In a message dated 2008/05/30, tchannel1@............ writes:

> The second problem:  At 20 seconds, I was missing all but large 
> earthquakes, those were coming through just fine. Chris suggest the arm might be 
> sticking.  I could see nothing which resembled sticking.

Hi Ted,

       1) what top bearing / flexure are you using?
       2) what bottom bearing / flexure are you using?
       3) have you checked both the coil and the damping magnets for 
whiskers? They tend to grow with time and can cause the beam to stick. I use one of 
the pen lasers to check that the gaps are clear.

> I don't know if this was the cause and effect, but I fixed it:
>  
> First this is not the best Lehman I have ever built.  Its okay, and with 
> little effort I got the 20 seconds, but I am not sure I could get 22.  However 
> at 20 something was sticking.  
> I think at 20 it was so near infinity, it was almost flat. At infinity it 
> would never move to center, the arm would just stay where you placed it. At 20 
> seconds it would move to center, if the event was large enough to overcome 
> the inertia, but smaller earthquakes would not do this.

       This is typical of a problem with the suspension system. I don't know 
what you expected to 'see', but you have just given a good description of what 
is observed. I suspect that you will not have fixed the problem by shortening 
the period, just reduced it a bit. The stiction, from whatever cause, will 
still be there.

> I reduce the period to 15 seconds, and it is working great.  I record 6.2M 
> Iceland, and 4.1M California.   I could, now increase the period to 16,17,18 
> etc and retest.   I will see how it performs at 15 for a while.

       Regards,

       Chris Chapman   
In a me=
ssage dated 2008/05/30, tchannel1@............ writes:



The second problem:  At 20=
 seconds, I was missing all but large earthquakes, those were coming through=
 just fine. Chris suggest the arm might be sticking.  I could see nothi=
ng which resembled sticking.




Hi Ted,



       1) what top bearing / flexure are you u=
sing?

       2) what bottom bearing / flexure are yo=
u using?

       3) have you checked both the coil and t=
he damping magnets for whiskers? They tend to grow with time and can cause t=
he beam to stick. I use one of the pen lasers to check that the gaps are cle=
ar.



I don't know if this was the c=
ause and effect, but I fixed it:

 

First this is not the best Lehman I have ever built.  Its okay, and wi=
th little effort I got the 20 seconds, but I am not sure I could get 22.&nbs=
p; However at 20 something was sticking.  

I think at 20 it was so near infinity, it was almost flat. At infinity it w=
ould never move to center, the arm would just stay where you placed it. At 2=
0 seconds it would move to center, if the event was large enough to overcome=
 the inertia, but smaller earthquakes would not do this.




       This is typical of a problem with the=20=
suspension system. I don't know what you expected to 'see', but you have jus=
t given a good description of what is observed. I suspect that you will not=20=
have fixed the problem by shortening the period, just reduced it a bit. The=20=
stiction, from whatever cause, will still be there.



I reduce the period to 15 seco=
nds, and it is working great.  I record 6.2M Iceland, and 4.1M Californ=
ia.   I could, now increase the period to 16,17,18 etc and retest.=
   I will see how it performs at 15 for a while.




       Regards,



       Chris Chapman