Hi, Jon, I like to use the old style Lehman design. The device I built =
with
the best results was the PSN San Jose Lehmans which used a 1-in square
aluminum rod L=3D 80cm and 80-ounce lead mass. They had a natural period =
of
20-seconds with minimal setup effort. The total boom length was 100cm =
and
they used a brass plate mounted at the end of the boom for the damping. =
I
was very happy with the performance. One of the issues you will face is =
the
selection of the gauge of the upper support wire. I tried to use #8 =
machine
(piano) wire but it kept breaking during use. I increased the wire gauge =
to
#10 machine wire and the wire life was about 1-year before rust causes =
the
wire to snap under tension.=20
=20
When I moved here to Aptos, California I was forced to reduce the length =
of
the boom to L=3D60cm because of limited space as seen in the photos in =
the
link below and the resulting natural period is now typically 10-12 =
seconds.
Needless to say, the performance these Lehmans are marginal and I live =
near
the ocean and the wave action causes them to osculate during storms. =
When I
have the time I'm going to pull them out and build one L=3D80cm device.=20
=20
My suggestion is to use the calculation seen on this list (sorry I don't
have the calculation for a natural period of a garden gate Lehman at =
hand.
Maybe somebody on the list does??) to determine the boom length you =
desire.
Then plan the design based on that calculation. You said you wanted a =
device
with a period of 20-second and as pointed out below by Chris, L is based =
on
the length of the boom measured from the pivot point to the center of =
the
mass weight (Chris, do you have this calculation??). I have found that
setting the device up this way also reduces the impact of ground =
deformation
(boom does not remains level and centered) over time which improves the
overall operation of the device keeping the boom off the stops.=20
http://pw2.netcom.com/~shammon1/AptosStn.htm
=20
=20
Regards, Steve Hammond Aptos California,  PSN San Jose.
=20
 -----Original Message-----
From: psn-l-request@.............. [mailto:psn-l-request@............... =
On
Behalf Of ChrisAtUpw@.......
Sent: Wednesday, February 13, 2008 7:08 PM
To: psn-l@..............
Subject: Re: How much mass ?



In a message dated 2008/02/13, jonfr@......... writes:



Hi all
I am continuing to planning the build of an Lehman sensor. But I am
wondering what the ideal mass is going to have to be. But I am hoping =
for
at as many seconds as I can. Preferably around 20 seconds.=20



Hi Jon,

       The mass is not important in a Lehman, only the length between =
the
hinge and the centre of mass =3D L ~56 cm. You need to keep the arm =
rigid but
light. Between 1/2 and 1 kg is about right, certainly no more. We use =
brass
rod. I use 3" x 1" Aluminum U channel, 1/8" Al plate and SS bolts. The
magnets are NdFeB, 1" square x 1/8" thick for the sensor and 1" x 1/2" x
1/4" thick for the damper, 4 off each type

       Have a look at our Lehman school seismometer. The dimensions and
construction are shown. Don't use any of the constructions shown on psn. =
See
http://www.bgs.ac.uk/education/school_seismology/seismometer.html
       I don't suppose that you are feeling rich, but MUTR sell them for
=A3290 + carriage.
~IK 33,282 + carriage. I don't know about your import duty or tax. This
includes the sensor, the electronics and the PSU. It plugs into a PC
computer and runs under AmaSeis. You can get 30 seconds, although they =
only
claim 20.

       Let me know if you need more information.

       Regards,

       Chris Chapman=20








Hi,=20
Jon, I like to use the old style Lehman design. The device I built =
with the=20
best results was the PSN San Jose Lehmans which used a 1-in square =
aluminum=20
rod L=3D 80cm and 80-ounce lead mass. They had a natural =
period of=20
20-seconds with minimal setup effort. The total boom length was 100cm=20
and they used a brass plate mounted at the end of the =
boom for=20
the damping. I was very happy with the performance. One of the issues =
you will=20
face is the selection of the gauge of the upper support wire. =
I tried to=20
use #8 machine (piano) wire but it kept breaking during use. I increased =
the=20
wire gauge to #10 machine wire and the wire life was about =
1-year=20
before rust causes the wire to snap under =
tension. 
 
When I=20
moved here to Aptos, California I was forced to reduce the length of the =
boom to=20
L=3D60cm because of limited space as seen in the photos in the link =
below and the=20
resulting natural period is now typically 10-12 seconds. Needless to =
say, the=20
performance these Lehmans are marginal and I live near the =
ocean and=20
the wave action causes them to osculate during storms. When I have the =
time I'm=20
going to pull them out and build one L=3D80cm device. =

 
My=20
suggestion is to use the calculation seen on this list (sorry I don't =
have the=20
calculation for a natural period of a garden gate Lehman at hand. =
Maybe=20
somebody on the list does??) to determine the boom length you desire. =
Then plan=20
the design based on that calculation. You said you wanted a device with =
a period=20
of 20-second and as pointed out below by Chris, L is based on the length =
of the=20
boom measured from the pivot point to the center of the mass weight =
(Chris, do=20
you have this calculation??). I have found that setting the device =
up this=20
way also reduces the impact of ground deformation (boom does not=20
remains level and centered) over time which improves the overall =
operation=20
of the device keeping the boom off the stops. 
http://pw2.netcom.c=
om/~shammon1/AptosStn.htm
 
 
Regards, Steve Hammond Aptos California,  PSN San=20
Jose.
 
 -----Original Message-----
From: =
psn-l-request@.................
[mailto:psn-l-request@............... On Behalf Of=20
ChrisAtUpw@.......
Sent: Wednesday, February 13, 2008 7:08 =

PM
To: psn-l@..............
Subject: Re: How much =
mass=20
?


In a message dated 2008/02/13, jonfr@......... =
writes:


  Hi all
I am continuing to planning the build of an =
Lehman=20
    sensor. But I am
wondering what the ideal mass is going to have =
to be.=20
    But I am hoping for
at as many seconds as I can. Preferably =
around 20=20
    seconds. 


Hi=20
  Jon,

       The mass is not =
important in=20
  a Lehman, only the length between the hinge and the centre of mass =3D =
L ~56 cm.=20
  You need to keep the arm rigid but light. Between 1/2 and 1 kg is =
about right,=20
  certainly no more. We use brass rod. I use 3" x 1" Aluminum U channel, =
1/8" Al=20
  plate and SS bolts. The magnets are NdFeB, 1" square x 1/8" thick for =
the=20
  sensor and 1" x 1/2" x 1/4" thick for the damper, 4 off each=20
  type

       Have a look at our =
Lehman=20
  school seismometer. The dimensions and construction are shown. Don't =
use any=20
  of the constructions shown on psn. See=20
  =
http://www.bgs.ac.uk/education/school_seismology/seismometer.html
&nbs=
p;     =20
  I don't suppose that you are feeling rich, but MUTR sell them for =
=A3290 +=20
  carriage.
~IK 33,282 + carriage. I don't know about your import =
duty or=20
  tax. This includes the sensor, the electronics and the PSU. It plugs =
into a PC=20
  computer and runs under AmaSeis. You can get 30 seconds, although they =
only=20
  claim 20.

       Let me know if =
you need=20
  more information.

      =20
  Regards,

       Chris =
Chapman=20