PSN-L Email List Message

Subject: Lehman Construction Questions...
From: Brad Rogers rogers_psn@.............
Date: 9 Apr 2000 07:23:30 -0700

I'm in the early stages of constructing a Lehman type seimometer, and have a few questions for the list....
Can anyone give me a good explaination of the tradeoffs involved between:
1)  strength of the magnet used in the pickup on a Lehman seismometer, 
2) the number of windings in the pickup coil, 
3) the bit resolution of my A->D board?  
I have a 12 bit A->D board, and live in central New York state.  Will I gain additional sensitivity by increasing the strength 
of the magnet used in the pickup?  If I use a stronger magnet do I need to increase the number of windings in the coil?  I have
 a couple magnet options, with one being a 24 pound pull, and the second being a 30 pound pull.  The difference in cost is not 
great, but do I gain anything by using a stronger magnet?
My second line of questioning is the construction of the boom.  For teleseismic events I obviously want the longest period poss
ible.  Certainly the period can be lengthened by careful adjustment of the boom balance.  But I'm also assuming that this delic
ate balancing act can be simplified by:
1) slightly lengthening the boom, 
2) getting the lead mass as close to the outside end of the boom as possible, and 
3) putting the attachment point for the suspending wire as close to the outside end of the boom as possible.  
4) I'm also assuming that I want to get the pickup coil as far out on the boom as possible, in order to increase its range of m
At least these things would seem to be intuitively true.  Are they?  Or am I missing something.
  --Brad Rogers


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