In a message dated 2008/05/27, tchannel1@............ writes:

> The reason I know it is missing earthquakes is that I am running three 
> channels, One vertical spring, one tiltmeter, and one Lehman..........The Lehman 
> misses a lot of events the other two record.

Hi Ted,

       OK. I accept that. Which compass direction is the boom please?

> Yes I think part of it is directional.
>  
> Would you think this could account for it..............The Lehman is set for 
> twenty seconds, but at twenty seconds, I think it is maxed out, that is to 
> say,  I don't think I could get 21 seconds.
> Even 20 seconds, for this unit, may be too much.   I say that because it 
> drifts, sometimes falling 1/2" per week to the left or to the right.  I adjust 
> it back to  center and it stays only for a few days.
>  
> Might it simply be this unit is not capable of working at 20 seconds. The 
> sticking you mentioned.........could this be because it is overextended, 
> perhaps this machine is only capable of a WORKING PERIOD of 15 seconds?

       No. If it works at 20 seconds, it should continue to do so. I am 
talking about stiction in the bearings. When we first tried out our school 
seismometer, they had not fitted a hard counterplate on the end of the Aluminum boom 
as I had specified, just a SS bearing on the vertical. The bearing made a tiny 
dent on the end of the boom and it did not respond properly. I then glued a SS 
scalpel blade to the boom and it was like a different instrument!

       What happens if you try to increase the period beyond 20 seconds? The 
original type Lehmans with a knife edge for the bottom bearing had difficulty 
getting beyond about 12 seconds. The knife edge either failed or dug into the 
counterface. A really dumb idea.

>  When I set it to 20, and move the boom, it does go to center, but because 
> of the drift issue, it may not be capable of HOLDING CENTER. and perhaps this 
> is stopping its sensitivity to all but the larger earthquakes?

       The instability could have several causes. One of these is the 
suspensions. You have not reminded me what you are using for the top and bottom 
suspensions!

       The drift with time could be related to temperature changes, maybe 
direct movement of the adjustment screws or movement over the ground due to 
differential expansion of the frame. What metal are the adjustement screws? Is the 
threaded insert / nut made from the same metal? Do you have three SS metal 
plates glued to the floor to support the adustment screws? Or are the screws in 
direct contact with the floor? I fit a nut and a WAVY spring washer on TOP of 
the baseplate, so that the three adjustment screws are ALWAYS held in tension. 

       I am not 'being fussy'.  The axis offset is maybe 0.32 degree - 0.1" 
if the vertical is 18" high.
 
       It is much more likely to be direct tilt of the floor. Where is the 
seismometer located? What sort of floor is it? Is the seismometer right next to 
an outside wall? Which direction does the wall face?

       What are the dimensions of the Lehman, what is the boom made of and 
what is the mass?

       Regards,

       Chris Chapman   
In a me=
ssage dated 2008/05/27, tchannel1@............ writes:



The reason I know it is missing=
 earthquakes is that I am running three channels, One vertical spring, one t=
iltmeter, and one Lehman..........The Lehman misses a lot of events the othe=
r two record.




Hi Ted,



       OK. I accept that. Which compass direct=
ion is the boom please?



Yes I think part of it is dire=
ctional.

 

Would you think this could account for it..............The Lehman is set fo=
r twenty seconds, but at twenty seconds, I think it is maxed out, that is to=
 say,  I don't think I could get 21 seconds.

Even 20 seconds, for this unit, may be too much.   I say that bec=
ause it drifts, sometimes falling 1/2" per week to the left or to the right.=
  I adjust it back to  center and it stays only for a few days.
 

Might it simply be this unit is not capable of working at 20 seconds. The s=
ticking you mentioned.........could this be because it is overextended, perh=
aps this machine is only capable of a WORKING PERIOD of 15 seconds?



       No. If it works at 20 seconds, it shou=
ld continue to do so. I am talking about stiction in the bearings. When we f=
irst tried out our school seismometer, they had not fitted a hard counterpla=
te on the end of the Aluminum boom as I had specified, just a SS bearing on=20=
the vertical. The bearing made a tiny dent on the end of the boom and it did=
 not respond properly. I then glued a SS scalpel blade to the boom and it wa=
s like a different instrument!



       What happens if you try to increase the=
 period beyond 20 seconds? The original type Lehmans with a knife edge for t=
he bottom bearing had difficulty getting beyond about 12 seconds. The knife=20=
edge either failed or dug into the counterface. A really dumb idea.



 When I set it to 20, and move=
 the boom, it does go to center, but because of the drift issue, it may not=20=
be capable of HOLDING CENTER. and perhaps this is stopping its sensitivity t=
o all but the larger earthquakes?




       The instability could have several caus=
es. One of these is the suspensions. You have not reminded me what you are u=
sing for the top and bottom suspensions!



       The drift with time could be related to=
 temperature changes, maybe direct movement of the adjustment screws or move=
ment over the ground due to differential expansion of the frame. What metal=20=
are the adjustement screws? Is the threaded insert / nut made from the same=20=
metal? Do you have three SS metal plates glued to the floor to support the a=
dustment screws? Or are the screws in direct contact with the floor? I fit a=
 nut and a WAVY spring washer on TOP of the baseplate, so that the three adj=
ustment screws are ALWAYS held in tension. 



       I am not 'being fussy'.  The axis=20=
offset is maybe 0.32 degree - 0.1" if the vertical is 18" high.

 

       It is much more likely to be direct til=
t of the floor. Where is the seismometer located? What sort of floor is it?=20=
Is the seismometer right next to an outside wall? Which direction does the w=
all face?



       What are the dimensions of the Lehman,=20=
what is the boom made of and what is the mass?



       Regards,



       Chris Chapman