Ben and Chris,   This is great information,  Thanks, for your response.
  ----- Original Message -----=20
  From: ChrisAtUpw@..........
  To: psn-l@.................
  Sent: Monday, June 04, 2007 6:25 PM
  Subject: Re: Springs for Verticals


  In a message dated 2007/06/04, tchannel@.............. writes:


    Hi Everyone, I wanted to ask some questions about choosing a spring =
for a vertical spring sensor.

       Pictures three situations, three different springs being pulled =
straight down by a mass. No triangular modifications, just straight =
down.

    1 One is a strong spring like a screen door spring, pulled down by a =
large mass.

    2 One is a weak spring being pulled down by very little mass.

    3 One is a rubber band being pulled down by a small mass.

    Just for comparisons, say they all had the same period of 1.5 =
seconds.  Is one of the three better for recording earthquakes?  Two =
questions here: Is there an advantage in using a weak spring, or strong =
spring, if the resulting period is the same. =20


      You will only get this sort of period with quite large extensions. =
You need a certain mass for the motion not to be swamped by thermal or =
environmental agitation.=20

      It is the product M x T x Q, which is important for noise =
considerations, not just the mass.=20

      For a simple spring, the extension E =3D g x T^2 / (2 x Pi)^2, =
where T is the period. Thus to get a period of say 10 sec, you need an =
extension of ~25 metres....

  And Secondly is a rubber band spring every used?

         No. Rubber is highly temperature sensitive and it also has a =
lot of loss (hysteresis) associated with the extension.


    What prompted the question is seeing how small the spring in a =
geophones is. Is the idea to have the smallest spring which would move =
under the smaller stimulus.


         The coil springs used for seismometers have very specific =
characteristics. They are designed to have a zero or negative length =
when extended. The wire is twisted as it is wound and this makes the =
coils clamp tightly together. If you plot the length versus the load, =
the length stays constant up to a certain load and you then get an =
extension proportional to the load. A plot of the load / length graph =
can be extended back to zero load and the 'zero load extension' can be =
negative. You need a net zero length to get an 'infinite period' on a =
vertical seismometer.

         The changes in metal properties with temperature give a =
practical limit of about 5 seconds for ordinary steel springs and you =
may only have ~a 5 C Deg temperature range before re-balancing becomes =
necessary. The 'easy' way around this is to extend the period =
electronically, but the noise performance then becomes critical. A =
maximum period extension of about x10 max is practicable. It is not very =
difficult to extend a 2.5 second period sensor to 25 seconds. The use of =
Sm-Co or NdFeB magnets has enabled the output to be considerably =
increased over Alnico magnet systems, reducing problems with noise.

         Regards,

         Chris Chapman







 Ben and Chris,   This =
is great=20
information,  Thanks, for your response.

  
----- Original Message ----- 
  From:=20
  ChrisAtUpw@....... 
  
To: psn-l@.............. 
  
Sent: Monday, June 04, 2007 =
6:25 PM
  
Subject: Re: Springs for =
Verticals
  

In a=20
  message dated 2007/06/04, tchannel@..............=20
  writes:


  Hi Everyone, I wanted to ask some questions about =
choosing a=20
    spring for a vertical spring sensor.

   =
Pictures three=20
    situations, three different springs being pulled straight down by a =
mass. No=20
    triangular modifications, just straight down.

1 One is a =
strong spring like=20
    a screen door spring, pulled down by a large mass.

2 One is a weak =
spring being=20
    pulled down by very little mass.

3 One is a =
rubber band being=20
    pulled down by a small mass.

Just for =
comparisons, say they=20
    all had the same period of 1.5 seconds.  Is one of the three =
better for=20
    recording earthquakes?  Two questions here: Is there an =
advantage in=20
    using a weak spring, or strong spring, if the resulting period is =
the=20
    same.  

    You will only =
get this=20
  sort of period with quite large extensions. You need a certain mass =
for the=20
  motion not to be swamped by thermal or environmental agitation.=20
  

    It is the product M x T x Q, which is =
important for=20
  noise considerations, not just the mass. 

    =
For a=20
  simple spring, the extension E =3D g x T^2 / (2 x Pi)^2, where T is =
the period.=20
  Thus to get a period of say 10 sec, you need an extension of ~25=20
  metres....

And=20
  Secondly is a rubber band spring every=20
  used?

       No. Rubber is highly =

  temperature sensitive and it also has a lot of loss (hysteresis) =
associated=20
  with the extension.


  What prompted the question is seeing how small the =
spring in a=20
    geophones is. Is the idea to have the smallest spring which would =
move under=20
    the smaller stimulus.

       The=20
  coil springs used for seismometers have very specific characteristics. =
They=20
  are designed to have a zero or negative length when extended. The wire =
is=20
  twisted as it is wound and this makes the coils clamp tightly =
together. If you=20
  plot the length versus the load, the length stays constant up to a =
certain=20
  load and you then get an extension proportional to the load. A plot of =
the=20
  load / length graph can be extended back to zero load and the 'zero =
load=20
  extension' can be negative. You need a net zero length to get an =
'infinite=20
  period' on a vertical =
seismometer.

      =20
  The changes in metal properties with temperature give a practical =
limit of=20
  about 5 seconds for ordinary steel springs and you may only have ~a 5 =
C Deg=20
  temperature range before re-balancing becomes necessary. The 'easy' =
way around=20
  this is to extend the period electronically, but the noise performance =
then=20
  becomes critical. A maximum period extension of about x10 max is =
practicable.=20
  It is not very difficult to extend a 2.5 second period sensor to 25 =
seconds.=20
  The use of Sm-Co or NdFeB magnets has enabled the output to be =
considerably=20
  increased over Alnico magnet systems, reducing problems with=20
  noise.

      =20
  Regards,

       Chris=20
  Chapman
      =20