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

> >Is the seismometer right next to an outside wall?  (Yes) 
> >Which direction does the wall face? (East/West)

Hi Ted,

       A location close to a load bearing outside wall is likely to be 
effected by any movements of the wall, due to rainfall on the soil outside, the 
effect of the sun raising the wall temperature and also noise from winds. 

> >What are the dimensions of the Lehman, what is the boom made of and what 
> is the mass?  36" boom 1/2" alum rod.  Dim. 20"wide, 22" tall, 36" long.  The 
> mass is 2.5# Alum cylinder

       Thanks. What is the weight of the Aluminum cylinder, please - or 
alternatively it's length? Is it solid Al, 2.5" OD?

> Chris, I think I found the drift issue?  I added a wing nut to lock the 
> legs after adjustment.  I think they were moving over time?

       That could well be the problem. Threads are cut at about a 60 degree 
angle and are rarely a tight fit. If the thread and the nut / threaded insert 
are not of the same material, you get differential expansion which slides the 
threads radially. Similarly, unless the adjusting screw is under tension, any 
differential expansion of the frame relative to the Earth underneath will try 
to tilt the thread and cause it to slip diagonally. It is preferable to use a 
wavy spring washer between the wing nut / lock nut and the frame to keep a 
fairly constant tension. This will not be much effected by the small amounts of 
differential expansion of the base and the bolt. 

       Let us know if this cures your drift problems?

       Regards,

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



>Is the seismometer right ne=
xt to an outside wall?  (Yes) 

>Which direction does the wall face? (East/West)




Hi Ted,



       A location close to a load bearing outs=
ide wall is likely to be effected by any movements of the wall, due to rainf=
all on the soil outside, the effect of the sun raising the wall temperature=20=
and also noise from winds. 



>What are the dimensions of=
 the Lehman, what is the boom made of and what is the mass?  36" boom 1=
/2" alum rod.  Dim. 20"wide, 22" tall, 36" long.  The mass is 2.5#=
 Alum cylinder




       Thanks. What is the weight of the Alum=
inum cylinder, please - or alternatively it's length? Is it solid Al, 2.5" O=
D?



Chris, I think I found the dri=
ft issue?  I added a wing nut to lock the legs after adjustment. =20=
I think they were moving over time?




       That could well be the problem. Threads=
 are cut at about a 60 degree angle and are rarely a tight fit. If the threa=
d and the nut / threaded insert are not of the same material, you get differ=
ential expansion which slides the threads radially. Similarly, unless the ad=
justing screw is under tension, any differential expansion of the frame rela=
tive to the Earth underneath will try to tilt the thread and cause it to sli=
p diagonally. It is preferable to use a wavy spring washer between the wing=20=
nut / lock nut and the frame to keep a fairly constant tension. This will no=
t be much effected by the small amounts of differential expansion of the bas=
e and the bolt. 



       Let us know if this cures your drift pr=
oblems?



       Regards,



       Chris Chapman