Edward Cranswick wrote:

> PSN and others-
>     There is a large number of people in Turkey at right now -- several tens of
> millions -- who are in an excited state because of the Izmet Earthquake
> , and many would be interested in having some
> kind of seismograph to monitor weak/strong ground motions. I saw more cell-phones
> in Istanbul than I have seen anywhere else; the populace is definitely hardware
> conscious. The only thing they don't have is a cheap, reliable seismic data
> acquisition unit built in the same mode as a PC computer; you take it out of the
> box, plug it in, and it starts working. Something like a Kinemetrics K2, but an
> order-of-magnitude cheaper, i.e., less than a $1000.
>     So the interest is there; where is the device?
> -Edward

Hi Edward,

Glad you're back safely from Turkey and that disastrous quake
and environment.  Personally, I would think it was the toughest
job of any USGS personnel, for the time involved.  Probably a
minimum of 16 hour days while you were there would also be a
rough guess of a typical day.

(The below is for anyones response)

Anyway.....the one thing that kind of bothers me in understanding
of the basic sensors variety is;.. why is their so much headlong ties
with solid state type accelerometers, when specifically, the Hall
device can do the same job?   I understand solid state accelerometers
in themselves would probably take up alot less physical space, while
any Hall design would involve probably more space volume, i.e.,
added boom/mass and some kind of damping/feedback circuitry,
or additional eddy current magnets approaches.

Additional comparisons involve circuitry.  I've seen Mr. John R.
Evans circuit last year.  It has alot of individual parts and alot of
them involve nonstandard values.  The Hall circuit (devoloped by
Robert Lamb) I've used for over a year, has very few.  Not
withstanding some kind of damping mechanism, which could be
eddy current also, let alone the possiblity of other electronic means.

I always feel that the previous history of Hall devices has been
"limited" by previous extensive research available on the
web.  These devices all used just standard 2 pole magnets in
various configurations.  Specifically, when 4 pole magnets are
involved, the magnitude of sensitivity, bi-direction, velocity all improve
dramatically...but I admit the circuit I use "overdrives" the Hall
devices considerably, from the original design standard per
gauss field....around 500, and not, 13-14K at or near the junction
of the 4 pole magnets divisions.  I suspect the linearity is affected
somewhat, but I would think it wouldn't matter excessively.  On
the other hand, no Hall has failed electronically either.

A rough comparison of my Hall devices with my old Sprengnether
coil/magnet computer output shows a fair similarity, but, the coil
magnets seem to win with distances over afew thousand miles
from the epicenter of quakes.  The likely difference is that the
Hall are connected to a simple hanging pendulum (S-G design),
whereas the coil magnet seismometers are amplified mechanically
per their boom tilt angle with seismic signal arrival.

I admit I'am bypassing the standard coil/magnet sensors in much
discussion here.  For amateurs, coils and magnets appropriation
is difficult, whereas, Hall sensors are available (even at Radio
Shack, on special order).

I admit I lack alot of electronics knowledge, and educational
background to go into much detail on specifics, but I hope that
someone can explain the difference and the why-fors reasonably.
I understand the added complexity of creating mini-seismos to
fit Hall sensors over that of ready made accelerometers, but I
would think its readily possible to do just that.

Other thoughts especially with commercial companys, is that
possibly they are "tied up" into standard approaches, and any
new means involves expenditures they may not want to get
into.  Also, the idea of alot of manufacturing is too just shove
the product "out the door", for profit, regardless of quality
control....good basic idea....bad product....

On the other hand, if its just a seismometer for private curiosity,
I would think that any people interested, could derive some
benefit from using Hall devices.  Any more extensive calibration
involving knowledable engineering would likely have to be done
by volunteers or any commercial interprise pursuing it.

Perhaps the worst aspect of using Hall sensors, is the need for
4 pole magnet/s (one can use one or two, per each sensor).
All of my sources have been surplus outlets.  Alot of the thinner
4 pole magnets are difficult to remove from their original
assemblys.  Whether or not, commercial 4 pole pieces are
still available now, is an unknown.

For more Hall sensor details, visit the following web site:

http://www.geocities.com/ResearchTriangle/Facility/1739/index.html

Am looking forward to any replys,...I try to stay open to thoughts
........really.....ha.

Thanks, Meredith Lamb











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