In a message dated 13/12/2008, gel@................. writes:

The  spring constant changes with temperature for the typical materials used 
for  springs. For instance, for carbon steel, the modulus of elasticity is 
29.5E6  psi at 70 degrees F and then changes to 28.8E6 psi at 200 degrees F. This 
 means that the spring constant changes with temperature. Is this a problem  
that I should be concerned about. My vertical seismometer is located in the  
wine cellar and so the temperature does not vary during the day, but the  
temperature may change 10 degrees F with the seasons. 


Hi Gary,
 
    This normally limits the maximum period that you  can set a steel spring 
vertical to about 6 seconds. The Tc of the modulus is  about  -2.4x10^-4 / C 
Deg, which is large. If you try to make a vertical  with a very long natural 
period, it will collapse as the room temperature  rises.
 
    The 'work around' for a simple amateur system is to  make a vertical 
sensor with a 1.5 to 2 second period and then fit a  1/f^2 low frequency boost 
amplifier to recover the long period  signals. You can get at least a factor of 
x10 this way before you run into noise  problems, more if you use quad NdFeB 
magnet + coil sensor. I run a sensor of  this general type and it picks up the 
Rayleigh waves very nicely. 
 
    You can get much greater period extensions if you  use a position sensor 
and force feedback - see the SG type pendulum  seismometers on psn. You don't 
have to use the RF oscillator + tuned sensor  circuit. Large area Si 
photodiodes, 10 to 20 sq mm, and a tungsten filament lamp  will work OK. Small 
photodiodes and photo transistors both have too high noise  levels. 
 
    The professionals now tend to use N-SpanC  springs and force feedback 
systems to get periods to 360 seconds. The old  Sprengnether verticals used an 
Elinvar spring and got stable operation to  30 seconds. 
 
    Sean Morrissey developed a force feedback vertical  with a steel leaf 
spring which was OK for about +/-10 deg. It is  all linked on psn, if you look!
 
    Regards,
 
    Chris Chapman






In a message dated 13/12/2008, gel@................. writes:
<=
FONT=20
  style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000 size=
=3D2>The=20
  spring constant changes with temperature for the typical materials used fo=
r=20
  springs. For instance, for carbon steel, the modulus of elasticity is 29.5=
E6=20
  psi at 70 degrees F and then changes to 28.8E6 psi at 200 degrees F. This=20

  means that the spring constant changes with temperature. Is this a problem=
=20
  that I should be concerned about. My vertical seismometer is located in th=
e=20
  wine cellar and so the temperature does not vary during the day, but the=20
  temperature may change 10 degrees F with the seasons. =


Hi Gary,
 
    This normally limits the maximum period that yo=
u=20
can set a steel spring vertical to about 6 seconds. The Tc of the modulus is=
=20
about  -2.4x10^-4 / C Deg, which is large. If you try to make a vertica=
l=20
with a very long natural period, it will collapse as the room temperature=20
rises.
 
    The 'work around' for a simple amateur system i=
s to=20
make a vertical sensor with a 1.5 to 2 second period and then fit a=20
1/f^2 low frequency boost amplifier to recover the long period=20
signals. You can get at least a factor of x10 this way before you run into n=
oise=20
problems, more if you use quad NdFeB magnet + coil sensor. I run a sensor of=
=20
this general type and it picks up the Rayleigh waves very nicely. 
 
    You can get much greater period extensions if y=
ou=20
use a position sensor and force feedback - see the SG type pendulum=20
seismometers on psn. You don't have to use the RF oscillator + tuned sensor=20
circuit. Large area Si photodiodes, 10 to 20 sq mm, and a tungsten filament=20=
lamp=20
will work OK. Small photodiodes and photo transistors both have too high noi=
se=20
levels. 
 
    The professionals now tend to use N-SpanC=20
springs and force feedback systems to get periods to 360 seconds. The old=20
Sprengnether verticals used an Elinvar spring and got stable operation=20=
to=20
30 seconds. 
 
    Sean Morrissey developed a force feedback verti=
cal=20
with a steel leaf spring which was OK for about +/-10 deg. It=
 is=20
all linked on psn, if you look!
 
    Regards,
 
    Chris Chapman