In a message dated 15/07/2009, paleoartifact@......... writes:

Outside of buying anything.... versus home building,  the old  Georgia Tech 
vertical (Classroom
Demonstration model) still remains as a quite interesting experimental  
setup; especially because they claim it is capable of much longer  periods. It 
is rather strange that after some 11 years of its  existence on the web that 
there is still no known private individual that has  garnered up a web site 
on such with their opinion of merit.
Of course the original design was of wood and some metal, and lacked a  
basic viable sensor and dampening; but the spring and pivot arrangement was  
the main very interesting approach idea for a long period  seismometer.
For readers it is at: _http://quake.eas.gatech.edu/Instruments/LPVERT0.htm_ 
(http://quake.eas.gatech.edu/Instruments/LPVERT0.htm) 



Hi Meredith,
 
    I agree that this is a good design. The problem  with all amateur 
vertical sensors is the spring. Steel coil springs have quite a  high temperature 
coefficient of the modulus, so you are limited to periods of  less than 
about 6 seconds for simple systems, unless you add a full force  feedback loop. 
If you try to make a longer natural period, the system will  simply 
collapse as the room temperature increases.
 
    The second limitation is in the extension springs  that you can buy. 
They all have a relatively low initial tension, whereas  seismometer springs 
require quite a high initial tension. You can wind springs  with a much 
higher initial tensions by either twisting the wire as it is wound  onto a 
mandrel, or by pre-bending the wire in the direction of the spring axis,  as 
LaCoste did.
 
    This design can also be compensated for atmospheric  pressure noise - 
the principle source of noise by a large factor in  uncompensated vertical 
systems.
 
    Regards,
 
    Chris Chapman






In a message dated 15/07/2009, paleoartifact@......... writes:

  
Outside of buying anything.... versus home building,  the old=
 
  Georgia Tech vertical (Classroom
  
Demonstration model) still remains as a quite interesting experimen=
tal 
  setup; especially because they claim it is capable of much longer 
  periods. It is rather strange that after some 11 years of its=
 
  existence on the web that there is still no known private individual tha=
t has 
  garnered up a web site on such with their opinion of merit.
  
Of course the original design was of wood and some metal, and lacke=
d a 
  basic viable sensor and dampening; but the spring and pivot arrangement=
 was 
  the main very interesting approach idea for a long period 
  seismometer.
  
For readers it is at: http://quake.eas.gatech.edu/Instruments/LPVERT0.htm<=
/DIV>

Hi Meredith,
 
    I agree that this is a good design. The probl=
em 
with all amateur vertical sensors is the spring. Steel coil springs have=
 quite a 
high temperature coefficient of the modulus, so you are limited to periods=
 of 
less than about 6 seconds for simple systems, unless you add a full force=
 
feedback loop. If you try to make a longer natural period, the system will=
 
simply collapse as the room temperature increases.
 
    The second limitation is in the extension spr=
ings 
that you can buy. They all have a relatively low initial tension, whereas=
 
seismometer springs require quite a high initial tension. You can wind spr=
ings 
with a much higher initial tensions by either twisting the wire as it is=
 wound 
onto a mandrel, or by pre-bending the wire in the direction of the spring=
 axis, 
as LaCoste did.
 
    This design can also be compensated for atmos=
pheric 
pressure noise - the principle source of noise by a large factor in=
 
uncompensated vertical systems.
 
    Regards,
 
    Chris Chapman