PSN-L Email List Message

Subject: Re: Adjusting Lehman Resonant Period
From: ChrisAtUpw@.......
Date: Sat, 7 Jun 2008 19:50:23 EDT


=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
    

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