results I'am sure.
I guess you could also call it a "metal" detector of sorts (ha).
It can be interesting in itself, but one really needs a remote
vault...and not in the city as I am located.

> I think I'd vote for the original SG variable capacitance detector.
> Today you can buy ICs which do all the detection.  For one source see
> "Universal Capacitive ReadoutTM IC (MS3110)" at:
> I don't know how much it costs, but in volume this can't be that
> expensive.  I also know there is at least a couple of European companies
> that makes a similar IC.
> Just some idle musings.

Of course, the old coil and magnet sensor approach is the ultimate in
simplicity forever it seems......but even there, finding/making
or buying a "very good" sensitive coil is the hardest part I suggest.

Have to go along with the dual magnet (4 pole) approach that Chris
noted also, it seems to increase the coil sensitivity some rough ~
2 times over that of a single magnet.  Adding another attracting
4 pole magnet on the side of the coil (via a normal garden gate
seismo), raises the sensitivity higher to about 4-6 times of a single
magnet.  Such magnets also seem to do well with eddy current damping
schemes also.  Of course the best coil magnet scheme is with a
"speaker" type setup, but for most amateurs, its really tough to
do this.

Don't know "absolutely" for sure, but I think its hard to beat a Hall
setup for being about the cheapest "general", and reliable seismo one
could have with all cost aspects considered.

Take care, Meredith Lamb


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