## PSN-L Email List Message

Subject: Re: Springs for Verticals
From: "tchannel" tchannel@..............
Date: Mon, 4 Jun 2007 18:34:27 -0600

``` 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

----- 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:
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 =
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 =
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

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