In a message dated 12/06/2007, jonfr@......... writes:

I have  finished the basic lehman type seismometer build. A picture of it
can be  seen  here,
http://www.jonfr.com/myndir/v/geology/seismometer/p6120001.jpg.html

What  currently is missing is decent magnetic damping and a decent mass.
But I  also would like an opinion on this design that I am using.

The plot  from this seismometer can be seen on my webpage.



Hi Jon,
 
    I can't see from your photo what sort of top and  bottom bearings you are 
using. Ball on a plane or crossed cylinder systems are  both easy to make and 
give excellent performance. You can also use flexing piano  wire.
    You are unlikely  to get long periods easily / reliably if you use knife 
edge or point in a cup  joints.
    
    It is much better if you put a crossbar immediately  behind the mass and 
use a V suspension from the ends up to the top bearing. This  stops the arm 
trying to rotate about it's long axis. Go to a shop which sells  fishing tackle 
and buy some nylon coated 7 strand SS trace wire, preferably  rated at 30 lbs 
or over. This can be attached by loops and crimp tubes, usually  sold with the 
reel of wire. You can buy it rated up to 200 lbs, but this is  excessive for 
a Lehman.
 
    A magnetic damping system for a Lehman can be seen  at 
_http://jclahr.com/science/psn/chapman/lehman/index.html_ 
(http://jclahr.com/science/psn/chapman/lehman/index.html)  You  do need rectangular damping magnets NOT round ones.
 
    You can also use a couple of Alnico magnets firmly  attached to the 
baseplate with a vertical copper or aluminum sheet in between,  as on the first 
Lehman design. However, the rectangular NdFeB quad magnet design  is much 
cheaper, more powerful and easier to adjust. If you use a  horizontal damping tongue, 
you simply slide the magnet housing over it  until you get adequate damping. 
I suggest that you do not even  consider oil damping. It is temperature 
sensitive, relatively difficult to  adjust and potentially quite messy.
 
    I suggest that you use a mass of about 1 kg,  preferably non magnetic. I 
use round brass rod or square / rectangular  brass bar. It is better if you 
can extend the base, say with L angle  steel or Al to the far end of the mass 
and put the vertical period adjust screw  there. In the photo shown, the mass is 
on the end of the arm, then comes the  coil + sensor magnet, then the 
suspension fitting, then the horizontal damping  blade and the sliding damping 
magnet. It is made as a single unit.
 
    How long an arm are you using?
 
    Regards,
 
    Chris Chapman



   






In a message dated 12/06/2007, jonfr@......... writes:
<=
FONT=20
  style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000 size=
=3D2>I have=20
  finished the basic lehman type seismometer build. A picture of it
can b=
e=20
  seen=20
  here,
http://www.jonfr.com/myndir/v/geology/seismometer/p6120001.jpg.ht=
ml

What=20
  currently is missing is decent magnetic damping and a decent mass.
But=20=
I=20
  also would like an opinion on this design that I am using.

The plot=
=20
  from this seismometer can be seen on my webpage.


Hi Jon,
 
    I can't see from your photo what sort of top an=
d=20
bottom bearings you are using. Ball on a plane or crossed cylinder systems a=
re=20
both easy to make and give excellent performance. You can also use flexing p=
iano=20
wire.
    You are unlikely=20
to get long periods easily / reliably if you use knife edge or point in a cu=
p=20
joints.

    
    It is much better if you put a crossbar immedia=
tely=20
behind the mass and use a V suspension from the ends up to the top bearing.=20=
This=20
stops the arm trying to rotate about it's long axis. Go to a shop which sell=
s=20
fishing tackle and buy some nylon coated 7 strand SS trace wire, preferably=20
rated at 30 lbs or over. This can be attached by loops and crimp tubes, usua=
lly=20
sold with the reel of wire. You can buy it rated up to 200 lbs, but this is=20
excessive for a Lehman.
 
    A magnetic damping system for a Lehman can be s=
een=20
at http://jcla=
hr.com/science/psn/chapman/lehman/index.html You=20
do need rectangular damping magnets NOT round ones.
 
    You can also use a couple of Alnico magnets fir=
mly=20
attached to the baseplate with a vertical copper or aluminum sheet in betwee=
n,=20
as on the first Lehman design. However, the rectangular NdFeB quad magnet de=
sign=20
is much cheaper, more powerful and easier to adjust. If you use a=20
horizontal damping tongue, you simply slide the magnet housing over it=20
until you get adequate damping. I suggest that you do not even=20
consider oil damping. It is temperature sensitive, relatively difficult=
 to=20
adjust and potentially quite messy.
 
    I suggest that you use a mass of about 1 kg,=20
preferably non magnetic. I use round brass rod or square / rectang=
ular=20
brass bar. It is better if you can extend the base, say with L ang=
le=20
steel or Al to the far end of the mass and put the vertical period adjust sc=
rew=20
there. In the photo shown, the mass is on the end of the arm, then comes the=
=20
coil + sensor magnet, then the suspension fitting, then the horizontal dampi=
ng=20
blade and the sliding damping magnet. It is made as a single unit.
 
    How long an arm are you using?
 
    Regards,
 
    Chris Chapman