=20
In a message dated 07/06/2008, gel@................. writes:
I=E2=80=99m the process of adjusting the resonant period of my first Lehman=
=20
seismometer. Are there other parameters besides the angle that I should be=20=
aware in=20
achieving a long period. How about the position of the mass, should I move=20=
the=20
mass towards the end of the pendulum. This assumes I have a good mounting=20
method for the support wire and a low friction bearing.
Hi Gary,
=20
You can use C of G mass to pivot lengths between about 1and 3 ft. The=20
longer lengths are easier to set up and to adjust. With short lengths you a=
re=20
very sensitive to ground tilt variations. The increase in period with lengt=
h=20
is low above 3 ft and the arm becomes difficult to make rigid and the=20
temperature + air movements (noise) more difficult to control / reduce. If=20=
you wanted=20
a vertical pendulum with a 4 second period, you would need a 12 ft long=20
arm...
I use a V support wire made from SS fishing trace onto a crossbar near=20
the mass to ensure that the mass + arm do not rotate about it's axis.=20
Otherwise you need to have the top wire support a few inches above the arm=20=
and the=20
damping plate level with the C of G of the mass. If you don't do this, the=20=
arm=20
will oscillate in rotation when a quake comes along. This motion is NOT=20
damped and it can give you a false output signal as the coil rocks in the m=
agnet=20
field. Check the background trace with a FFT for any peaks of a few Hz.
Lehmans are extremely sensitive to air current movements / drafts. You=20
need a thermally insulated box with all the joint lines airtight / sealed.=20=
You=20
can buy clear mylar film for photocopying transparancies. Use this to make=20
two layer windows with an air gap in between.
The mounting bolts and nuts need to be of the same metal. I use a wavy=20
washer + an extra nut on top of the baseplate to ensure that the three=20
levelling screws are always held in tension and do not move as the temperat=
ure=20
changes.=20
=20
Regards,
=20
Chris Chapman
=20
=20
In a message dated 07/06/2008, gel@................. writes:
<=
FONT=20
style=3D"BACKGROUND-COLOR: transparent" face=3DArial color=3D#000000 size=
=3D2>
I=E2=80=99m the process of adjusting the resonant per=
iod of my=20
first Lehman seismometer. Are there other parameters besides the angle tha=
t I=20
should be aware in achieving a long period. How about the position of the=20
mass, should I move the mass towards the end of the pendulum. This assumes=
I=20
have a good mounting method for the support wire and a low friction=20
bearing.
Hi Gary,
You can use C of G mass to pivot lengths betwee=
n=20
about 1and 3 ft. The longer lengths are easier to set up and to adjust. With=
=20
short lengths you are very sensitive to ground tilt variations. The=20
increase in period with length is low above 3 ft and the arm becomes difficu=
lt=20
to make rigid and the temperature + air movements (noise) more difficult to=20
control / reduce. If you wanted a vertical pendulum with a 4 second period,=20=
you=20
would need a 12 ft long arm...
I use a V support wire made from SS fishing tra=
ce=20
onto a crossbar near the mass to ensure that the mass + arm do not rotate ab=
out=20
it's axis. Otherwise you need to have the top wire support a few inches abov=
e=20
the arm and the damping plate level with the C of G of the mass. If you=
=20
don't do this, the arm will oscillate in rotation when a quake comes along.=20=
This=20
motion is NOT damped and it can give you a false output signal as the coil r=
ocks=20
in the magnet field. Check the background trace with a FFT for any peaks of=20=
a=20
few Hz.
Lehmans are extremely sensitive to air current=20
movements / drafts. You need a thermally insulated box with all the joint li=
nes=20
airtight / sealed. You can buy clear mylar film for photocopying transparanc=
ies.=20
Use this to make two layer windows with an air gap in between.
The mounting bolts and nuts need to be of the s=
ame=20
metal. I use a wavy washer + an extra nut on top of the baseplate to ensure=20=
that=20
the three levelling screws are always held in tension and do not move a=
s=20
the temperature changes.
Regards,
Chris Chapman