In a message dated 2007/07/26, tchannel1@............ writes:

> With the rod+magnets+nuts there was no more room for a mass.  I tried 
> addition mass and it pulled the spring too long.  I wanted to limit the total 
> height to 7', but cut it down to 5'....I think at 7' the period was 2.4, and at 
> 5' it was 2.2?

Hi Ted,

       So why not chop a few turns off the spring? ! Maybe make two, with 
half length springs?

> Paul C. suggested I use two ring magnets, one entering the top of the coil, 
> and the other exiting the bottom of the coil.
> I started with one magnet 1/2 (deep inside the coil), but I did not like the 
> results.  

       This would give ~zero output at the centre of the coil.
       
I then moved the magnet to the top of the coil, just entering, that worked 
better.  I 
> then added another magnet, two together, 1 and 1/2 magnets inside the top 
> of the coil. This worked well. To try Paul's idea, I would just need to open 
> it up and move the spacing nuts and magnets.

       You would probably need the magnets mounted in opposition, but this 
arrangement should reduce environmental magnetic forces on the seismometer. 

             For the sensor you would probably do to better to make an 
analogue if a LVDT. Wind two coils end to end and connect them in opposition. Then 
place the magnet(s) in the centre. The output can be made nearly independent of 
the vertical position over the length of the magnet(s) / length of the coils, 
whichever is shorter.

       You could make an anti rotation, anti pendulum swing damper with two 
pairs of bar magnets in a cross inside a copper tube, or four copper plates. 
You could also try square Al tube, or bent sheet? You could probably adjust this 
to supply the vertical damping as well?

       You might find that about a 2 second period is optimal? This should 
bring in the P waves nicely, but reduce thermal and microseism disturbances. 
Have you tried putting thermal insulation around the tube?

       Regards,

       Chris Chapman
   
In a me=
ssage dated 2007/07/26, tchannel1@............ writes:



With the rod+magnets+nuts there=
 was no more room for a mass.  I tried addition mass and it pulled the=20=
spring too long.  I wanted to limit the total height to 7', but cut it=20=
down to 5'....I think at 7' the period was 2.4, and at 5' it was 2.2?=




Hi Ted,



       So why not chop a few turns off the spr=
ing? ! Maybe make two, with half length springs?



Paul C. suggested I use two rin=
g magnets, one entering the top of the coil, and the other exiting the botto=
m of the coil.

I started with one magnet 1/2 (deep inside the coil), but I did not like th=
e results.  




       This would give ~zero output at the ce=
ntre of the coil.

       

I then moved the magnet to the top of the coil, just entering, that worked b=
etter.  I 

then added another magnet, two=
 together, 1 and 1/2 magnets inside the top of the coil. This worked well. T=
o try Paul's idea, I would just need to open it up and move the spacing nuts=
 and magnets.




       You would probably need the magnets mou=
nted in opposition, but this arrangement should reduce environmental magneti=
c forces on the seismometer. 



             For=
 the sensor you would probably do to better to make an analogue if a LVDT. W=
ind two coils end to end and connect them in opposition. Then place the magn=
et(s) in the centre. The output can be made nearly independent of the vertic=
al position over the length of the magnet(s) / length of the coils, whicheve=
r is shorter.



       You could make an anti rotation, anti p=
endulum swing damper with two pairs of bar magnets in a cross inside a coppe=
r tube, or four copper plates. You could also try square Al tube, or bent sh=
eet? You could probably adjust this to supply the vertical damping as well?<=
BR>


       You might find that about a 2 second pe=
riod is optimal? This should bring in the P waves nicely, but reduce thermal=
 and microseism disturbances. Have you tried putting thermal insulation arou=
nd the tube?



       Regards,



       Chris Chapman