=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 =20In 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