=20
In a message dated 22/03/2008, gel@................. writes:

I=E2=80=99m  looking to finally build my Lehman seismometer and I was wonder=
ing what=20
are  the latest recommendations in the construction. I=E2=80=99ve downloaded=
 many of=20
the  changes and recommendations that people have submitted. I have one=20
question,  is the pickup coil and magnet still the best method to detect mot=
ion. I =20
thought I saw some mention of using Hall-Affect devices as sensors. Any hint=
s =20
are much appreciated.


Hi Gary,
=20
    I don't know what counts as 'new'?
=20
    Hall effect devices are fine for 1 to 2 second  pendulums / SG systems,=20
but not too good for Lehmans. They only have a  measurement range of +/- 0.5=
 to=20
+/- 1 mm and Lehmans suffer from tilt drift with  time of several mm.
=20
    You would be much better using a rectangular  coil + quad NdFeB magnet=20
system. See drawings at=20
_http://jclahr.com/science/psn/chapman/lehman/index.html_ (http://jclahr.com=
/science/psn/chapman/lehman/index.html)  The  quad magnet=20
system gives about 10x the output of older Alnico U magnet + coil  systems=20
with much smaller coils. It is also ~cheaper. Put the coil on the arm  and t=
he=20
damping and sensor magnets on the frame. Brass is heavy, non magnetic,  easy=
 to=20
machine and makes a good mass of 1 to 2 lbs. 3/4" square or round  Al tube,=20
about 2 ft long, makes a good boom - definitely keep it rigid, but  light in=
=20
comparison to the mass.
    You can use SS ball bearing on a flat SS blade  suspension or crossed SS=
=20
shoulder bolts or crossed tungsten carbide drill  shanks. You can also use=20
flex wire and crossed 8 thou music wire systems. The  all work well. Avoid p=
oint=20
in a cup or knife edge suspensions.
    I make a T frame out of 3" x 1" x 1/8" U channel Al  with 1/8" thick=20
corner plates, bolted together with 1/4" SS marine bolts.  Ordinary steel an=
d=20
brass will corrode Al quite rapidly. This is 'easy' to do and  works well. Y=
ou=20
mount all the components on the one frame, which you then tilt  to adjust th=
e=20
balance and set the period. Avoid the older systems which use  separately mo=
unted=20
damping and sensor components. Use a cross bar near the mass  and a V top=20
wire suspension. I use 40 lb fishing trace. This prevents the arm  from rota=
ting=20
during a quake. Try to mount the damping blade ~level with the C  of G of th=
e=20
mass - underneath the arm.
    Use magnetic damping; definitely avoid oil damping.  Oil is only good fo=
r=20
about a +/-2 C Deg temperature range, less than average  daily room=20
temperature variations and it is messy. Magnetic damping is easy  to set up=20=
and adjust=20
and it is not effected by temperature. It is also  clean.
=20
    Also have a look at=20
_http://www.bgs.ac.uk/education/school_seismology/seismometer.html_=20
(http://www.bgs.ac.uk/education/school_seismology/seismometer.html)  This  i=
s the UK school seismometer system and uses top and bottom=20
crossed tungsten  carbide roller suspensions. The mass is on the end of the=20=
arm=20
followed by the  sensor magnet and the mounted coil, the suspension block an=
d the=20
damper blade +  sliding damper magnet block system. To set it up, you adjust=
=20
the cross balance  first, then set the period to ~25 seconds by adjusting th=
e=20
frame tilt and then  slide the damper magnet over the damping blade till you=
=20
get 0.7 critical  damping.=20
=20
    Hope that is helps.
=20
    Regards,
=20
    Chris Chapman



  =20






In a message dated 22/03/2008, gel@................. writes:
<=
FONT=20
  style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000 size=
=3D2>
  
I=E2=80=99m=20
  looking to finally build my Lehman seismometer and I was wondering what ar=
e=20
  the latest recommendations in the construction. I=E2=80=99ve downloaded ma=
ny of the=20
  changes and recommendations that people have submitted. I have one questio=
n,=20
  is the pickup coil and magnet still the best method to detect motion. I=20
  thought I saw some mention of using Hall-Affect devices as sensors. Any hi=
nts=20
  are much appreciated.

Hi Gary,
 
    I don't know what counts as 'new'?
 
    Hall effect devices are fine for 1 to 2 second=20
pendulums / SG systems, but not too good for Lehmans. They only have a=20
measurement range of +/- 0.5 to +/- 1 mm and Lehmans suffer from tilt drift=20=
with=20
time of several mm.
 
    You would be much better using a rectangul=
ar=20
coil + quad NdFeB magnet system. See drawings at http://jcla=
hr.com/science/psn/chapman/lehman/index.html The=20
quad magnet system gives about 10x the output of older Alnico U magnet + coi=
l=20
systems with much smaller coils. It is also ~cheaper. Put the coil on the ar=
m=20
and the damping and sensor magnets on the frame. Brass is heavy, non magneti=
c,=20
easy to machine and makes a good mass of 1 to 2 lbs. 3/4" square or rou=
nd=20
Al tube, about 2 ft long, makes a good boom - definitely keep it rigid,=
 but=20
light in comparison to the mass.
    You can use SS ball bearing on a flat SS blade=20
suspension or crossed SS shoulder bolts or crossed tungsten carbide drill=20
shanks. You can also use flex wire and crossed 8 thou music wire systems. Th=
e=20
all work well. Avoid point in a cup or knife edge suspensions.
    I make a T frame out of 3" x 1" x 1/8" U channe=
l Al=20
with 1/8" thick corner plates, bolted together with 1/4" SS marine bolts.=20
Ordinary steel and brass will corrode Al quite rapidly. This is 'easy' to do=
 and=20
works well. You mount all the components on the one frame, which you then ti=
lt=20
to adjust the balance and set the period. Avoid the older systems which use=20
separately mounted damping and sensor components. Use a cross bar near the m=
ass=20
and a V top wire suspension. I use 40 lb fishing trace. This prevents the ar=
m=20
from rotating during a quake. Try to mount the damping blade ~level with the=
 C=20
of G of the mass - underneath the arm.
    Use magnetic damping; definitely avoid oil damp=
ing.=20
Oil is only good for about a +/-2 C Deg temperature range, less than average=
=20
daily room temperature variations and it is messy. Magnetic damping is=20=
easy=20
to set up and adjust and it is not effected by temperature. It is also=20
clean.
 
    Also have a look at h=
ttp://www.bgs.ac.uk/education/school_seismology/seismometer.html Th=
is=20
is the UK school seismometer system and uses top and bottom crossed tungsten=
=20
carbide roller suspensions. The mass is on the end of the arm followed by th=
e=20
sensor magnet and the mounted coil, the suspension block and the damper blad=
e +=20
sliding damper magnet block system. To set it up, you adjust the cross balan=
ce=20
first, then set the period to ~25 seconds by adjusting the frame tilt and th=
en=20
slide the damper magnet over the damping blade till you get 0.7 critical=20
damping. 
 
    Hope that is helps.
 
    Regards,
 
    Chris Chapman