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=20
hinge and the centre of mass =3D L ~56 cm. You need to keep the arm rigid bu=
t=20
light. Between 1/2 and 1 kg is about right, certainly no more. We use brass=20=
rod. I=20
use 3" x 1" Aluminum U channel, 1/8" Al plate and SS bolts. The magnets are=20
NdFeB, 1" square x 1/8" thick for the sensor and 1" x 1/2" x 1/4" thick for=20=
the=20
damper, 4 off each type

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




Hi Jon,



       The mass is not important in a Lehman,=20=
only the length between the hinge and the centre of mass =3D L ~56 cm. You n=
eed to keep the arm rigid but light. Between 1/2 and 1 kg is about right, ce=
rtainly 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 s=
ensor and 1" x 1/2" x 1/4" thick for the damper, 4 off each type



       Have a look at our Lehman school seismo=
meter. The dimensions and construction are shown. Don't use any of the const=
ructions shown on psn. See http://www.bgs.ac.uk/education/school_seismology/=
seismometer.html

       I don't suppose that you are feeling ri=
ch, but MUTR sell them for =A3290 + carriage.

~IK 33,282 + carriage. I don't know about your import duty or tax. This incl=
udes the sensor, the electronics and the PSU. It plugs into a PC computer an=
d runs under AmaSeis. You can get 30 seconds, although they only claim 20.


       Let me know if you need more informatio=
n.



       Regards,



       Chris Chapman