In a message dated 16/11/2008, gel@................. writes:
For the last few months I have been working on a vertical seismometer.
Hi Gary,
There is a reasonably good vertical design on John's website at
_http://jclahr.com/science/psn/hill/index.html_
(http://jclahr.com/science/psn/hill/index.html)
But I don't like either the suspension knife blade or the oil damping.
It is quite easy to use either a twin wire or foil suspension in tension or a
twin blade/foil suspension in compression. A bit of weak spring on the input
suspension doesn't matterin a vertical. Magnetic damping is vastly preferable
to oil. It is quite cheap, easy to adjust and clean.
See _http://jclahr.com/science/psn/chapman/as1%20damping/index.html_
(http://jclahr.com/science/psn/chapman/as1%20damping/index.html)
The longest period you can expect to get from a steel spring vertical is
about 6 seconds. The elastic properties of steel are strongly temperature
dependant and attempts at longer period suspensions simply collapse with small
changes in temperature. 6 seconds is fine if you want to study microseisms,
but it is not good for the surface waves from earthquakes.
However, you can make a 1.5 to 2 second vertical quite easily as shown
above and then extend the period to 15 to 20 seconds to cover the common Love
and Rayleigh surface waves at about 20 seconds, using an extra gain of x100.
If you do this digitally, you will need a 16 bit ADC to give you enough
signal resolution, with the normal sesimic background gain set at over 100
counts.
If you use an electronic period compensating amplifier, this needs to be
low noise, but you can use a 12 bit ADC with it (just!).
Sorry, but it is only fair to warn you that you WON'T get extended
periods with the vertical compression spring that you are trying to use. The
spring + the geometry are both wrong. See Bob's spring calculations?
Regards,
Chris Chapman
In a message dated 16/11/2008, gel@................. writes:
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style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000 size=
=3D2>For the=20
last few months I have been working on a vertical=20
seismometer.
Hi Gary,
There is a reasonably good vertical design on=20
John's website at
http://jclahr.com/sci=
ence/psn/hill/index.html
But I don't like either the suspension knife bl=
ade=20
or the oil damping. It is quite easy to use either a twin wire or foil=20
suspension in tension or a twin blade/foil suspension in compression. A bit=20=
of=20
weak spring on the input suspension doesn't matterin a vertical. Magnetic=20
damping is vastly preferable to oil. It is quite cheap, easy to adjust and=20
clean.
See http=
://jclahr.com/science/psn/chapman/as1%20damping/index.html
The longest period you can expect to get f=
rom=20
a steel spring vertical is about 6 seconds. The elastic properties of steel=20=
are=20
strongly temperature dependant and attempts at longer period suspensions sim=
ply=20
collapse with small changes in temperature. 6 seconds is fine if you want to=
=20
study microseisms, but it is not good for the surface waves from=20
earthquakes.
However, you can make a 1.5 to 2 second vertica=
l=20
quite easily as shown above and then extend the period to 15 to 20 seco=
nds=20
to cover the common Love and Rayleigh surface waves at about 20 seconds, usi=
ng=20
an extra gain of x100.
If you do this digitally, you will need a 16 bi=
t=20
ADC to give you enough signal resolution, with the normal sesimic=20
background gain set at over 100 counts.
If you use an electronic period compensating=20
amplifier, this needs to be low noise, but you can use a 12 bit ADC with it=20
(just!).
Sorry, but it is only fair to warn you=20
that you WON'T get extended periods with the vertical compression sprin=
g=20
that you are trying to use. The spring + the geometry are both wrong. S=
ee=20
Bob's spring calculations?
Regards,
Chris Chapman