In a message dated 04/12/2001, david_j_findlay@............ writes:

> I'm in Brisbane Australia, and looking at building a Lehmann sensor because 
>    > 
> I live about 400km from an tremor prone area, Bundaberg - Harvey Bay. I'd 
> like to be able to pick up stuff in that area, as well as around the rest 
> of the world. 
> I was going to use timber to build my Lehmann unit with a dowel for the 
> boom 
> and fishing line for the suspension of the boom. It will use a very strong 
> magnet for the pickup coil, and magnet horizontal damping at the end. 

Hi there David,

       I think that you are just inviting trouble with a wooden construction. 
Even if you could get it to work, it won't be stable enough to stay in 
adjustment. I suggest that you use metal to build the frame, a metal boom and 
piano wire suspension. You can use slotted angle steel / aluminium or steel 
water pipe or copper / stainless steel water pipe. With copper pipe you can 
use soldered joints or compression fittings. With stainless steel you use 
compression fittings. With galvanised steel, you need to be able to thread 
the pipe and use threaded fittings. The bottom hinge joint on the boom can be 
a ball bearing or a suspension wire. Suggest that you look around the psn.net 
sites for photo examples.
 
> mounted on buried concrete 
> besser blocks to hold it stable. 

       You would be much better casting a square / rectangular slab using a 
cement and sand mixture, no gravel. This gives the least movement while the 
mix cures slowly over some months. Make it about twice the dimensions of the 
seis.

> Is there anything I haven't thought of, and will this be able to do what I'm 
> interested in? What length pendulum should I use?

       The old commercial seis used about 18" boom, but 12" seems OK. See 
John Cole's mini Lehman.

       Regards,

       Chris Chapman


In a message dated 04/12/2001, david_j_findlay@............ writes:



I'm in
 Brisbane Australia, and looking at building a Lehmann sensor because 

it looks pretty easy to build.
   


I live about 400km from an tremor prone area, Bundaberg - Harvey Bay. I'd like to be able to pick up stuff in that area, as
 well as around the rest of the world. 

I was going to use timber to build my Lehmann unit with a dowel for the boom 

and fishing line for the suspension of the boom. It will use a very strong 

magnet for the pickup coil, and magnet horizontal damping at the end. 




Hi there David,



       I think 
that you are just inviting trouble with a wooden construction. Even if you could get it to work, it won't be stable enough to s
tay in adjustment. I suggest that you use metal to build the frame, a metal boom and piano wire suspension. You can use slotted
 angle steel / aluminium or steel water pipe or copper / stainless steel water pipe. With copper pipe you can use soldered join
ts or compression fittings. With stainless steel you use compression fittings. With galvanised steel, you need to be able to th
read the pipe and use threaded fittings. The bottom hinge joint on the boom can be a ball bearing or a suspension wire. Suggest
 that you look around the psn.net sites for photo examples.

 

It will be in a small enclosure in the ground
, 
mounted 
on buried concrete besser blocks to hold it stable. 




       You woul
d be much better casting a square / rectangular slab using a cement and sand mixture, no gravel. This gives the least movement 
while the mix cures slowly over some months. Make it about twice the dimensions of the seis.



Is there anything I haven't thought of, and will this b
e able to do what I'm interested in? What length pendulum should I use?




       The old 
commercial seis used about 18" boom, but 12" seems OK. See John Cole's mini Lehman.



       Regards,




       Chris Chapman