Hi Dave, it's Steve and after reading your posts for the last few weeks
I thought I might just ask a couple of basic questions. I'm uncertain
about one point; is this your first Lehman?  If so, here a couple of
thing I always do to just get mine into operation. Did you try the WUWA
test? (Walk up walk away) Can you step back and then wait, then walk up
to the side of the seismo, wait a few sec. and then walk away? The boom
should take about three cycles, loosing 1/3 on each pass and then come
back to rest at center (the starting point) and only move for natural
background noise. If not, the next thing I do is place a simple bubble
level on the boom and make sure the mass end is not up. The boom must be
slightly below level. If it is up it just osc or it may go to rest to
the left or right boom stops. When adjusting the period I always open
the damping mags up and start with the boom being well below level. I
then raise the mass end until I get about a 10 sec period and I then
adjust the damping mags so that the boom comes to rest at center after
three cycles. If I have just replaced the support wires or any key parts
I let it sit for a day or two before trying to extend the period. From
that point on it is just a matter of a few slight tweaks on the leveling
screws and the boom wire turnbuckle to extend the period to 12-16
seconds where I try to keep it during normal operation. I hope you find
this helpful. 
 
Regards, Steve Hammond PSN San Jose, Aptos CA
-----Original Message-----
From: psnlist-request@..............
[mailto:psnlist-request@............... On Behalf Of Christopher Chapman
Sent: Sunday, August 07, 2011 8:56 AM
To: psnlist@............... dave.nelson@...............
Subject: Re: new seismo









From: Dave Nelson 
To: psnlist 
Sent: Sat, 6 Aug 2011 23:47
Subject: Re: new seismo


hi guys



**** Am still getting nowhere with this system  !!  :(



**** Rebuilt the dampening system and it works well for the 
natural period of the seismo. That is, have the boom swinging 
naturally, ~ 7 sec period. Slide up the dampening magnets to 
the aluminium vane and the natural osc stops within a cycle. 
That part looks ok.
.
Hi Dave, 
    I would not call getting your damper actually operating, from 
being totally useless, exactly 'going nowhere' ?
    Now you need to increase the period to about 25 seconds and re 
set the damping to 0.7 critical. A period of 7 seconds is only 
useful if you just want to observe microseisms ! You need to be 
able to pick up the ~20 second surface waves at their full 
amplitude.
.


    To set the damping accurately, I stick a label with a vertical 
line on it onto the mass and view the line with a magnifying glass 
through a transparent plastic ruler. I deflect the mass 10 mm and 
release it. The mass moves back toward the balance point and 
usually past it. I check how far it moves past the balance point. 
I increase / decrease the damping until the mass goes 1/2 mm past 
the balance point and then moves slowly back to zero without any 
oscillations. The arm is then set to 0.7 critical damping. The 
amount of damping needed decreases as the set period is increased.
    


****The problem is when viewed on SDR there is an ongoing osc of ~ 10 Hz

that slowly dies off over another ~ 30 sec till the trace is basically 

flatlining. (of course you can't see the 10Hz osc visibly.)

****With the dampening in place, the seismo is VERY responsive to 
hi freq (>1Hz) noise and any of this noise, walking around the 
house near the seismo, vehicular traffic out on the road, etc etc 
just shows up as a large amplitude of this 10Hz. osc that then dies 
off back to the flatline ( + - a small data count).



****If I remove the sensor coil from the arm there is NO ~ 10 Hz osc 
visible. That sort of indicates that its not some extraneous signal 
being picked up. But on the other hand, I can't imagine how a 
supposed long period device is even able to osc at ~ 10 Hz ??

A pic of the Lehman http://www.sydneystor
 mcity.com/IMGP2023a.jpg





     Can you show us a new photo of the modified Lehman, please ? 
    As shown, the coil will be very sensitive to vertical motions 
of the mass. You can eliminate most of this with a quad magnet in 
a soft iron frame. GUESS : the oscillations are coming from the 
vertical motion of the mass bending the long baseplate - you have 
something like a 'tuning fork' construction.
    If you have only one magnet disk, or one top and bottom, you 
need to offset the edge of the magnets to the centre of the coil. 
Otherwise you WON'T get positive and negative signals from the 
coil - only same polarity signals for movements in BOTH directions, 
with a null when the magnets are centralised !! 
.
    Regards,
    Chris Chapman








Hi=20
Dave, it's Steve and after reading your posts for the last few weeks I =
thought I=20
might just ask a couple of basic questions. I'm uncertain about one =
point; is=20
this your first Lehman?  If so, here a couple of thing I always do =
to just=20
get mine into operation. Did you try the WUWA test? (Walk up walk =
away) Can=20
you step back and then wait, then walk up to the side of the seismo, =
wait a few=20
sec. and then walk away? The boom should take about three cycles, =
loosing 1/3 on=20
each pass and then come back to rest at center (the starting point) and =
only=20
move for natural background noise. If not, the next thing I do is place =
a simple=20
bubble level on the boom and make sure the mass end is not up. The boom =
must be=20
slightly below level. If it is up it just osc or it may =
go to=20
rest to the left or right boom stops. When adjusting the period I always =
open=20
the damping mags up and start with the boom being well below level. =
I=20
then raise the mass end until I get about a 10 sec period and I =
then adjust=20
the damping mags so that the boom comes to rest at center after =
three=20
cycles. If I have just replaced the support wires or any key parts I let =
it sit=20
for a day or two before trying to extend the period. From that point on =
it is=20
just a matter of a few slight tweaks on the leveling screws and the boom =
wire=20
turnbuckle to extend the period to 12-16 seconds where I try to keep it =
during=20
normal operation. I hope you find this =
helpful. 
 
Regards, Steve Hammond PSN San Jose, Aptos =
CA

-----Original =
Message-----
From:=20
psnlist-request@.............. [mailto:psnlist-request@............... =
On=20
Behalf Of Christopher Chapman
Sent: Sunday, August 07, =
2011 8:56=20
AM
To: psnlist@..................
dave.nelson@...............
Subject: Re: new=20
seismo



  

  

  

  

  

  

  

  

  

  

  

  
From:=20
  Dave Nelson <dave.nelson@...............>
To: psnlist=20
  <psnlist@..............>
Sent: Sat, 6 Aug 2011 =
23:47
Subject: Re:=20
  new seismo


  
hi guys

**** Am still getting nowhere with this system  !!  :(

**** Rebuilt the dampening system and it works well for the =
natural period of the seismo. That is, have the boom swinging 
naturally, ~ 7 sec period. Slide up the =
dampening magnets to =
the =
aluminium vane and the natural osc stops within a cycle. 
That part looks ok.
.
Hi Dave, =
    I =
would not call getting your damper actually operating, from =
being =
totally useless, exactly 'going nowhere' =
?
    =
Now you need to increase the period to about 25 seconds and re =
set =
the damping to 0.7 critical. A period of 7 seconds is only =
useful if you just want to observe microseisms ! You need to be =
able =
to pick up the ~20 second surface waves =
at their full 
amplitude.
.
    To set the damping =
accurately, I stick a label with a vertical 
line on =
it onto the mass and =
view the line with a magnifying glass =
through a transparent =
plastic ruler. I deflect the mass 10 mm =
and 
release it. The =
mass moves back toward the balance point =
and 
usually past it. =
I check how far it moves past the balance =
point. 
I increase / =
decrease the damping until the mass goes =
1/2 mm past 
the balance point and then moves slowly back to zero without any =
oscillations. =
The arm is then set to 0.7 critical =
damping. The 
amount of damping =
needed decreases as the set period is =
increased.
    
****The problem is when viewed on =
SDR there is an ongoing osc of ~ 10 Hz 
that slowly dies =
off over another ~ 30 sec =
till the trace is basically=20
flatlining. (of course you =
can't see the 10Hz osc visibly.)
****With the dampening in place, the seismo is VERY responsive to 
hi freq (>1Hz) noise and any of this =
noise, walking around the 
house near the seismo, vehicular traffic out on the road, etc etc 
just shows up as =
a large =
amplitude of this 10Hz. osc that then dies 
off back to the =
flatline ( + - a small data count).

****If I remove the sensor coil from the arm there is NO ~ 10 =
Hz osc 
visible. That sort of =
indicates that its =
not some extraneous signal 
being picked up. But on the =
other hand, I can't imagine how a 
supposed =
long period device is even able to =
osc at ~ 10 Hz ??
A pic of the Lehman http://www.sydneystormcity.com/IMGP2023a.jpg


     Can you show us a new =
photo of the modified Lehman, please ? 
    As shown, the coil will be =
very sensitive to vertical motions 
of the mass. You can eliminate most of this with a =
quad magnet in 
a soft iron frame. GUESS : the oscillations are coming from the =
vertical =
motion of the mass bending the long baseplate - you have =
something like =
a 'tuning fork' construction.
    If you have only one magnet disk, or one top and =
bottom, you 
need to offset the edge of the magnets to the centre of the =
coil. 
Otherwise you WON'T get positive and negative signals from the =
coil - only =
same polarity signals for movements in BOTH =
directions, 
with a null when the magnets are centralised !! =
.
    =
Regards,
    =
Chris Chapman