In a message dated 24/01/2009, rog@.......... writes:

I have  just joined the list and am looking for suggestions for good 
examples and  instructions for home made vertical seismometers.



Hi Robert,
 
    It is only possible to make a simple vertical  seismometer with a stable 
period of less than about 6 seconds. This is due to  the large temperature 
coefficient of Young's Modulus for steel. If you try to  make one with a much 
longer period, it simply collapses as the room temperature  rises a few degrees. 
Elinvar and Ni-SpanC springs were developed to overcome the  problem, but you 
also need a quite critical design, first described in 1934 by  LaCoste. See 
the references on _http://psn.quake.net/infoequip.html_ 
(http://psn.quake.net/infoequip.html) 
 
    Another approach is to make a simple seismometer  with a period of maybe 
1.5 to 2 seconds and then selectively compensate the F^2  signal response drop 
off below resonance with a 1/F^2 amplifier. You can  extend the period by up 
to about x20 this way, but you need to design very low  noise electronics. See 
_http://jclahr.com/science/psn/hill/index.html_ 
(http://jclahr.com/science/psn/hill/index.html)   but  DO use magnetic damping, NOT oil!
 
    The other approach to get longer stable periods is  to use a similar 
setup, but to provide a 3 element force feedback  system. Look at the STM-8 on 
psn, designed by Sean Morrissey 
_http://www.eas.slu.edu/People/STMorrissey/index.html_ (http://www.eas.slu.edu/People/STMorrissey/index.html)  and  
_http://www.bryantlabs.net/seismo.html_ (http://www.bryantlabs.net/seismo.html) 
 
    I don't want to discourage you, but it is MUCH  easier to make a good 
horizontal seismometer with a period of up to 30 seconds.  See 
_http://www.bgs.ac.uk/schoolseismology/_ (http://www.bgs.ac.uk/schoolseismology/)  and  click on 
'seismometer'. See also the Volksmeter at _http://www.rllinstruments.com/_ 
(http://www.rllinstruments.com/) 
 
    Regards,
 
    Chris Chapman
    






In a message dated 24/01/2009, rog@.......... writes:
<=
FONT=20
  style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000 size=
=3D2>I have=20
  just joined the list and am looking for suggestions for good 
examples=20=
and=20
  instructions for home made vertical seismometers.
<=
/DIV>

Hi Robert,
 
    It is only possible to make a simple vertical=20
seismometer with a stable period of less than about 6 seconds. This is due t=
o=20
the large temperature coefficient of Young's Modulus for steel. If you try t=
o=20
make one with a much longer period, it simply collapses as the room temperat=
ure=20
rises a few degrees. Elinvar and Ni-SpanC springs were developed to overcome=
 the=20
problem, but you also need a quite critical design, first described in 1934=20=
by=20
LaCoste. See the references on http://psn.quake.net/infoequip.=
html
 
    Another approach is to make a simple seismomete=
r=20
with a period of maybe 1.5 to 2 seconds and then selectively compensate the=20=
F^2=20
signal response drop off below resonance with a 1/F^2 amplifier. You ca=
n=20
extend the period by up to about x20 this way, but you need to design very l=
ow=20
noise electronics. See http://jclahr.com/sci=
ence/psn/hill/index.html  but=20
DO use magnetic damping, NOT oil!
 
    The other approach to get longer stable periods=
 is=20
to use a similar setup, but to provide a 3 element force feedback=20
system. Look at the STM-8 on psn, designed by Sean Morrissey http://www.eas=
..slu.edu/People/STMorrissey/index.html and=20
http://www.bryantlabs.net/sei=
smo.html
 
    I don't want to discourage you, but it is MUCH=20
easier to make a good horizontal seismometer with a period of up to 30 secon=
ds.=20
See http://www.bgs.ac.uk/schools=
eismology/ and=20
click on 'seismometer'. See also the Volksmeter at http://www.rllinstruments.com/
 
    Regards,
 
    Chris Chapman